Chapter 24 Principals of Inheritance and Variation Part 1 by TEACHING CARE online coaching classes

 

 Introduction.

The term genetics was coined by Bateson (1960). Genetics is the study of principles and mechanism of heredity and variations. The resemblance amongst offspring is never 100% (except in monozygotic twins) due to reshuffling of chromosomes and their genes. Same genetic traits are present in monoparental individuals formed through asexual reproduction or mitosis. Such individuals are called ramets while the whole group of similar individuals is called clone.

 

Father of genetics (classical genetics)	Mendel
Father of modern genetics/Animal genetics	Bateson
Father of experimental genetics/Drosophila genetics	Morgan
Father of human & physiological genetics	Garrod
Father of quantitative inheritance	Kolreuter
Father of Neurospora genetics	Dodge
Father of Eugenics	Francis Galton
Father of Indian genetics	M. S. Swaminathan

 

 Heredity.

Heredity is the study of transmission of characters and variations from one generation to the next.

• Basis of heredity : Heredity involves the transfer of chromosomes from parents to offspring or one individual to another. Therefore, chromosome is the base of heredity. The physical basis of heredity are genes while chemical basis of heredity is

(2)  Pre-Mendelian view points

• Vapour and fluid theory : Greek philosopher, Pythagoras proposed that some moist vapour is given out from the brain, nerves and all other parts of the body during On account of these vapours, the offspring exhibits similarities with the male parents.
• Semen theory : Empedocles, suggested that both parents produce semen which arises directly from their various body The semen from both the parents gets mixed and produces a new individual.
• Preformation theory : Antony von Leeuwenhoek was the first to observe human sperms. This theory believes that one of the sex cells or gametes either sperm or egg, contained within itself the entire organism in perfect miniature form. Miniature form was called as ‘homunculus’. The theory was supported by Malpighi, Hartosoeker and
• Particulate theory : Maupertuis proposed that the body of each parent gives rise to minute particles. These particles unite together to form the daughter
• Encasement theory : Charles Bonnet and his supporters presumed that every female contains within her body miniature prototypes of all the creatures which would descend from her, one generation within the other, somewhat like a series of chines This was named as encasement theory.

 

 

 

• Theory of epigenesis : Wolgg proposed that the germ cells contain definite but undifferentiated substances, which after fertilization, become organised into various complex body organs that form the adult. This idea was referred to as
• Pangenesis theory : This theory was proposed by Charles Darwin according to this theory every cell, tissue and organ of animal body produces minute invisible bodies, called gemmules or pangenes. They can produce
• Weismann theory of germplasm : August Weismann (1889) suggested the theory of continuity of germplasm. He described reproductive cells as germplasm and rest of the body as The germplasm forms the bridge of life between successive generations and is passed on from one generation to the next.

• Evidences against blending theory : Thus individual would represent the mixture of both the parents. The prevailing view of in pre-mendelian era was blending The hereditary material was thought of as being analogous to a fluid. Under this concept, the progeny of a black and white animal would be uniformly grey. The further progeny from crossing the hybrids among themselves would be grey, for the black and white hereditary material, once blanded, could never be seperated again. Pattern of inheritance shown by atavism also speaks against blending theory. The traits of sex do not blend in unisexual organisms.
• Basic features of inheritance : In the middle of 18^th century, Carolus Linnaeous a Swedish taxonomist and two German plant breeders Kolreuter and Gaertner performed artificial cross pollination in plants and obtained hybrid Kolreuter obtained experimental evidence that inherited traits tended to remain discrete, although his observations were similar to mendel but he was not able to interpret them correctly. Mendel’s great contribution was to replace the blending theory with particulate theory. Few essential features of inheritance are : –
• Traits have two alternative
• Traits are represented in the individual by distinct particles which do not blend or
• Traits may remain unexpected for one or more generations and reappear later
• Traits may remain together in one generation and separate in a later
• One alternative of a trait may express more often then the

 Variations.

Variations are differences found in morphological, physiological and cytological behaviouristic traits of individuals belonging to same species race and family. They appear in offspring or siblings due to : –

• Reshuffling of genes/chromosomes by chance separation of chromosomes
• Crossing over
• Chance combination of chromosomes during meiosis and

Types of variations

• Somatic variations : These variations influence the somatic or body cells. They appear after birth and are, also called acquired characters, modifications or acquired variations. Somatic variations are non-inheritable and usually disappear with the death of the individual. They are formed due to three reasons e. environmental factors, use and disuse of organs, and conscious efforts.
  • Environmental factors : They have lesser effect on animals as compared to plants. Important environmental factors are as follows:

 

 

 

• Medium : Amphibious or emergent aquatic plants possess heterophylly, e. different types of submerged, floating and emerged leaves, e.g. Ranunculus aquatilis, Limnophila heterophylla and this meristic activities are due to change in depth and medium of water.
• Light : Partial shade causes elongation of
• Temperature : Plants of hot areas have extensive roots but smaller Human skin becomes darker with increase in environmental temperature.
• Nutrition : Honey bee larva feeding on royal jelly develops into queen while the ones obtaining ordinary nourishment (bee bread) grow into
• Water : Water deficiency leads to several modifications in plants like succulente, spines, reduced leaves, thick bark, hair
  • Use and disuse of organs : In higher animals and human beings, greater use of an organ leads to its better development as compared to other organs which are less used, g., stronger muscular body in a wrestler.
  • Conscious efforts : Acquired variations due to conscious efforts include education, training of pets boring of pinna, bonsai,
• Germinal variations : They are inheritable variations formed mostly in germinal cells which are either already present in the ancestors or develop a new due to mutations. Germinal variations are of two types, continuous and discontinuous

• Continuous variations : They are fluctuating variations and also called recombinations because they are formed due to recombination of alleles as found in sexual reproduction. Darwin (1859) based his theory of evolution on continuous
• Discontinuous variations : They are mutations, which are ultimate source of organic variations. Discontinuous variations are caused by chromosomal aberrations, change in chromosome number and gene In pea seed coat colour changes gray to white is an example of spontaneous mutation.

Importance of variations

• Variations continue to pile up forming new species with time.
• They are essential in the struggle for
• Adaptability is due to
• Variations allow breeders to improve races of plants and
• Discontinuous variations introduce new
• Inbreeding between closely related organisms reduces

 Important terms used in inheritance studies.

• Gene (Mendel called them factor) : In modern sense an inherited factor that determines a biological character of an organism is called gene (functional unit of hereditary material).
• Allelomorphs or alleles : Alleles, the abbreviated form of term allelomorphs (meaning one form or the other) indicates alternative forms of the same e.g., Tall TT and dwarf tt are alternation forms of the same gene etc.

 

 

 

• Gene locus : It is the portion or region on chromosome representing a single The alleles of a gene are present on the same gene locus on the homologous chromosomes.
• Wild and mutant alleles : An original allele, dominant in expression and wide spread in the population is called wild An allele formed by a mutation in the wild allele, recessive in expression and less common in the population is termed as mutant allele.
• Homozygous : Both the genes of a character are identical is said to be homozygous or genetically pure for that character. It gives rise to offspring having the same character on self-breeding e.g. TT (Homozygous dominant) or tt (Homozygous recessive).
• Heterozygous : Both the genes of a character are unlike is said to be heterozygous or Such organisms do not breed true on self fertilization e.g. Tt

If we know the number of heterozygous pairs we can predict the following:

Number of types of gametes = 2ⁿ

Number of F₂ phenotype = 2ⁿ   (Where n is the number of heterozygous pairs).

Number of F₂ genotype = 3ⁿ

• Genotype : The genotype is the genetic constitution of an organism. TT, Tt and tt are the genotypes of the organism with reference to these particular pairs of
• Phenotype : Expresses the characters of individuals like form, sex, colour and behaviour etc.
• Pure line : Generations of homozygous individuals which produce offsprings of only one type e. they breed true for their phenotype and genotype.
• Monohybrid, dihybrid and polyhybrid : When only one allelic pair is considered in cross breeding, it is called monohybrid cross. Similarly when two allelic pairs are used for crossing, it is called dihybrid cross and more than two allelic pairs in a cross are called polyhybrid
• Reciprocal cross : The reciprocal crosses involve two crosses concerning the same characteristics, but with reversed
• Genome : Total set of genes (DNA instructions) in the haploid set of chromosomes and inherited as unit from parents to offspring is called
• Gene pool : All the genotypes of all organisms in a population form the gene
• F₁ Generation : F₁ or first filial generation is the generation of hybrids produced from a cross between the genetically different individuals called
• F₂ Generation (Bateson, 1905) : F₂ or second filial generation is the generation of individuals which arises as a result of inbreeding or interbreeding amongst individuals of F₁
• Punnet square : It is a checker-board used to show the result of a cross between two organisms, it was devised by geneticist, C. Punnet (1927). It depicts both genotypes and phenotypes of the progeny.
• Back cross : It is cross which is performed between hybrid and one of its parents. In plant breeding, back cross is performed a few times in order to increase the traits of that
• Test cross : It is a cross to know whether an individual is homozygous or heterozygous for dominant The individual is crossed with recessive parent. The ratio will be 50% dominant and 50% recessive in

 

 

 

case of hybrid or heterozygous individual. In case of double heterozygote (e.g., RrYy) crossed with recessive (rryy) the ratio will be 1:1:1:1 test cross help to find out genotype of parents.

(F₁ Hybrid)                                                                                 (Recessive parent)

 

 

(Gametes)

 

 

 

 

 

 

1                           :                     1

Fig : Showing test cross

 

• Self cross/selfing : It is the process of fertilization with pollen or male gametes of the same individual.
• Theory of probability : (i) Out of the two alternate events, the probability of occurrence of each one of them is 50%.
• Two events are independent if occurrence of one does not affect the probability of occurrences of the
• The probability of joint occurrence of two independent events is the product of their individual
• For an event, which can happen through two independent pathways, the probability of its occurrence is the sum of separate
• Observed Vs expected results : Experimental results conform to the ones expected through the theory of probability if the size of the sample is small but they tend to approach the latter if the sample size is
• Hybrid : The organism produced after crossing of two genetically different individuals is called

(23)  Heredity and variations in sexual and asexual reproduction

• Sexual reproduction : Variations are common in animals and plants which reproduce by sexual means. The reason for this is that the sexual reproduction is biparental, involves meiosis and fertilization, and the offspring receives some traits from father and some from
• Asexual reproduction : Those organisms which reproduce by asexual means eq. bacteria, amoeba, euglena, rose The asexual reproduction is monoparental, involves mitosis and the organism produced by it, inherits all the traits of its single parent. With the result, it is almost a carbon copy of the parent and is known as ramet. A group of ramets is called a clone.

Differences between clone and offsprings

 

Characters	Clone	Offspring
Type of reproduction	Clone is the product of asexual reproduction	Offspring is the product of sexual reproduction
Number of parents	Clone is monoparental	Offsprings biparental	is	derived	from	two	parents	thus

 

 

 

Cell division	Clone is formed by mitosis. meiosis does not occur.	Meiosis takes place prior to formation of gametes
Resemblance	Clone exactly resembles the parent	Offspring differs from parents.

 Mendel’s predecessors.

A number of scientists had worked on plant hybridization during the 18^th and 19^th centuries prior to the mendel. Some of the more notable scientists among them are Joseph Koelreuter, John Goss, Gaertner, Darwin, Herbert, and Naudin Koelreuter conducted extensive studies on hydridization between various species of Nicotiana (Tabacco) between 1761 and 1767, he noted the uniformity and heterosis in F₁ (First ficial generation) and appearance of increased variations in F₂. Koelreuter also observed that the hybrids were intermediate between their parents and that hybrids from reciprocal crosses were indistinguishable. Knight and goss conducted experiments on edible pea (Pisum sativum) much before Mendel but failed to formulate the laws of inheritance.

 Mendelian period.

Introduction : Gregor Johann Mendel (1822-1884) first “geneticist”, also known as father of genetics was born in 1822 in Silisian, a village in Heizendorf (Austria). In 1843, he joined Augustinian monastry at Brunn (then in Austria, now Brno Czechoslovakia). In 1856, Mendel got interested in breeding of Garden pea (Pisum sativum). He selected pure breeding varieties or pure lines of pea. Breeding experiments were performed between 1859 – 1864. The results were read out in two meetings of Natural History Society of Brunn in 1865 and published in 1866 in “Proceedings of Brunn Natural History Society” under the topic “Experiments in Plant Hybridisation”. Mendel died in 1884 without getting any recognition during his lifetime.

In 1900, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tshermak of Austria came to the same findings as were got by Mendel. Hugo de Vries found the paper of Mendel and got it reprinted in ‘Flora’ in 1901. Correns converted two of the generalisations of Mendel into two laws of heredity. These are law of segregation and law of independent assortment.

• Reasons for Mendel’s success : The reasons of his success can be discussed as follows:
  • Method of working : He maintained the statistical records of all the experiments and analysed He selected genetically pure (pure breed line) and purity was tested by self-crossing the progeny for several generations.
  • Selection of material : Mendel selected garden pea as his experimental material because it has the following

It was an annual plant. Its short life–cycle made it possible to study several generations within a short period and has perfect bisexual flowers containing both male and female parts. The flowers are predominantly self-pollinating because of self-fertilization, plants are homozygous. It is, therefore, easy to get pure lines for several generations and also easy to cross because pollens from one plant can be introduced to the stigma of another plant by removing anthers (emasculation) and bagging. In addition to that there was one reason more for his success. He studied seven pairs of characters which were present on four different pairs of chromosomes.

• Selection of traits : Mendel selected seven pairs of contrasting characters as listed in the Luckily all were related as dominant and recessive.

List of seven pairs of contrasting characters in pea plant

 

S.No.	Character	Dominant	Recessive
(1)	Stem length	Tall	Dwarf

 

 

 

 

(2)	Flower position	Axial	Terminal
(3)	Pod shape	Inflated	Constricted
(4)	Pod colour	Green	Yellow
(5)	Seed shape	Round	Wrinkled
(6)	Seed colour	Yellow	Green
(7)	Seed coat colour	Grey	White

 

• Mendel’s experiments
  • Monohybrid cross : Experiments with garden pea for single pair of contrasting
• Procedure : Mendel crossed pure tall and dwarf The plants belonged to F₁ generation all tall were self- pollinated. The plants of F₂ generation were both tall and dwarf, in approximate 3:1 ratio phenotypically and 1:2:1 genotypically. On, self-pollination, the tall plants of F₂ only 1/3–rd breed true for tallness, the rest 2/3–rd produced tall and dwarf in the ratio of 3:1 (F₃ generation). It means F₂ generation consisted of three types of plants (instead of apparent two types) –

Tall homozygous (Pure) 1 25% TT

Tall heterozygous (Hybrid) 2        50%   Tt

Dwarf homozygous (Pure)

1     25%   tt

 

Hence it is to be said that in F₂ generation 50% plants passes parental combination while 50% are new combination.

Fig : Mendel’s monohybrids crosses between tall and dwarf pea plants

 

• Mendel’s explanation : Mendel explained above results by presuming that Tallness and dwarfness are determined by a pair of contrasting factors or determiners (now these are called genes). A plant is tall because it possesses determiners for tallness (represented by T) and a plant is dwarf because it has determiners for dwarfness (represented by t). These determiners occur in pairs and are received one from either On the basis of this behaviour the tallness is described as dominant character and dwarfness as recessive (law of dominance). The determiners are never contaminated. When gametes are formed, these unit factors segregate so that each gamete gets only one of the two alternative factors. When F₁ hybrids (Tt) are self pollinated the two entities separate out and unite independently producing tall and dwarf plants (law of segregation).
  • Dihybrid cross (Crosses involving two pairs of contrasting characters)

 

 

 

 

• Procedure : Later on Mendel conducted experiments to study the segregation and transmission of two

 

pairs of contrasting characters at a time. Mendel found that a cross between round yellow and

MALE                                       FEMALE

 

GREEN

 

wrinkled green seeds (P₁) produced only round and yellow seeds in F₁ generation, but in F₂ four types of combinations were observed. These are

YELLOW

ROUND

YYRR

PARENTS

yyrr

WRINKLED

 

F1 GENERATION

Round yellow 9 Parental combinations Round green 3 Non-parental combinations

YR                     YR                      yr                     yr

GAMETES OF PARENTS

EGGS

 

Wrinkled yellow 3 Non-parental combination Wrinkled green 1 Parental combination

Thus the offsprings of F₂ generation were

YyRr

ALL YELLOW ROUND

 

produced in the ratio of 9 : 3 : 3 : 1 phenotypically

YR                      Yr                     yR                     yr

 

F1 GAMETES

and 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 genotypically. This ratio is called dihybrid ratio.

The results can be represented as follows:

Mendel represented round character of seed by R and wrinkled by r. Similarly he designated the yellow character by Y and green by y. Therefore, it was a cross between RRYY and rryy.

• Mendel’s explanation : Mendel explained the results by assuming that the round and yellow characters are dominant over wrinkled and green so that all the F₁ offsprings are round yellow. In F₂– generation since all the four characters were assorted out independent of the others, he said that a pair of alternating  or   contrasting   characters   behave

independently of the other pair i.e., seed colour is

 

POLLEN

 

YYRR   YELLOW ROUND   YYRr   YELLOW ROUND   YYRR   YELLOW ROUND   YyRr   YELLOW ROUND   YYRr   YELLOW ROUND   YYrr   YELLOW WRINKLED   YyRr   YELLOW ROUND   Yyrr   YELLOW WRINKLED   YyRR   YELLOW ROUND   YyRr   YELLOW ROUND   yyRR   GREEN ROUND   yyRr   GREEN ROUND   YyRr   YELLOW ROUND   Yyrr   YELLOW WRINKLED   yyRr   GREEN ROUND   yyrr   GREEN WRINKLED

YR

 

 

 

 

Yr

 

 

 

 

yR

 

 

 

 

yr

YR                    Yr

 

 

 

 

 

 

 

EGGS

yR                     yr

 

 

 

 

 

 

 

 

 

 

 

 

independent of seed coat.

Therefore, at the time of gamete formation

Fig : Mendel’s dihybrid-oross between pea plants having yellow round seeds and green wrinkled seeds

 

genes for round or wrinkled character of seed coat assorted out independently of the yellow or green colour of the seed. As a result four types of gametes with two old and two new combinations i.e., RY, ry Ry, rY are formed from the F₁ hybrid. These four types of gametes on random mating produced four types of offsprings in the ratio of 9:3:3:1 in F₂ generation (Law of Independent Assortment).

 

 

 

Factor for Seed Colour             Factor for Seed Shape                              Genotype of Gametes

(Forked-line method showing formation of four types of gametes from a F₁ – dihybrid for seed colour and seed shape)

• Trihybrid cross : The offsprings shows 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 ratio is found in trihybrid This suggests that a di, tri, or polyhybrid cross is actually a combination of respectively two, three or more monohybrid crosses operating together.
• Mendel’s laws of inheritance : Mendel’s law are still true because these take place in sexually reproducing organisms or parents are of pure He enunciated two major laws of inheritance i.e., law of segregation and law of independent assortment.

• Law of segregation (Purity of gametes) : The law of segregation states that when a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote (hybrid) the two members of the allelic pair remain together without being contaminated and when gametes are formed from the hybrid, the two separate out from each other and only one enters each gamete as seen in monohybrid and dihybrid cross. That is why the law of segregation is also described as law of purity of
• Law of independent assortment : If the inheritance of more than one pair of characters (two pairs or more) is studied simultaneously, the factors or genes for each pair of characters assort out independently of the other Mendel formulated this law from the results of a dihybrid cross.

 

Important Tips                                                                                                                                                                                                            

• Cytogenetics is Integrated study of cytology and genetics to find cytological basis for various events of This term was coined by Muller.
• J. Hammerlings proved that nucleus controls the heredity by a experiment on acetabularia (A unicellular green algae)
• Only sexually derived organisms are called offspring or siblings (offsprings at different births) g., brother and sister.
• Variations due to environment are known as ecophenotypes.
• Every test cross is back cross but every back cross is not a test cross.
• Back cross is used by breeders as a rapid method of making homozygous.
• When the two genetic loci produce identical phenotypes in cis and trans position they are considered to be pseudoalleles.
• Somaclonal variations are produced in tissue culture during differentiation of
• In thalassemia, the b chain of haemoglobin is changed due to frame shift mutation as a result, bone marrow is not
• Bateson coined the term Genetics, allele, F₁, F₂, homozygous heterozygous and epistasis. He is also known as father of animal genetics.
• The basis of genetic counseling is

 

 

 

• In heredity, the genes are obtained from father and
• Nucleus and chromosomes are stained by hematoxylin.
• Johannsen coined the term genotype, phenotype, pure line.
• The term hybrid vigour (heterosis) given by
• In mitosis, the daughter cells resemble each other and also the parent cell, in meiosis they differ not only from parent cell in having half the number of chromosomes, but also differ among themselves qualitatively in genetic constitution due to crossing over, independent assortment and
• Mendelian genetics is also called as forward genetics.
• Mendel either avoided the result or could not conduct independent assortment between pod form and stem
• Mendel also observed that flower colour and colour of the seed coat may not assort
• Mendel failed to produce same results of his experiments of pea in Hawkweed (Hieracium) and
• Mendel’s typical monohybrid phenotypic ratio was 3 : 1 which was in reality a hidden 1 : 2 : 1 ratio of
• Mendelism gave well-defined principles even in early stage in compare to Darwinism.
• Mendel did not recognize the linkage phenomenon in his experiments because characters, he studied were located on different
• Mendelian factor are separated during Anaphase- I in Meiosis- I.
• If Mendel had studied the seven traits using a plant with 12 chromosomes instead of 14, He would not have discovered the law of independent
• Law of filial regression was postulated by
• Mendel didn’t imagine of
• Mendel in his experiments on pea considered quantity in relation to
• In bound seeds (RR/Rr) starch branching enzyme (SBE -1) is found but it is absent in wrinkled seeds or in rr
• It was thought previously that seven traits in pea studied by Mendel were located on seven different chromosomes but recent studies proves that these are on four
• The genes for seed form in pea was present on chromosomes no.
• Independent assortment is shown by the allels present on different

 

 

 Interaction of genes.

Genes interaction is the influence of alleles and non-alleles on the normal phenotypic exprssion of genes. It is of two types.

• Inter–allelic or intra–genic gene interaction : In this case two alleles (located on the same gene locus on two homologous chromosomes) of gene interact in such a fashion to produces phenotypic expression q. co- dominance, multiple alleles.

 

 

 

 

• Incomplete dominance (1:2:1 ratio) : After Mendel, several cases were recorded where F₁ hybrids were not related to either of the parents but exhibited a blending of characters of two This is called incomplete

dominance or blending inheritance.

Example : In 4-O’clock plant, (Mirabilis jalapa), when plants with red flowers (RR) are crossed with plants having white flowers (rr) the hybrid F₁ plants (Rr) bear pink flowers. When these

 

GAMETES

F₁ plants with pink flowers are self pollinated they develop red (RR), pink (Rr) and white (rr) flowered plants in the ratio of 1:2:1 (F₂ generation).

Example : In Snapdragon or dog flower (Antirrhinum majus) the dominant character of leaf (Broadness) and flower (Red) shows incomplete dominance over recessive characters (Narrowness and white) in dihybrid cross.

 

 

 

 

  P₁

 

 

 

 

 

 

 

 

 

F₁

 

 

Fig : Incomplete dominance of flower colour in Mirabillis jalapa

 

 

• Codominance (1:2:1        ratio)      :       In codominance, both the genes of an allelomorphic pair

F₂

express themselves equally in F₁ hybrids. 1:2:1 ratio both

genotypically as well as phenotypically in F₂ generation.

Example : Codominance of coat colour in cattle.

In cattle gene R stands for red coat colour and gene r stands for white coat colour. When red cattle (RR) are crossed with white cattle (rr), the F₁ hybrids

 

 

 

 

 

 

 

 

 

 

 

Fig : Inheritance of coat colour in cattle

 

have roan coloured skin (not the intermediate pink). The roan colour is actually expressed by a mixture of red and white hairs, which develop side by side in the heterozygous F₁ hybrid. In F₂ generation red, roan and white appear in the ratio of 1 : 2 : 1. The phenotypic ratio equal to genotypic ratio RR, Rr, rr (1 : 2 : 1).

 

 

 

 

Example : Codominance in andalusian fowl

In andalusian fowl a cross between pure black and pure white varieties results in blue hybrids.

Example : Codominance of blood alleles in man

• MN blood type in man is an example of The persons with MN genotype produce both antigen M and N and not some intermediate product indicating that both the genes are functional at the same time.
• In ABO blood group system gene A and B responsible for blood group A and B are codominant. The hybrid has AB blood

Differences between incomplete dominance and codominance

 

Incomplete dominance	Codominance
Effect of one of the two alleles is more conspicuous.	The effect of both the alleles is equally conspicuous.
It produces a fine mixture of the expression of two alleles.	There is no mixing of the effect of the two alleles.
The effect in hybrid is intermediate of the expression of the two alleles.	Both the alleles produce their effect independently, e.g., IA and IB, HbS and HbA.

 

• Non–allelic or inter-genic gene interaction : Here two or more independent genes present on same

 

or different chromosomes, interact to produce a new expression e.g. epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes etc.

P₁ Generation

 

Gametes

White Flower CC pp

 

White Flower cc PP

 

×

 

• Complementary genes (9 : 7 ratio) : The complementary genes are two pairs of nonallelic dominant genes (e. present on separate gene loci), which interact to produce only one phenotypic trait, but neither of them if present alone produces the phenotypic trait in the absence of other.

Example : Complementary genes for flower colour in sweet pea. In sweet pea (Lathyrus odoratus) the purple colour of flowers is dependent on two nonallelic complementary genes C and P. Gene C produces an enzyme that catalyzes the formation of colourless chromogen for the formation of anthocyanin pigment. Gene P controls the production of an enzyme, which catalyzes the transformation of this chromogen into anthocyanin. These genes are complementary to each other. It means the

pigment anthocyanin is produced by two-biochemical

F₁ Generation

Coloured Flower

Cc Pp

 

G E CP Cp cP cp   CP CC PP coloured CC Pp coloured Cc PP coloured Cc Pp coloured   Cp CC Pp coloured CC pp white 1 Cc Pp coloured Cc pp white 2   cP Cc PP coloured Cc Pp coloured cc PP white 3 cc Pp white 4   cp Cc PP coloured Cc pp white 5 cc Pp white 6 cc pp white 7

reactions and the end product of first reaction forms the substrate for the other.

If a plant possesses dominant gene C and P, it produces purple flowers. But if a plant has a genotype

F   Generation =      Coloured Flower : 9

2

White Flower        : 7

Fig : The results of an experiment to show the operation of complimentary genes in the production of flower colour in sweet pea (Lathyrus)

 

CCpp, it produces the raw material but is unable to convert it into anthocyanin. Therefore, it produces white flowers. Similarly, if it possesses dominant gene P, but no dominant C (ccPP), it produces white flowers because gene P can convert colourless chromogen into anthocyanin but cannot form chromogen.

 

 

 

 

 

• Supplementary genes (9 : 3 : 4 ratio) : Supplementary genes are two independent pairs of dominant genes. Which interact in such a way that one dominant gene will produce its effect whether the other is present or not. The second dominant when added changes the expression of the first one but only in the presence of first one. In rats and guinea pigs coat colour is governed by two dominant genes A and C, the agouti-coloured guinea pigs have genotype CCAA. The black mice possess factor for black colour C but not the gene A for agouti colour. If gene for black colour is absent agouti is unable to express itself and mice with a genotype ccAA are albino. Here presence of gene C produced black colour and addition of gene A change its expression to agouti
• Epistasis (Inhibiting genes) : Epistasis is the interaction between nonallelic genes (Present on separate loci) in which one-gene masks, inhibits or

P₁ Generation

 

 

Gametes

 

G Gametes G E CA cA Ca ca   CA CC AA agouti 1 Cc AA agouti 2 CC Aa agouti 3 Cc Aa agouti 4   cA Cc AA agouti 5 cc AA albino 6 Cc Aa agouti 7 cc Aa albino 8   Ca CC Aa agouti 9 Cc Aa agouti 10 CC aa Black 11 Cc aa Black 12   ca Cc Aa agouti 13 cc Aa albino 14 Cc aa Black 15 cc aa albino 16

F₁ Generation

 

Agouti       ×

CC AA

 

 

agouti Cc Aa

 

 

 

 

 

 

 

 

 

 

 

 

Agouti : 9

Albino cc aa

 

 

 

 

 

suppresses the expression of other gene. The gene that suppresses the other gene is known as inhibiting or epistatic factor and the one, which is prevented from

F₂ Generation =   Black  : 3

Albino : 4

Fig : Interaction of supplementary genes in mice for coat colour

 

exhibiting itself, is known as hypostatic. Although, it is similar to dominance and recessiveness but the two factors occupy two different loci. Therefore, while dominance involved intragenic or interallelic gene suppression, the epistasis involves intergenic suppression. Epistasis can be of the following types – dominant epistasis, recessive epistasis.

• Dominant epistasis (12:3:1 or 13:3 ratio) : In dominant epistasis out of two pairs of genes the dominant allele, (e., gene A) of one gene masks the activity of other allelic pair (Bb). Since the dominant epistatic gene A exerts its epistatic influence by suppressing the expression of gene B or b, it is known as dominant epistasis.

Example – Dominant epistasis in dogs : In dogs white coat colour appears to be dominant. It develops due to the action of epistatic gene I which prevents the formation of pigment, controlled by hypostatic gene B.

P₁ Generation

 

 

Gametes

F₁ Generation

Brown Dog   ×

bb ii

 

E G BI bI Bi bi   BI BB II white 1 Bb II white 2 BB Ii white 3 Bb Ii white 4   bI Bb II white 5 bb II white 6 Bb Ii white 7 bb Ii white 8   Bi BB Ii white 9 Bb Ii white 10 BB ii black 11 Bb ii black 12   bi Bb Ii white 13 bb Ii white 14 Bb ii black 15 bb ii brown 16

All White Bb Ii

 

 

 

 

 

 

 

 

 

 

 

 

 

 

White : 12

White Dog BB II

 

The hypostatic gene B produces black coat while its hypostatic allele b produces brown coat colour only when gene I is recessive. The progeny of dominant gene I does not allow them to function and results in white colour.

F₂ Generation =     Black  : 3

Brown : 1

Fig : Interaction of inhibiting genes in dog for coat colour showing dominant epistasis

 

When two white coat dogs are crossed, they produce white, black and brown in the ratio of 12 : 3 : 1 The white dogs in this case possess gene for black or brown colour but does not produce the pigment because of the presence

 

 

 

 

of gene I in dominant state. Similar phenomena have been seen in fruit colour in cucurbita as summer squash and coat colour in chickens.

• Recessive epistasis (9:3:4 ratio) : Epistasis due to recessive gene is known as recessive epistasis, e., out of the two pairs of genes, the recessive epistatic gene masks the activity of the dominant gene of the other gene

 

 

 

 

 

AaBb     ×

×                                                          Parents

 

 

Triangular AaBb

Top Shaped aabb

Triangular AABB

                  

F₁

 

 

AB Ab aB ab A B AABB Triangular AABb Triangular AaBB Triangular AaBb Triangular Ab AABb Triangular AAbb Triangular AaBb Triangular Aabb Triangular aB AaBB Triangular AaBb Triangular aaBB Triangular aaBb Triangular ab AaBb Triangular Aabb Triangular aaBb Triangular aabb Top shaped

F₂

locus. The dominant A expresses itself only when the epistatic locus C also has the dominant gene if the epistatic locus has recessive gene c, gene A fails to express.

×

Example : In mice agouti colour, characterised by banding of hairs is controlled by gene A, which is hypostatic to recessive allele c. The dominant epistatic gene C in absence of A gives black coloured mice and in presence of

 

 

 

dominant gene A gives agouti, but dominant gene A is unable

P₁ Generation

 

 

Gametes

Rose Comb RR pp

pea Comb rr PP

 

to produce agouti colour in absence of gene c. Therefore, recessive c gene acts as epistatic gene A.

• Duplicate genes ( 15 : 1 ratio) : Sometimes two pairs of genes located on different chromosomes determine the same These genes are said to be duplicate of each other. The dominant triangular fruit shape of Capsella bursa pastoris (shepherd’s purse) is determined by two pairs of genes, say A and B. If any of these genes is present in dominant form, the fruit shape is triangular. In double recessive forms the fruits are top shaped and thus we get a 15 (triangular) : 1(top shaped) ratio in F₂ generation.
• Collaborator genes : In collaboration two gene pairs, which are present on separate loci but influence the same trait, interact to produce some totally new trait or phenotype that neither of the genes by itself could

Example : Inheritance of combs in poultry, where two genes control the development of comb.

 

F₁ Generation

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

F₂ Generation =

Walnut comb Rr Pp

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

RP Rp rP rp   RP RR PP walnut 1 RR Pp walnut 2 Rr PP walnut 3 Rr Pp walnut 4   Rp RR Pp walnut 5 RR pp rose 6 Rr Pp walnut 7 Rr pp rose 8   rP Rr PP walnut 9 Rr Pp walnut 10 rr PP pea 11 rr Pp pea 12   rp Rr Pp walnut 13 Rr pp rose 14 rr Pp pea 15 rr pp single 16

Walnut : 9

Rose   : 3

Pea      : 3

Single : 1

 

 

• Gene R gives rise to rose comb.

Fig : Inheritance of rose and pea comb in poultry

 

• Gene P produces pea comb. Both rose and pea combs are dominant over single
• Gene R and P for rose and pea comb together produce a new phenotype the walnut

 

 

 

 

Wyandotte variety of domestic chicken possesses rose comb, whereas Brahmas have pea comb. Bateson crossed rose-combed Wyandottes and pea–combed Brahmas. The F₁ chickens developed walnut comb, a phenotype not expressed in either parent. When F₁ chickens mated among themselves, the resultant F₂ chickens exhibited the familiar dihybrid ratio 9 : 3 : 3 : 1. These four phenotypes were – walnut comb, rose comb, pea comb and single comb in the ratio of 9 : 3 : 3 : 1. Out of these four phenotypes two phenotypes were different from those expressed in the parents.

The analysis of F₂ results indicates that the presence of two dominant genes R and P results in the walnut comb. The double recessive (rrpp) genotype produces single comb. The rose comb develops when dominant gene for rose comb is present and dominant gene for pea comb is absent (RRpp) whereas pea comb develops when gene for rose comb is recessive and gene for pea comb (P) is dominant (rrPP).

(vi)  Pleiotropic effect of genes

(a) Lethal genes : Certain genes are known to control the manifestation of some phenotypic trait as well as

affect the viability of the organism. Some

 

1. Monohybrid Crosses
   1. Brown mice yy

 

 

2. Yellow mice

×

 

Brown mice yy

×

Brown mice yy

 

 

Yellow mice

other genes have no effect on the appearance of the organism but affect the viability alone. These genes are known as lethals or semilethals depending upon their influence. Complete lethal genes in

 

1/4 Yellow mice (Dies) YY

1. Test Cross

2/4 Yellow mice

Yy

1/4 Brown yy

homozygous condition kill all or nearly all homozygous individuals, while in case of

semilethal   genes   some   homozygous

 

Yellow mice             ×

Brown mice

individuals are able to survive. The lethal

 

 

 

 

Parents

1/2 Yellow                                 1/2 Brown

genes are always recessive for their lethality and express the lethal effect only in homozygous condition.

 

 

Gametes F₁

Generation

 

YY

Dies as embryo

Yellow (Yy)

 

y and y

 

 

 

 

Yy Yellow

Yellow (Yy) y and y

 

 

YY

Yellow

 

 

 

 

 

 

Yy yy Non Yellow

Dominant lethals : The dominant lethal genes are lethal in homozygous condition and produce some defective or abnormal phenotypes in heterozygous condition. Their most serious effect in heterozygous may also cause death.

 

Fig : Inheritance of lethal gene-Y in mice

Following are the examples of dominant lethal genes.

 

Example – Yellow lethal in mice : A well known example of such lethals is from mice, given by Cuenot. He found that the yellow mice never breed true. Whenever the yellow mice were crossed with yellow mice, always yellow and brown were obtained in the ratio of 2:1. A cross between brown and brown mice always produced brown offsprings and a cross between brown and yellow produced yellow and brown in equal proportions.

In 1917, Stiegleder concluded that yellow mice are heterozygous. The homozygous yellow (1/4^th of the total offsprings) dies in the embryonic condition. When there unborn ones are added to the 2:1 ratio of yellow and brown, these form typical 3:1 ratio. Cuenot suggested that gene Y has a multiple effect.

 

 

 

 

Note :

• It controls yellow body colour and has a dominant
• It affects viability and acts as a recessive

Example – Inheritance of sickle cell anaemia in man : The disease sickle cell anaemia is caused by a gene (Hb^S), which is lethal in homozygous condition but has a slight detectable effect in the heterozygous condition.

The homozygous for this gene (Hb^s/ Hb^s)

 

 

 

Gametes

Sickle cell carrier (Heterozygous) Hb^A/Hb^S

Hb^A                         Hb^S

Sickle cell carrier Hb^A/Hb^S (Heterozygous)

Hb^A                     Hb^S

generally die of fatal anaemia. The heterozygotes or carriers for Hb^s, (i.e., Hb^A / Hb^s) show signs of mild anaemia as their R.B.C. become sickle shaped in oxygen deficiency.

A marriage between two carriers, therefore, results in carrier and normal offsprings in the ratio of 2:1. The variation in 3:1 ratio is due to the

 

Progeny

Hb^A/Hb^A Normal

Hb^A/Hb^S Carrier

Hb^S/Hb^A Carrier

Hb^S/Hb^S Fatal anemia

death of homozygous and incomplete dominance

of normal gene over recessive gene for sickle cell

 

(Disease free) 1

(Exhibit sckle cell anemia) 2

(Usually dies) X

anaemia in which glutamic acid is substituted by

 

Fig : Diagram depicting inheritance of lethal gene for sickle cell anaemia

valine in b chain. Which is disorder in Africans that reduce oxygen uptake.

 

Example – Brachyphalangy : Persons exhibiting brachyphalangy have short fingers apparently having two joints in their fingers, the middle bone being greatly shortened and often fused with one of the other two bones of the finger. Mohr described one case where one child was born without any fingers or toes and did not survive. Two other children showed short fingers and one was normal. This is exact 1 : 2 : 1 ratio.

Example – Huntington’s chorea in man : The gene causing Huntington’s chorea in man can express itself even when a single dominant allele is present. In both homo and heterozygous condition, the gene expresses itself only at middle–age, usually after forty years. The person suffers from muscular failure, mental retardation and finally death. Thus a dominant gene in heterozygous condition may also produce lethal effect. The gene is transmitted to next generation only because it expresses itself only after the start of reproductive period.

Recessive lethals : The recessive lethals produce lethal effect only in homozygous condition. Their heterozygotes are normal. Therefore, recessive lethals remain unnoticed in the population but are established in the population because female are carrier for lethal gene. These are detected only when two heterozygous persons get married.

Example – Tay Sach’s lethal : The recessive lethal gene for Tay Sach’s disease causes death of young children only in homozygotes which are unable to produce enzymes needed for normal fat metabolism. The accumulation of fat in nerve sheaths hampers transmissions of nerve impulse leading to poor muscular control and mental deficiency.

• Qualitative inheritance : Qualitative inheritance or monogenic inheritance is that type of inheritance in which one dominant allele influences the complete trait, so that two such allele do not change the phenotype. Here dominant allele is
• Quantitative inheritance : Quantitative inheritance or polygenic inheritance can be defined as, ‘two or more different pairs of alleles which have cumulative effect and govern quantitative characters. The quantitative

 

 

 

inheritance is due to incomplete dominance. It has been suggested that the multiple gene inheritance may have following characteristics:

• The effects of each contributing gene are cumulative or
• Each contributing allele in a series produces an equal
• There is no dominance
• Epistasis does not exist among genes at different
• No linkage is involved in the
• Effects of environment are absent or may be

Example – Human skin colour : This character was studied by Davenport, 1913 in the marriages between negroes and whites. The F₁ offsprings arising as a result of these marriages are called as mulattoes. The human skin colour is determined by two pairs of genes, P₁ and P₂. A negro having very dark skin with four colour genes i.e., P₁P₁ P₂P₂, when married to a white with no colour gene (p₁p₁ p₂p₂) produce mulattoes with only two colour genes. These mulattoes show intermediate type of skin colour. If this mulatto is married to a similar genotype, the inheritance of pigment forming gene in F₂ offspring shall be as under:

 

Very dark	–        4 Colour genes	–        One
Intermediates	–        3 Colour genes	–        Four
	–        2 Colour genes	–        Six
	–        1 Colour genes	–        Four
White	–        No colour gene	–        One

If the mulatto is married to a pure white (test cross), the distribution of skin colour shall be as follows: 25% offsprings with two colour genes (P₁ and P₂),

50% offsprings with one colour gene (P₁ or P₂) 25% offsprings with no colour gene

If the mulatto is married to a negro (back cross) the distributions of skin colour shall be as under: 25% offspring with four colour genes

50% offspring with three colour genes 25% offsprings with two colour genes

Example – Ear size in maize : Emerson and East (1913) studied the for the ear length in maize. The size difference between two strains of maize is generally due to two or more pairs of genes. If it is due to two pairs of genes, the ratio of different sizes shall be 1 : 4 : 6 : 4 : 1. But, if it is due to three pairs of genes, the size ratio shall be 1 : 6 : 15 : 20 : 15 : 6 : 1.

Example – White spotting in mice : This trait is also polygenic for it is governed by two or more pairs of gene. Depending upon the involvement of the number of gene pairs for determine the trait, the ratios also vary in the manner stated above.

 

 

 

Example – Grain colour in wheat : This character was examined by Nilsson-Ehile 1908, which is similar as polygenic gene of skin colour in human. When a red grain was crossed with a white, the F₁ offspring produced light red grains due to incomplete dominance of red over white. The ratio is come out as 1 : 6 : 15 : 20 : 15 : 6 : 1.

• Multiple alleles : The multiple alleles can be defined as a set of three, four or more allelomorphic genes or alleles, which have arisen as a result of mutation of the normal gene and which occupy the same locus in the homologous Characters of multiple alleles are following –

• Multiple alleles occupy the same locus with in the homologous It means only one member of the series is present in a given chromosome.
• Since only two chromosomes of each type are present in each diploid cell, only two genes of the multiple series are found in a cell and also in a given
• The gametes contain only one chromosome of each types, therefore, only one allele of the multiple series in each
• Crossing over does not occur in the multiple
• Multiple alleles control the same character, but each of them is characterised by different

Sturtevent has summarised it that they carry the same function but with varying degree of efficiency.

• The multiple alleles of a series are more often related as dominant and recessive. More commonly, the normal gene is dominant to all other mutant alleles. Even the intermediate members of the series may be related as dominant and recessive, or they may exhibit Therefore, multiple alleles act in some way to control the various steps in a chemical reaction.

Examples : Coat colour in rabbit, blood group in human beings.

• Pedigree analysis : As man is not a suitable material for genetic research, the human genetics is studied from different point of Pedigree analysis is one such method based on Mendelism. It was started by Galton.

 

 

2                                                                                                                             3              4
3	4	5	6		7	8	9		10	11	12
			3		4	5	6		7

Fig : Pedigree analysis of three generations

 

A pedigree is a record of inheritance of certain genetic traits for two or more generations presented in the form of a diagram or family tree or case history or genealogy. Pedigree analysis is a system of analysis of pedigree to find out the possibility of absence/presence of a particular trait in the progeny. It is mainly employed in domesticated animals and men also. The person from whom the case history of a pedigree starts is called Proband (called Propositus or Prosipitus if it is male and Proposita if it is female). The children are called Sibs. Empty/open circles and squares represent normal female ( ) and normal male ( ) solid/shaded (=/<) symbols stand for those which bear the trait under study. ◑ or ¤ represent carrier normal female having recessive allele of the trait under study

 

 

 

identical twins non-identical twins. ◨ This is never used as man can never be a carrier. Horizontal line shows marriage line. All siblings are connected to a horizontal line below parents in order of birth. Only X-linked genes show criss cross inheritance.

Important Tips

• Fair child’s sweet William is hybrid flower.
• In polygenic inheritance some offsprings may exhibit more extremes than either parent or For example, some children are shorter or taller than either parent or any of their more remote ancestors. The same is true with respect to intelligence. These are called transgressive variations.

 

• Formula of number of genotypes in case of multiple allelism is:

n (n + 1) = n

2

alleles .

 

• In ABO blood group three alleles regulate blood group leading to the formation of six Landsteiner discovered A, B, O, blood groups.
• During serological test in which antihuman serum is mixed with blood of another animal, blood of chimpanzee gives the thickest
• Dominance is a phenomenon and not a low because of the existence of incomplete dominance and
• Heterosis in plants is obtained by crossing in unrelated parents and known as hybrid
• Expressivity is the degree of effect produced by penetrant
• Nilsson-Ehle (1908) was the first scientist to prove quantitative inheritance.

 

 

 Cytoplasmic inheritance/Extrachromosomal inheritance.

The fact that nucleus contains the units of inheritance was proposed by Oscar Hertwig in 1870’s. The mechanism was clearly understood with the development of Mendel’s law of inheritance. Further researchers proposed that cytoplasm also contains the hereditary material. The evidence for cytoplasmic inheritance was first presented by Correns in Mirabilis Jalapa and by Baur in Pelargonium zonale in 1908. Later on Ruth Sager (1954) described cytoplasmic inheritance of streptomycin resistance in chlamydomones in other animals and plants certain characters are inherited independent of the chromosomal genes (Non-chromosomal genes). The cytoplasm in such cases contain self perpetuating hereditary particles formed of DNA. These may be mitochondria, plastids or foreign organism, etc. The total self duplicating hereditary material of cytoplasm is called plasmon and the cytoplasmic units of inheritance are described as plasmogenes.

• Criteria for cytoplasmic inheritance : The cases of cytoplasmic inheritance are found to exibit maternal influence. The reason is very simple. Very little cytoplasm is contained in the sperm cell of an animal. Most of the cytoplasm is contributed to the zygote by the ovum or egg. Hence if there are hereditary units in the cytoplasm, these will be transmitted to the offsprings through the egg. The offspring, therefore will exhibit maternal This could be explained further by following example.

• Maternal influence on shell coiling in
• Inheritance of sigma particles in
• Breast tumour in
• Plastid inheritance in Mirabilis (4 O’ clock plant).
• Plastid inheritance in Oenothera.
• Male sterility in plants – q. maize.

 

 

 

 

• Inheritance of kappa particles in Paramecium : Sonneborn and his associates have described the transmission of some cytoplasmic particles known as kappa particles and their relation to nuclear genes in Paramecium aurelia. Individuals of particular race of paramecium aurelia called killer strain destroy other races of paramecia by secreating some toxic substance into the water in which they live. This substance is known as Although kappa particles are cytoplasmic particles and transmitted strictly through the cytoplasm.

Chromosomes and Genes.

The chromosomes are capable of self–reproduction and maintaining morphological and physiological properties through successive generations. They are capable of transmitting the contained hereditary material to the next generation. Hence these are known as ‘hereditary vehicles’.

(1)  Discovery of chromosomes

Hofmeister (1848) : First observed chromosomes in microsporocytes (microspore mother cells) of

Tradescantia.

Flemming (1879) :         Observed splitting of chromosomes during cell division and coined the term, ‘chromatin’.

Roux (1883) : He believed the chromosomes take part in inheritance.

W.Waldeyer (1888) : He coined the term ‘chromosome’.

Benden and Boveri (1887) : They found a fixed number of chromosomes in each species.

(2)  Kinds of chromosomes

• Viral chromosomes : In viruses and bacteriophages a single molecule of DNA or RNA represents the viral
• Bacterial chromosomes : In bacteria and cyanobacteria, the hereditary matter is organized into a single large, circular molecule of double stranded DNA, which is loosely packed in the nuclear It is known as bacterial chromosome or nucleoid.
• Eukaryotic chromosomes : Chromosomes of eukaryotic cells are specific individualized bodies, formed of deoxyribonucleo proteins (DNA + Proteins).

• Chromosomal theory of inheritance: It was proposed independently by Sutton and Boveri in 1902. The chromosome theory of inheritance proposes that chromosomes are

 

vehicles of hereditary information and expression as Mendelian factors or genes.

• Bridge between one generation to the next are sperm and

Fig : Diagram showing chromosome cycle : M-Metaphase, A-Anaphase, G-Growth phase I, S-Synthetic phase, G₂ – Growth phase II

 

 

 

 

• Both sperm and ovum contribute equally in heredity. Sperm provides only nucleus for fertilization. Therefore, heredity must be based in nuclear
• Nucleus possesses Therefore, chromosomes must carry hereditary characters.
• Chromosomes, like hereditary factors are particulate structures, which maintain their number, structure and individuality in organisms from generation to

• Chromosomes number : Chromosome number is n = 2 in Mucor hiemalis, 2n=4 in plant Haplopappus gracilis. Chromosome number is 14 (n=7) in Pea, 20 in Maize, 46 in human beings. Maximum number of chromosomes is known for Adder’s Tongue Fern (Ophioglossum reticulatum, 2n = 1262) and Aulocantha (2n=1600). Number of chromosomes is not related to complexity or size of organism g., Domestic Fowl and Dog both possess 78 chromosomes. Study of chromosome structure is performed at metaphase and study of chromosome shape at anaphase.
• Chromosome cycle and cell cycle : Chromosomes exhibit cyclic change in shape and size during cell In the non-dividing interphase nucleus, the chromosomes form an interwoven network of fine twisted but uncoiled threads of chromatin, and are invisible. During

cell division the chromatin threads condense into compact structures by helical coiling.

• Chromosome structure : Different regions (structures) recognized in chromosomes are as
• Pellicle : It is the outer thin but doubtful covering or sheath of the chromosome.
• Matrix : Matrix or ground substance of the chromosome is made up of proteins, small quantities of RNA and It has one or two chromonemata (singular – chromonema) depending upon the state of chromosome.
• Chromonemata : They are coiled threads which form the bulk of A chromosome may have one (anaphase) or two (prophase and metaphase) chromonemata. There are three view points about the constitution of chromonema and chromosome.

 

• A Primary Constriction and Centromere (kinetchore) : A part of the chromosome is marked by a

Fig : Structure of chromosome

 

constriction. It is comparatively narrow than the remaining chromosome. It is known as primary constriction. The primary constriction divides the chromosome into two arms. It shows a faintly positive Feulgen reaction, indicating presence of DNA of repetitive type. This DNA is called centromeric heterochromatin.

• Centromere : Centromere or kinetochore lies in the region of primary constriction. The microtubules of the chromosomal spindle fibres are attached to the centromere. Therefore, centromere is associated with the chromosomal movement during cell Kinetochore is the outermost covering of centromere.
• Secondary constriction or nucleolar organizer : Sometimes one or both the arms of a chromosome are marked by a constriction other than the primary During interphase this area is associated with the

 

 

 

 

nucleolus and is found to participate in the formation of nucleolus. It is, therefore, known as nucleolar organizer region or the secondary constriction.

Nucleolar organizer region (NOR) : In certain chromosomes, the secondary constriction is (In human beings 13, 14, 15, 20 and 21 chromosome are nucleolar organizer) intimately associated with the nucleolus during interphase. It contains genes coding for 18S

and 28S ribosomal RNA and is responsible for the formation of nucleolus. Therefore, it is known as nucleolar organizer region (NOR).

• Telomeres : The tips of the chromosomes are rounded and sealed and are called telomeres which play role in Biological clock. The terminal part of a chromosome beyond secondary constriction is called satellite. The chromosome with satellite is known as sat chromosome, which have repeated base
• Chromatids : At metaphase stage a chromosome consists of two chromatids joined at the common In the beginning of anaphase

 

when centromere divides, the two chromatids acquire independent centromere and each one changes into a chromosome.

Fig : Nucleolus organizer or secondary constriction and its association with necleolus

 

• Types of chromosomes based on number of centromeres : Depending upon the number of centromeres, the chromosomes may be:
• Monocentric with one
• Dicentric with two centromeres, one in each
• Polycentric with more than two
• Acentric without centromere. Such chromosomes represent freshly broken segments of chromosomes, which do not survive for
• Diffused or non-located with indistinct centromere diffused throughout the length of The microtubules of spindle fibres are attached to chromosome arms at many points. The diffused centromeres are found in insects, some algae and some groups of plants (e.g. Luzula).
• Types of chromosomes based on position of centromere : Based on the location of centromere the chromosomes are categorised as follows:
  • Telocentric : These are rod-shaped chromosomes with centromere occupying a terminal position. One arm is very long and the other is

 

 

 

 

• Acrocentric : These are rod-shaped chromosomes having subterminal centromere. One arm is very long and the other is very
• Submetacentric : These are J or L shaped chromosomes with centromere slightly away from the mid-point so that the two arms are unequal.
• Metacentric : These are V-shaped chromosomes in which centromere lies in the middle of chromosomes so that the two arms are almost

• Molecular organisation of chromosome : Broadly speaking there are two types of models stating the relative position of DNA and proteins in the

Multiple strand models : According to several workers (Steffensen 1952, Ris 1960) a chromosome is thought to be composed of several DNA protein fibrils, chromatids are made up by several DNA protein fibrils and atleast two chromatids form the chromosome.

Single strand models : According to Taylor, Du prow etc. The chromosome is made up of a single DNA- protein fibril. There are some popular single strand models.

• Folded fiber model : Chromosomes are made up of very fine fibrils 2 nm – 4 nm in thickness. As the diameter of DNA molecule is also 2 nm (20Å). So it is considered that a single fibril is a DNA molecule. It is also seen that chromosome is about a hundred times ticker than DNA whereas the length of DNA in chromosome is several hundred times that of the length of So it is considered that long DNA molecule is present in folding manner which forms a famous model of chromosome called folded fibre model which given by E.J. Dupraw (1965).
• Nucleosome model : The most accepted model of chromosome or chromatin structure is the ‘nucleosome model’ proposed by Kornberg and Thomas (1974). Nucleosomes are also called core particles or Nu-bodies. The name nucleosome was given by Outdet etal. The nucleosome is a oblate particle of 55Å height and 110Å diameter. Woodcock (1973) observed the structure of chromatin under electron microscope. He termed each beaded structure on chromosome as nucleosome. Nucleosome is quasicylindrical structure made up of histones and DNA.

• Structural proteins (histones) : Histones are main structural protein found in eukaryotic cells. These are low molecular weight proteins with high proportion of positively charged basic amino acids arginine and

 

Types of histones : These are five different types of histones that fall into two categories.

Fig : A-C Nucleosome : A Units of nucleosome,

B Nucleosome model C Solenoid model

 

 

 

 

Nucleosomal histones : These are small proteins responsible for coiling DNA into nucleosome. These are

 

H 2 A, H 2 B, H3

and

H ₄ (two molecule of each four histone protein form a octamer structure). These form the inner

 

core of nucleosome.

H₁-histones or linker histon protein : These are large (about 200 amino acids) and are tissue specific. These are present once per 200 base pairs. These are loosely associated with DNA. H₁ histones are responsible for packing of nucleosomes into 30 nm fiber.

Functions of  histones : Histones in eukaryotic chromosomes serve some functions.

• These either serve as structural elements and help in coiling and packing of long DNA
• Transcription is possible only by dissolution of histones in response to certain molecular

• DNA in nucleosome : Nucleosome is made of core of eight molecules of histones wrapped by double

 

helical DNA with 1 3 turns making a repeating unit. Every 1 3

turn of DNA have 146 base pairs. When H₁ protein

 

4                                                  4

is added the nucleotide number becomes 200. DNA which joins two nucleosome is called linkar DNA or spacer DNA.

• Solenoid model : In this model the nucleosomal bead represents the first degree of coiling of It is further coiled to form a structure called solenoid (having six nucleosome per turn). It represents the second degree of coiling. The diameter of solenoid is 300Å. The solenoid is further coiled to form a supersolenoid of 2000-4000Å diameter. This represent the third degree of coiling. The supersolenoid is perhaps the unit fiber or chromonema identified under light microscopy. The solenoid model was given by Fincy and Klug 1976. A Klug was awarded by noble prize in 1982 for his work on chromosome.
• Dangier-String or Radial Loop Model : (Laemmli, 1977). Each chromosome has one or two interconnected scaffolds made of nonhistone chromosomal The scaffold bears a large number of lateral loops all over it. Both exit and entry of a lateral loop lie near each other. Each lateral loop is 30 nm thick fibre similar to chromatin fibre. It develops through solenoid coiling of nucleosome chain with about six nucleosomes per turn. The loops undergo folding during compaction of chromatin to form chromosome.

• Giant chromosomes : These chromosomes are of two
• Polytene chromosome : Polytene chromosome was described by Kollar (1882) and first reported by Balbiani (1881). They are found in salivary glands of insects (Drosophila) and called as salivary gland chromosomes. These are reported in endosperm cells of embryosac by Malik and Singh (1979). Length of this chromosome may be upto 2000m The chromosome is formed by somatic pairs between homologous chromosomes and repeated replication or endomitosis of

 

chromonemata.  These   are   attached   to   chromocentre.  It   has pericentromeric heterochromatin. Polytene chromosomes show a large

Fig : Polytene chromosome showing balbiani ring

 

number of various sized intensity bands when stained. The lighter area between dark bands are called interbands. They have puffs bearing Balbiani rings. Balbiani rings produce a number of m-RNA, which may remain stored temporarily in the puffs, are temporary structures.

 

 

 

 

• Lampbrush chromosomes : They are very much elongated special type of synapsed or diplotence chromosome bivalents already undergone crossing over and

first observed by Flemming (1882). The structure of lampbrush chromosome was described by Ruckert (1892). They are found in oocyte, spermatocytes of many animals. It is also reported in Acetabularia (unicellular alga) by Spring et.al. in 1975. In urodele oocyte the length of lampbrush

 

chromosome is upto 5900mm. These are found in pairs consisting of homologous chromosomes jointed at chiasmata (meiotic prophase–I). The chromosome has double main axis

Fig : A part of main axis with a pair of lateral loops of a lampbrush chromosome showing synthesis of RNA

 

due to two elongated chromatids. Each chromosome has rows of large number of chromatid giving out lateral loops, which are uncoiled parts of chromomere with one–many transcriptional units and are involved in rapid transcription of mRNA meant for synthesis of yolk and other substances required for growth and development of meiocytes. Some mRNA produced by lampbrush chromosome is also stored as informosomes i.e., mRNA coated by protein for producing biochemicals during the early development of embryo. Length of loop may vary between 5- 100 mm.

(11)  Other types of eukaryotic chromosomes.

• B-chromosomes (Wilson, 1905) : They are supernumerary or extra chromosomes which are mostly heterochromatic, smaller than normal and show slower B-chromosomes may get lost. In excess, they may result in loss of vigour.
• M-chromosomes : They are minute but functional chromosomes (0.5mm or less). Which occur is some bryophytes and
• L-chromosomes : The chromosomes found only in germ-line cells, which are eliminated during formation of somatic cells. In Mainstor 36 chromosomes in female and 42 chromosomes in male are eliminated during development of somatic They are also called E–chromosomes.
• Sex chromosomes : Sex chromosomes are those chromosomes whose presence, absence or particular form determines the sex of the individual. Sex chromosomes are also called idiochromosomes/allosomes. Besides determining sex, these chromosomes also control a number of morpho-physiological traits called sex-linked Chromosomes other than sex chromosomes are known as autosomes. Autosomes determine morpho- physiological traits of the organisms, which are similar in both the sexes and are not sex-linked.

The two sex chromosomes in an individual may be morphologically similar/homomorphic (e.g. XX) or different/heteromorphic (e.g. XY). The morphologically different chromosome is androsome. (e.g., Y-chromosome) or male determining in same organisms (e.g., mammals) and gynosome or female determining in others (e.g., W- chromosome in birds). Individuals having homomorphic sex chromosomes produce similar gametes. They are, therefore, homogametic (A+X, A+X in human females). Individuals with heteromorphic sex chromosomes produce two types of gametes. They are heterogametic (A+X, A+Y in human males). Some sex chromosomes are heterochromatic (Y–chromosome in males and one X–chromosome in females) and are called heterochromosomes/heterosomes. Other chromosomes are called euchromosomes though the latter term is also applied for autosomes.

 

 

 

(12)  Functions of chromosomes

• Chromosomes are link between parents and
• They contain genes and hence hereditary
• Sex chromosomes determine
• Chromosomes control cell growth, cell division, cell differentiation and cell metabolism through directing synthesis of particular proteins and
• Haploid and diploid chromosome number determine gametophytic and sporophytic
• Chance separation, crossing over and random coming together of chromosomes bring about
• New species develop due to change in number, form and gene complements of
• Karyotype : It is chromosome complement of a cell–organism providing description of number, types and characteristics of chromosomes. Idiogram is a karyotype consisting of photograph or diagram of all the metaphasic chromosomes arranged in homologous pairs according to decreasing length, thickness, position of centromere, shapes etc, with sex chromosomes placed at the end (but at position I in Drosophila).
• Human karyotype : Tijo and Levan (1956) of Sweden found that human cells have 23 pairs or 46 22 pairs or 44 chromosomes are autosomes and the last or 23rd pairs is that of sex chromosomes, XX in females and XY in males.

 

Term ‘gene’ was given by Johannsen (1909) for any particle to which properties of Mendelian factor or determiner can be given. T.H Morgan (1925) defined gene as ‘any particle on the chromosome which can be separated from other particles by mutation or recombination is called a gene. In general, gene is the basic unit of inheritance.

According to the recent information a gene is a segment of DNA which contains the information for one enzyme or one polypeptide chain coded in the language of nitrogenous bases or the nucleotides. The sequence of nucleotides in a DNA molecule representing one gene determines the sequence of amino acids in the polypeptide chain (the genetic code). The sequence of three nucleotides reads for one amino acid (codon).

• Gene action : Gene act by producing enzymes. Each gene in an organism produces a specific enzyme, which controls a specific metabolic activity. It means each gene synthesizes a particular protein which acts as enzyme and brings about an appropriate

(i) One gene one enzyme : This theory was given by Beadle and Tatum (1958), while they were working on red mould or Neurospora (ascomycetes fungus). Which is also called Drosophila of plant kingdom. Wild type Neurospora grows in a minimal medium (containing sucrose, some mineral salts and biotin). The asexual spores i.e. conidia were irradiated with x-rays or UV-rays (mutagenic agent) and these were crossed with wild type. After crossing sexual fruiting body is produced having asci and ascospores. The ascospores produced are of 2 types –

• The ascospores, which are able to grow on minimal medium called ‘prototrophs’.
• Which do not grow on minimal medium but grow on supplemented medium called ‘auxotrophs’.

 

 

 

• Molecular structure of gene : Gene is chemically DNA but the length of DNA which constitutes a gene, is controversial 3 term e. cistron, muton and recon were given by Seymour Benzer to explain the relation between DNA length and gene.
  • Cistron or functional gene or gene in real sense : Cistron is that particular length of DNA which is capable of producing a protein molecule or polypeptide chain or enzyme
  • Muton or unit of mutation : Muton is that length of DNA which is capable of undergoing mutation. Muton is having one or part of
  • Recon : Recon is that length of DNA which is capable of undergoing crossing over or capable of Recon is having one or two pairs of nucleotides.
  • Complon : It is the unit of complementation. It has been used to replace cistron. Certain enzymes are formed of two or more polypeptide Whose active groups are complimentary to each other.
  • Operon : Operon is the combination of operator gene and sequence of structure genes which act together as a unit. Therefore it is composed of several The effect of operator gene may be additive or suppresive.
  • Replicon : It is the unit of Several replicons constitute a chromosome.

(3)  Some specific terms

• Transposons or Jumping genes : The term ‘transposon’ was first given by Hedges and Jacob (1974) for those DNA segments which can join with other DNA segments completely unrelated and thus causing illegitimate These DNA segments are transposable and may be present on different place on main DNA. The transposons are thus also called Jumping genes. Hedges and Jacob reported them in bacteria. But actual discovery of these was made by Barbara Mc Clintock (1940) in maize and she named them as controlling elements in maize or mobile genetic elements in maize. For this work, she was awarded nobel prize in (1983).
• Retroposons : The term was given by Rogers (1983) for DNA segments which are formed from RNA or which are formed by reverse transcription under the influence of reverse transcriptase enzyme or RNA dependent DNA polymerase

RNA ¾¾^Rev¾^erse¾^tran¾^scr¾^ipta¾^se ® DNA (Retroposon)

 

Note :

• About 10% of DNA of genome in primates and rodents is of this

• Split genes or interrupted genes : Certain genes were reported first in mammalian virus and then in eukaryotes by R. Roberts and P. Sharp in (1977) which break up into pieces or which are made of segments called exons and These are called split genes or interrupted genes.

Split gene = Exons + Introns

In mRNA formed from split gene exons are present and not corresponding to introns. So in split genes, exons carry genetic information or informational pieces of split genes are exons.

• Pseudogenes or false genes : DNA sequences presents in multicellular organisms, which are useless to the organism and are considered to be defective copies of functional genes (cistrons) are called pseudogenes or false These have been reported in Drosophila, mouse and human beings.

 

 

 

Important Tips

• Rarely a functional centromere is absent and the whole surface of chromosome functions as Such a chromosome is called holocentric.
• Inheritance is based on particles (genes).
• Genetically identical progeny is produced when the individual produces identical
• Gene flow is spread of genes from one breeding population to another by
• The genes, which enhance the effect of other gene, is also known as
• Single copy genes : Represented only once in the whole genome.
• Multigenes : A group of nearly similar
• Sutton and Winiweter (1900) expressed that number of chromosome is reduced to half in meiosis and doubled in
• Flemming clarified the chromosomal events involving mitosis and transfer of it from parent to
• A human diploid cell has about 100000 genes on its 46 chromosomes, out of which only 5-15% (average 10%) genes are expressed at a
• H₁, H_2a, H_2b protein of nucleosome rich in lysine amino acid and H₃, H₄ rich in arginine
• Sometimes two satellites are present in a chromosome these chromosome are called tandem SAT-Chromosomes.
• SAT Chromosomes are used as marker chromosomes.
• Deletion is common to acentric
• Lampbrush chromosomes are larger than polytene chromosomes.
• Lampbrush loops and polytene puff are
• Plasma genes occur in plastids, mitochondria, plasmid, sigma particle & kappa
• Hyper chromism is presence of some chromosome more than
• The former gene which have been mutated to such as extent that they can not be transcribed further in m RNA are called

Pseudogenes.

• Chromosomal theory of inheritance in the present form was modified by B. Bridges.
• Genes modify the effect of other gene called
• Super numerary chromosomes formed due to non-disjunction at the time of meiosis and called
• 3-11 nucleotide sequence of ribosome recognisition site on mRNA is called SD sequence or shine Dalgrano
• The term gene refers to a portion of DNA Gene is formed of Which can synthesis a single protein.
• Number of genes on a chromosome is
• In a chromosomes the protein content is
• Spring al. In 1975 reported lampbrush chromosome in Acetabularia.
• The drug mercaptolethanol when applied early in mitosis, interferes with the centriole apparatus, it therefore affects mitosis by disrupting the spindle
• Genetic drift is the random change in gene
• An allele is said to be dominant if it is expressed only in both homozygous & heterozygous
• Holandric genes are genes located on non- homologous segment of Y
• Cytochimera means cell having different chromosomes other than vegetative
• Translocation is a type of chromosomal aberration where a part of one chromosome is exchanged between non homologous
• The genetic basis of evolution (particular adaptation) was demonstrated in bacteria by Lederberg and E. M. Lederberg.
• The factors controlling change in gene frequencies are natural selection, mutation, migration, and genetic
• Gene flow is described as the transfer of gene between population, which differ genetically from one another but can