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Genetic Basis Of Inheritance

A. ENOTYPE-Phenotype concept

In order to make definite distinction between hereditary and environmental variations, Johannsen (1909) formulated the genotype-phenotype concept. According to him, the genotype of an individual represents sum total of heredity. On the other hand, phenotype represents features, which are produced by interaction between genotype and environment.

”A genotype can thus exhibit different phenotypes under different conditions. This is referred to as individual’s norm of reaction to the environment. Therefore, similar genotypes may not have the same phenotype. Conversely, similar phenotypes do not necessarily mean same genotype.

”As an interesting case, we may consider the example of Himalayan albino rabbits, which are characterized by black colour on feet, ears, nose and tail, the remaining body being white. If  hairs from white part were plucked and the rabbit was allowed to stay at a cold place, the developing hairs were found to be black rather than white. Such changes due to different environments are normally of an adaptive nature.

Phenocopies

”When two genotypes produce the same phenotype due to different environments, one is called the phenocopy of the other, because they differ genotypically.

”Phenocopy (Goldschmidt, 1935) is nonhereditary phenotypic modification, caused by special environmental condition, that mimics a similar phenotype. For instance, in Drosophila melanogaster, normal body colour is brown and a heredity variant has yellow colour. It was observed that brown and yellow flies reproduce sincerely, irrespective of changes in the environment. Cases are known where normal larvae (brown), when raised on food containing silver salts, develop into yellow flies. This is a phenocopy of yellow mutant, but would give rise to brown flies in normal environment.

B. 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.

(1) 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 e.q. co-dominance, multiple alleles.

(i) 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 parents. This is called incomplete dominance or blending inheritance.

(ii) Codominance (1:2:1 ratio) : In codominance, both the genes of an allelomorphic pair express themselves equally in F1 hybrids. 1:2:1 ratio both genotypically as well as phenotypically in F2generation.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 F1hybrids 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).

                                           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, HbSand HbA.

(2) 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.

(i) Complementary genes (9 : 7 ratio) : The complementary genes are two pairs of nonallelic dominant genes (i.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.

(ii) 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 Aand C,the agouti-coloured guinea pigs have genotype CCAA. The black mice possess factor for black colour Cbut 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 ccAAare albino. Here presence of gene Cproduced black colour and addition of gene Achange its expression to agouti colour.

(iii) Epistasis (Inhibiting genes) : Epistasis is the interaction between nonallelic genes (Present on separate loci) in which one-gene masks, inhibits or 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 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.

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

(b) Recessive epistasis (9:3:4 ratio) : Epistasis due to recessive gene is known as recessive epistasis, i.e., out of the two pairs of genes, the recessive epistatic gene masks the activity of the dominant gene of the other gene locus. The dominant Aexpresses itself only when the epistatic locus C also has the dominant gene if the epistatic locus has recessive gene c, gene Afails to express.

(iv) Duplicate genes (15:1 ratio) : Sometimes two pairs of genes located on different chromosomes determine the same phenotype. These genes are said to be duplicateof each other. The dominant triangular fruit shape of Capsella bursa pastoris(shepherd’s purse) is determined by two pairs of genes, say Aand 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 a15 (triangular) : 1(top shaped)ratio in F2generation.

(v) 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 produce.

(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 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 homozygous condition kill all or nearly all homozygous individuals, while in case of semilethal genes some homozygous individuals are able to survive. The lethal genes are always recessive for their lethality and express the lethal effect only in homozygous condition.

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. Following are the examples of dominant lethal genes.

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.

(vii) 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 monogene.

(viii) 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:

(a) The effects of each contributing gene are cumulative or additive.

(b) Each contributing allele in a series produces an equal effect.

(c) There is no dominance involved.

(d) Epistasis does not exist among genes at different loci.

(e) No linkage is involved in the process.

(f) Effects of environment are absent or may be ignored.

Human skin colour : This character was studied by Davenport, 1913 in the marriages between negroes and whites. The F1offsprings arising as a result of these marriages are called as mulattoes. The human skin colour is determined by two pairs of genes, P1and P2. A negro having very dark skin with four colour genes i.e., P1P1 P2P2, when married to a white with no colour gene (p1p1p2p2) 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 F2offspring 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 (P1and P2),

50% offsprings with one colour gene (P1or 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

(ix) 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 chromosomes. Characters of multiple alleles are following –

(a) Multiple alleles occupy the same locuswith in the homologous chromosomes. It means only one member of the series is present in a given chromosome.

(b) 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 individual.

(c) The gametes contain only one chromosome of each types, therefore, only one allele of the multiple series in each gamete.

(d) Crossing over does not occur in the multiple alleles.

(e) Multiple alleles control the same character, but each of them is characterised by different manifestation. Sturteventhas summarised it that they carry the same function but with varying degree of efficiency.

(f) 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 codominance. Therefore, multiple alleles act in some way to control the various steps in a chemical reaction.

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

C. TYPES OF VARIATIONs

”Variations are of two types regarding the nature of cells affected, somatic and germinal (blastogenic).

”Somatic variations affects the somatic cells of an organism. These are neither inherited from the parents nor transmitted to the next generation. These are acquired by an individual during its own life and is lost with its death (i.e., called the acquired variations or Modifications). These are produced by three types of factors: environment, use and disuse of organs, and conscious efforts.

”Germinal variations or Blastogenic variations affects the germ cells of an organism and is, consequently, inheritable. These are received by the individual from the parents and is transmitted to the next generation.

The germinal variations further are of two types regarding the degree of difference among the individuals : discontinuous and continuous.

DISCONTINUOUS VARIATIONs

”These are also called as sports, saltations or mutations.

”There are certain characteristics within a population that exhibit a limited form of variation. Variation in this case produces individuals showing clearcut differences with no intermediates between them e.g., blood groups in humans, wing lengths in Drosophila, melanic and light forms in Biston betularia, style length in Primula and sex in animals and plants.

”Characteristics showing discontinuous variations are usually controlled by one or more major genes that may have two or more allellic forms and their phenotypic expression is relatively unaffected by environmental conditions.

CONTINUOUS VARIATIOns

”These are also called as Fluctuating variations i.e., they fluctuate on either side (both plus and minus) of a mean or average for the species.

”Many characteristics in a population show a complete gradation from one extreme to the other without any break and is referred to as continuous variations. The continuous variation can be explained by certain characteristics e.g., mass, linear dimension, shape and colour of organs and organisms. The frequency distribution for characteristics exhibiting continuous variation is a normal distribution curve. Most of the organisms in the population fall in the middle of the range with approximately equal numbers showing the two extreme forms of the characteristics. Characteristics exhibiting continuous variation are produced by the combined effects of many genes i.e., polygenes, and environment factors.

INFLUENCE OF THE ENVIRONMENT ON VARIATIONs

”The ultimate factor determining a phenotypic characteristic is the genotype. At the moment of fertilization the genotype of the organisms is determined but the subsequent degree of expression allowed to this genetic potential is influenced greatly by the action of the environmental factors during the development of the organisms.

SOURCES OF VARIATIONs

”The sources of genetic variations are as follows:

❒ Crossing over : The reciprocal crossing over of genes between chromatids of homologous chromosomes may take place during prophase I (pachytene sub stage) of meiosis. This produces new linkage groups and so provides a major source of genetic recombination of alleles.

❒ Independent assortment : The orientation of the chromatids of homologous chromosomes (bivalents) on the equatorial spindle during metaphase I of meiosis determines the direction in which the pairs of chromatids move during the anaphase I. This orientation of the chromatids is random. During metaphase the orientation of pairs of chromatids once more is random and determines which chromosomes migrate to opposite poles of the cell during anaphase II. These random orientations and the subsequent independent assortment i.e., segregation of the chromosomes gives rise to a large calculable number of different chromosomal combinations in the gametes.

Random fusion of gametes :In sexual reproduction the fusion of gametes (both male and female gametes) is completely random. So any male gamete is potentially capable of fusing with any female gamete.

D. Heredity

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

(1) 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 DNA.

(2) Pre-Mendelian view points

(i) 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 coitus. On account of these vapours, the offspring exhibits similarities with the male parents.

(ii) Semen theory: Empedocles, suggested that both parents produce semen which arises directly from their various body parts. The semen from both the parents gets mixed and produces a new individual.

(iii) 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 Roux.

(iv) Particulate theory : Maupertuis proposed that the body of each parent gives rise to minute particles. These particles unite together to form the daughter individual.

(v) 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 boxes. This was named as encasement theory.

(vi) 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 epigenesis.

(vii) Pangenesis theory : This theory was proposed by Charles Darwinaccording to this theory every cell, tissue and organ of animal body produces minute invisible bodies, called gemmules or pangenes. They can produce offsprings.

(viii) 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 somatoplasm. The germplasm forms the bridge of life between successive generations and is passed on from one generation to the next.    

(3) Evidences against blending theory:Thus individual would represent the mixture of both the parents. The prevailing view of in pre-mendelian era was blending theory. 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.

(4) Basic features of inheritance :In the middle of 18thcentury, Carolus Linnaeousa Swedish taxonomist and two German plant breeders Kolreuterand Gaertnerperformed artificial cross pollination in plants and obtained hybrid offspring. 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 : –

(i) Traits have two alternative forms.

(ii) Traits are represented in the individual by distinct particles which do not blend or change.

(iii) Traits may remain unexpected for one or more generations and reappear later unchanged.

(iv) Traits may remain together in one generation and separate in a later generation.

(v) One alternative of a trait may express more often then the other />  

E. ABO BLOOD GROUPS IN HUMAN BEINGS

”K. Landsteiner (1900)has given the concept of ABO blood groups, on the basis of presence or absence of certain antigens. It was found that there are two antigens A or B and as a result of these, four groups of blood are present. With these antigens, there are two types of antibodies in the serum of

the blood. Antibodies in a particular individual will be found of those antigens that are absent in blood.

Presence of antigen and antibodies in human blood

”Antibodies in blood group A will be able to agglutinate RBC of the blood group B and vice versa. AB blood group will not agglutinate any other group due to the absence of antibodies. O blood group should be able to agglutinate all other three blood groups except its own.

Compatible donor and recipient ABO blood groups

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