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Multiple Factor Hypothesis/Polygenic Inheritance

(NOTES)

Qualitative traits are governed by the Mendel’s Laws of inheritance. These are clear cut

traits, resulting in two extremes such as the flower colour (purple or white), seed-shape

(round or wrinkled) and the seed colour (yellow or green). There is no intermediate

phenotype. Variation in qualitative trait is called discontinuous variation.

However, Some other important characters in cultivated plants and domestic animals,

such as the yield of seeds and fruits as well as the eggs, amount of milk or meat

produced by animals, etc., do not fall into two clear-cut classes and all gradations

appear between the two extremes), for example, between black and white skin-colour

of humans, between red and white colour of wheat grain, etc.). These characters are

called as quantitative characters. Variation in quantitative trait is called continuous

variation.

Such quantitative characters show a continuous variation. Mendel’s method of analysis

is hard to apply in such continuously varying characters because they seem to mix or

blend with each other instead of getting segregated in the offspring of hybrids.

The problem of the inheritance of quantitative character was taken up by the Swedish

botanist. H. Nilsson-Ehle (1908) and American scientist E.N. East (1910, 1916). These

investigators showed that this apparent ‘blending inheritance’ can be explained by

supposing that continuously varying characters are due to the combined or cumulative

action of several genes, each of which exerts a small effect on the same character. Such

genes are called the cumulative genes or additive genes or polygenes.

A cumulative gene is one, which if added to another identical or similar gene, affects

the intensity or degree of expression of a quantitative character. In other words, a

quantitative character is simultaneously governed by several genes (= polygenes), and

the effect or action of such genes is cumulative or additive in nature. This is also known

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as the multiple-factor hypothesis. Gene-pairs, which act in a cumulative way to result

into a quantitative trait, are known as multiple-factors. Since quantitative inheritance

is controlled by many genes, it is also known as polygenic inheritance.

Various shades of wheat grain

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Few examples of polygenic inheritance in both plants and animals are listed below:

1. Grain color in wheat

2. Ear length in maize

3. Ray size in flower heads of Compositae

4. Seed colour in wheat

5. Corolla length in tobacco (Nicotiana longiflora)

6. Skin color in human beings

7. Height of man

A few common examples of polygenic inheritance are described as below:

Kernel (seed) colour in wheat

The Swedish botanist, H. Nilsson-Ehle (1908) first studied the inheritance pattern of the

colour of the grain in wheat. He crossed two varieties of wheat, with red and white

grain colour. He found that all the F1 offspring were intermediate between red and

white (with light-red colour), demonstrating as if red seed-colour was incompletely

dominant over white seed-colour.

When the F1 hybrid-plants were self fertilized, the F2 progeny showed a ratio of 15 red

to 1 white. The red progeny, however, carried various shades between the red and

white (such as dark-red, medium-red, light-red and pink). The ratio 15:1 clarifies that

this was a dihybrid cross, in which two identical genes were involved for producing the

red colour.

In this example, two pairs of segregating genes are responsible for the color variation in

the wheat kernel. The red kernel wheat carries two pairs of dominant genes (two pairs

of alleles: R1R1R2R2) both of which contribute some quantity of redness to the grain.

These genes are duplicates of each other. The white kernel wheat had recessive alleles

of both these pairs (r1r1r2r2) and does not contribute anything to red coloration.

The F1 hybrid possesses two dominant genes (R1r1R2r2); hence, it is intermediate

between red and white. In the F2 generation, the color varies depending on the number

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of dominant genes that the offspring gets from the hybrid-parents, i.e. 4, 3, 2, 1 or zero.

See the Table given below:

Various phenotypes (and genotypes) have been shown in the above checker board

(Punnett square). Of the possible 16 types, the following will be the phenotypes

according to the number of dominant genes possessed by them. See the Table given

below:

S.

No.

Phenotypic ratio and phenotypes

Genotype and Number of dominant genes (colour-genes) carried by 5 types of individuals (5 phenotypes)

1 1/16 Red 4 colour-genes (R1R1R2R2): completely like the red grandparent

2 4/16 Dark-red 3 colour-genes (R1R1R2r2), (R1r1R2R2)

3 6/16 Medium-red 2 colour-genes (R1R1r2r2),(R1r1R2r2),(R1r1R2r2),(r1r1R2R2),(R1r1R2r2)

4 4/16 Light-red 1 colour-gene (R1r1r2r2), (r1r1R2r2)

5 1/16 White No colour-gene (r1r1r2r2): completely like the white grandparent

Phenotypic ratio =1:4:6:4:1

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Skin colour in man

It was idea of Davenport (1913) that the multiple-factor hypotheses could explain the

mode of inheritance of skin colour in man. His assumption was that the Negroes

differed from the whites in having 2 pairs of colour-genes (dominant genes) that did not

show complete dominance. He carried out his studies in Jamaica and Bermuda, where

intermarriages between coloured and white people were very common.

A marriage between a Negro (P1P1P2P2) and a white (p1p1p2p2) results in children having

intermediate shade (mulattoes) as a result of only 2 colour-genes (P1p1P2p2). When 2

mulattoes (F1 hybrids) marry they may have children showing different degree of

coloration, ranging from pure black to white (F2 generation).

In the F2 generation, 1/16 will be as dark as the Negro grandparent having 4 colour

genes. The rest 14/16 will show intermediate shades depending upon the number of

colour-genes contained by them. But all these gradations could be seen only when a

large number of children are born. In a small family, the parents of mulattoes will

produce a completely black or white child.

Various phenotypes (and genotypes) have been shown in the above checker board

(Punnett square). Of the possible 16 types the following will be the phenotype according

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to the number of dominant genes (colour-genes) possessed by them. See the Table

given below:

S. No. Phenotypic ratio and the phenotype

Genotype and Number of dominant genes (colour-genes) carried by 5 types of individuals (5 phenotypes)

1 1/16 Negro 4 colour-genes (P1P1P2P2): completely like the red grandparent

2 4/16 Dark 3 colour-genes (P1P1P2p2), (P1p1P2P 2)

3 6/16 Mullatoes 2 colour-genes (P1P1p2p2), (P1p1P2p2), (P1p1P2p2), (p1p1P2P2),

(P1p1P 2p2)

4 4/16 Fair 1 colour-gene (P1p1p2p2), (p1p1P2p 2)

5 1/16 White No colour-gene (p1p1p2p2): completely like the white grandparent

It has been proposed that the difference in the skin colour between Negros and Whites

is due to the presence of more than two pairs of colour-genes bringing about a

considerable variation in the skin colour. Gates has suggested for three where as Stern

for four, five or six pairs of genes. Other geneticists have estimated the colour-genes

number from two to twenty pairs but the exact number involved is still unknown.

Corolla length in tobacco (Nicotiana longiflora)

Edward East (1916) reported his studies on the inheritance of corolla length in

Nicotiana longiflora, a self-pollinated variety of tobacco. This character is governed by

multiple genes. He crossed a variety, the corolla of which had an average length of 52

mm, to a variety, the corolla of which had an average length of 70 mm. Both of these

varieties had long been inbred and therefore were of homozygous nature.

The marked differences in corolla lengths were heritable, pointing out that they are

controlled by genes rather than by environmental factors. East found that F1 hybrid was

intermediate with mean corolla length of 61 mm.

In F2 generation, a much larger variation for corolla length than in F1 generation was

observed. The variation was continuous (governed by quantitative genes) as well. East

raised 444 F2 plants and failed to get even a single plant similar to either of the parents.

This pointed out that more than 4 pairs of genes were involved in determining the

length of corolla in Nicotiana longiflora.

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East postulated that each dominant gene contributed towards the length of corolla

length of Nicotiana longiflora. According to him, the contribution of each dominant

gene can be calculated by the following formula:

(Maximum corolla length – Minimum corolla length)/Number of alleles involved)

For Example: (93.3 mm – 40.5 mm)/6 alleles = 52.8/6 = 8.8 mm per allele

Thus, in this example, each of the 6 dominant genes contributed 8.8 mm to corolla

length of Nicotiana longiflora.

Salient features of quantitative/polygenic inheritance

1. Quantitative traits are governed by multiple genes or polygenes.

2. Expression of polygenes results in continuous variation in phenotype of the individual

3. There is a marked effect of the environment on the expression of polygenes

4. Polygenes can be subjected to statistical analysis in order to assess their number regarding a particular trait.

5. Each gene produces a unit-effect or individual effect; hence, the effects of genes are additive or cumulative.

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6. Dominance is absent or partial. F1 hybrid shows blending in characters or in other words the F1 hybrid is intermediate.

7. Segregation of genes occurs at many loci and not at a single locus of homologous chromosomes.

8. Polygenes are independently assorted in F2 generation according to Mendelian inheritance, but the phenotype is in continuous range between the extreme limits of the parental phenotype.

9. The phenotypic proportion of F2 progeny is modified according to the number and nature of the genes.

10. Polygenes have pleotrophic effect, i.e. they may influence more than one trait.

Difference between Qualitative and Quantitative characters:

S. No. Qualitative inheritance

Quantitative inheritance

1 Qualitative traits are governed by major genes, whose effects are definite.

These traits are governed by minor genes. Their effect is additive.

2 These traits are not usually affected by environmental factors.

These traits are well affected by environmental factors.

3 Clear-cut difference between the phenotypes (two extremes)

No clear-cut difference between the phenotypes. The phenotype shows a spectrum between the two extremes.

4 Inheritance of These traits results in distinct phenotypic changes (Discontinuous variation).

Inheritance of These traits results in continuous variation. Quantitative characters may not be put up in to clear cut classes. Instead, they show a spectrum of the two extremes.

5 Single gene effect (monogenic inheritance): Effect of two alleles of a single is well detected.

Effect of several genes (polygenic inheritance): Effect of individual genes is too slight to be detected.

6 Concerned with mating of individuals and their progeny

Concerned with population of organisms consisting of all possible kinds of mating

7 Analyzed by making counts and ratios Statistical analysis gives estimates of population parameters such as mean, standard deviation, variance, etc.