The Main Examples of Polygenic Inheritance in both Plants and Animals are listed below:
1. Grain Color in Wheat:
It was Swedish scientist Nilsson – Ehle (1909) who first studied the inheritance pattern of the colour of the grain in wheat. In some types, the kernel color is red (aleurone color) and in others white and in still others which may be regarded as intermediates different, shades of red appear in the kernel color.
Nilsson – Ehle crossed a variety of wheat having dark red kernel with the one having white kernel. The red color is incompletely dominant over white hence the F hybrid had a kernel which was intermediate between red and white.
These F hybrids were bred among themselves to get the F2 generation. In the F, progeny the kernel color ranged from dark red, to white, between them there were at least three grades of color. These shades of color can be graded as follows
Red, Reddish, Intermediate (pink), light and white.
According to the law of probability, it can be assumed, that two pairs of segregating genes are responsible for the color variation in the wheat kernel. The red kernel wheat has two pairs of genes (two pairs of alleles) 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 (Q R,) and does not contribute anything to red coloration.
The F possesses two dominant genes (Rt r R2 r2) hence it is intermediate between red and white. In the F, generation the color varies depending on the number of dominant genes the offspring gets i.e. 4, 3, 2, 1 or zero.
In the above checker board, of the possible 16 types the following will be the phenotype according to the number of dominant genes possessed by them As per the above categorization, in the checker board, number I is red, 2,3, 5, and 9 are reddish, 4,5,6,10,11 and 13 are intermediate, 8, 12,14 and 15 are light and 16 is white.
The ratio is 1:4:6:4:1. In many instances this ratio is also represented as 15:1, since except for the last one all others have some shades of red Nilsson-Ehle found in certain other crosses of wheat concerning the kernel color, in the F2 progeny only one in every 4 was white, with at least 5 intergrades between red and white. He proposed that in this instance there were three pairs of genes (R, Rt R2R2 R3 R3) responsible for the kernel color.
2. Skin Color in Human Beings:
The interpretation regarding the skin color in Negroes and Caucasoid whites was bit of a problem to interpret from the point of view of genie inheritance davenport (1913) first successfully explained the skin color inheritance in terms of polygene.
He studied skin color of several people in Jamica and Muda. In these regions intermarriages between Negroes and whites was quite frequent and the children were called Mulattoes. He hypothesized that Negroes of central and Western Africa differ from whites in having two pairs of dominant alleles which are incompletely dominant over the genotype of whites.
In a cross between a Negro and a white, the F, called mulattoe is heterozygous to both P1 and P, with the result the skin color is intermediate. Just as in kernel color in wheat, here also the skin color depends on the number of dominant alleles.
It is as follows – Negro – allele four dominant genes and whites all the four recessive genes. Mulattoes – Two dominant genes. When two mulattoes marry among themselves (a mulattoe man and a mulattoe woman) in the F2 generation 16 possible types could be found among children.
Assuming that mulattoes are hybrids for both the color genes (P, and P,) there genotype should be while they get dominant allele from the Negro parent, the recessive alleles are obtained from the white parent.
Theoretically mulattoes both male and female produce four types of gametes which on random combination would give 16 types.
In the F2 progeny the skin color would be black (4 dominant genes), dark (3 dominant genes), Mulattoe (2 dominant genes), Fair (one dominant gene) and white (no dominant gene).
Physiologically the skin color in human beings depends upon the amount of melanin (a pigment) deposited on the skin. The amount of melanin depends upon the genes and its development depends upon the amount of sun light received by the skin. According to Curt Stern, the human skin colour depends on gene loci located at four to several different loci.
3. Height of Man:
The height of the body in man is another typical example of polygenic inheritance. The functioning of these genes however greatly depends on environmental factors. The genes control the functioning of the pituitary thus controlling growth.
An under secretion of the pituitary under the influence of a ‘dwarf gene’ would retard the growth.
4. Ear Length in Maize:
Emerson and East, the pioneers in the study of polygenic inheritance, observed many quantitative variations in ear size in maize they worked on two varieties of maize – long eared black Mexican sweet com and short eared Tom Thumb popcorn.
The first variety ranges in ear length from 12-21 cms with an average of 16.8 cms. The second variety has ears ranging in length from 5-8 cms, averaging to 6.6 cms. When these two varieties were crossed, the ear length in the F, progeny ranged from 9-15 cms averaging 12.1 cms.
Selling the F, progeny, F, was obtained much in the same way as in inheritance of kernel color in wheat.
5. Ray Size in Flower Heads of Compositae:
A possible occurrence of polygenic system operating in the ray development has been reported by the author (Sundara Rajan.) in the plant Bidenspilosa of Compositae. Here there are two extremes.
In the one represented by a normal plant, the rays in the heterogamous heads are of normal size i.e. they are formed by the fusion of three petals and the other 3 are reduced resulting in zygomorphy of the ray floret.
In the other extreme which is a recessive mutant, the rays are completely reduced; with the result all the five petals of the ray floret are equal in size like the disc floret. The floret becomes actinomorphic.
Between these, there are at least three intergradations representing cumulative interaction of these genes. The fully developed rays represent all the dominant genes. While the reduced rays represent all the recessive alleles.
Work is going on to hybridize the various grades to arrive at the possible number of cumulative genes involved in the ray development.