Explain polygenic inheritance with an example.

Explanation

Polygenic inheritance - Several genes combine to affect a single trait. A group of genes that together determine (contribute) a characteristic of an organism is called polygenic inheritance . It gives explanations to the inheritance of continuous traits which are compatible with Mendel.s Law .
The first experiment on polygenic inheritance was demonstrated by Swedish Geneticist H.Nilsson - Ehle (1909) in wheat kernels . Kernel colour is controlled by two genes each with two alleles, one with red kernel colour was dominant to white. He crossed the two pure breeding wheat varieties deark red and a white . Dark red genotypes `R_1R_1R_2R_2` and white genotypes are `r_1r_1r_2r_2`. In the `F_1` generation medium red were obtained with the genotype `R_1r_1R_2r_2 . F_1` wheat plant produces

four types of gametes `R_1R_2, R_1r_2,r_1R_2,r_1r_2` . The intensity of the red colour is determined by the number of R genes in the `F_2` generation.
Four R genes: A dark red kernel colour is obtained . Three R genes Medium - dark red kernel Medium - red kernel colour is obtained . One R gene : Light red kernel colour is obtained . Absence of R gene: Results in white kernel colour.
The R gene in an additive manner produces the red kernel colour . The number of each phenotype is plotted against the intensity of red kernel colour which produces a bell shaped curve . This represents the distribution of phenotype.
Conclusion :
Finally the loci that was studied by Nilsson - Ehle were not linked and the genes assorted independently. Later , researchers discovered the third gene that also affect the kernel colour of wheat. The three independent pairs of alleles were involved in wheat kernel colour . Nilsson - Ehle found the ration of 63 red : 1 white if F2 generation - 1 : 6: 15 : 20 : 15 : 6 : 1 in F2 generation.