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Explain polygenic inheritance with an example.

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Polygenic inheritance – Several genes combine to affect a single trait. A group of genes that together Dark Red 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 dark red and a white. Dark red genotypes F1 generation R1 R1 R2 R2 and white genotypes are r 1r1 r2 r2 – F1 generation medium red were obtained with the genotype R1 r1 R2 r2 . F1 wheat plant produces four types of gametes R1 R2 , R1 r2 , r1, r2 . The intensity of the red colour is determined by the number of R genes in the F2 generation. Four R 

genes: A dark red kernel colour is obtained. 

Three R genes: Medium – dark red kernel colour is obtained. 

Two R genes: 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 ratio of 63 red : 1 white in F2 generation – 1 : 6 : 15 : 20 : 15 : 6 : 1 in F2 generation.

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