Extensions of Mendelian Genetics (Deviation from Mendelism) - Polygenic Inheritance

Polygenic Inheritance:

• 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.
• For example - Height and skin colour in humans.
• Human skin colour is classic example of polygenic inheritance. In a polygenic trait, the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive. Let us assume that three genes A, B, and C control skin colour in humans with the dominant forms A, B and C responsible for dark skin colour and the recessive forms a, b and c for light skin colour. The genotype with all the dominant alleles (AABBCC) will have the darkest skin colour and that with all the recessive alleles (aabbcc) will have the lightest skin colour. As expected the genotype with three dominant alleles and three recessive alleles will have an intermediate skin colour. In this manner, the number of each type of alleles in the genotype would determine the darkness or lightness of the skin in an individual.
• Three genes produce seven phenotypes of skin colour due to the cumulative effect.
• Mulatto (sanwla) is intermediate with 3 dominant alleles and produces 8 different types of gametes.

Polygenic Inheritance in Wheat (Kernel colour):

• 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 R₁R₁R₂R₂ and white genotypes are r₁r₁r₂r₂.
• In the F₁ generation, medium red were obtained with the genotype R₁r₁R₂r₂. F₁ wheat plant produces four types of gametes R₁R₂, R₁r₂, r₁R₂, r₁r₂. The intensity of the red colour is determined by the number of R genes in the F₂ generation.
  

  Polygenic inheritance in wheat kernel colour

• 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. Another example: Height and skin colour in humans are controlled by three pairs of genes.
  

  The genetic control of colour in wheat kernels

Conclusion:

1. Finally, the loci that was studied by Nilsson – Ehle were not linked and the genes were assorted independently.
2. Later, researchers discovered the third gene that also affect the kernel colour of wheat.
3. The three independent pairs of alleles were involved in wheat kernel colour.
4. Nilsson – Ehle found the ratio of 63 red: 1 white in F₂ generation – 1: 6: 15: 20: 15: 6: 1 in F₂ generation.
   

   Polygenic inheritance in Wheat kernel

5. From the above results Nilsson – Ehle showed that the blending inheritance was not taking place in the kernel of wheat.
6. In F₂ generation plants have kernels with a wide range of colour variation. This is due to the fact that the genes are segregating and recombination takes place.
7. Another evidence for the absence of blending inheritance is that the parental phenotypes dark red and white appear again in F₂. There is no blending of genes, only the phenotype.
8. The cumulative effect of several pairs of gene interaction gives rise to many shades of kernel colour. He hypothesized that the two loci must contribute additively to the kernel colour of wheat.
9. The contribution of each red allele to the kernel colour of wheat is additive.