Question

Explain polygenic inheritance and multiple alleism with the help of suitable examples.

Answer 1

Polygenic inheritance occurs when one characteristic is controlled by two or more genes. One wheat variety with a gene for dark red colour that is homozygous dominant is crossed with another variety which has a homozygous recessive gene for white colour. The red colour in the wheat kernel is incompletely dominant over white hence the F${}_1$ hybrid has a kernel intermediate between red and white i.e. medium red. The offspring of the F${}_1$ generation are heterozygous dominant for both genes. Thus when F${}_1$ hybrids are crossed to get the F${}_2$ generation the kernel colour has a range dark red, medium dark red, medium red, light red and white in the ratio 1:4:6:4:1.

Multiple allelism refers to the presence of more than two contrasting alleles for a gene. Human blood group inheritance is the governed by three alleles namely I${}^A$, I${}^B$ and i. Alleles I${}^A$ and I${}^B$ show codominance as I${}^A$ I${}^B$ genotype produce AB blood group that exhibits characters of both I${}^A$ and I${}^B$ alleles. Allele 'i' is recessive to both I${}^A$ and I${}^B$; I${}^A$ i and I${}^B$ i phenotypes exhibit A and B blood groups respectively.