Explain Multiple Alleles / Dominance.

Explanation

When more than two alternative form called of a gene occupying the same locus on a chromosome in a population, these are known as multiple alleles.
= The ABO blood grouping is a good example of multiple alleles.
= In this case, more than two i.e. three alleles are present governing the same character.
= Multiple alleles can be found only when population studies are made.
= Occasionally, a single gene product may produce more than one effect.
= For example starch synthesis in pea seeds is controlled by one gene.
= - It has two alleles (B and b).
= Starch is synthesised effectively by BB homozygotes and therefore large starch grain are produced
= In contrast bb homozygote have lesser efficiency in starch synthesis and produce smaller starch grains.
= After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled.
= Heterozygotes produce round seeds and so B seems to be the dominant allele. But the starch grains produced are of intermediate size in Bb seeds.
= So if starch grain size is considered as the phenotype, then from this angle the alleles show incomplete dominance.
= Therefore, dominance is not an autonomous feature of a gene or the product that it has information for.
= - It depends as much on the gene product and production of a particular phenotype from this product as it does on the particular phenotype that we choose to examine.
= In case, more than one phenotype is influenced by the same gene.