Question

What are multiple alleles? Describe multiple alleles with the help of ABO blood group in man.

Answer 1

Hint: Until further studies and experiments were conducted, it was believed that there were only two alleles for a gene. However, for certain genes, more than two alleles are found to be present in the population. The major example of multiple allelism is ABO blood grouping in humans.

Complete answer:
Multiple alleles refer to the occurrence of three or more than three alleles for a particular gene. Alleles are different or contrasting forms of a gene. For example, for the gene encoding for height, one allele can be for tallness, whereas the other can be for dwarfness. But, in the case of multiple allelism, more than two forms of the gene can be present for the same locus.
The best example to understand multiple allelism is ABO blood grouping in humans.
A) The gene for blood grouping has 3 allelic forms – \[{I^A}\], \[{I^B}\], and \[{I^O}\].
B) Out of the 3 alleles, IA and IB are codominant and \[{I^O}\] is recessive.
C) These alleles code for surface antigens on RBCs. Allele A codes for antigen A, Allele B codes for antigen B, and Allele O codes for the absence of antigen.
D) Based on which two alleles the progeny inherits, the blood groups are determined in the following manner:
> A BLOOD GROUP: (\[{I^A}\], \[{I^A}\]) or (\[{I^A}\], \[{I^O}\])
> B BLOOD GROUP: (\[{I^B}\], \[{I^B}\]) or (\[{I^B}\], \[{I^O}\])
> AB BLOOD GROUP: (\[{I^A}\], \[{I^B}\])
> O BLOOD GROUP: (\[{I^O}\], \[{I^O}\])

Note:
Multiple allelism does not mean that 3 alleles are present at the same time, in the same locus. It just means that three or more than three alleles are present in the population for a particular gene. A human can only inherit 2 genes from their parents (one each).