Question

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

Answer 1

Hint: The term ‘multiple alleles’ refers to three or more alleles governing a single phenotype. Alleles are an alternative form of gene found on the same locus on homologous chromosomes. Multiple alleles can be understood only when population studies are conducted. Because, there can be many alleles for a single gene, but only two alleles will be found in a single organism.

Complete answer:
When there are three or more alleles for a single gene, governing a single phenotype, it is known as multiple allelism. Alleles are an alternative form of genes. Mostly, there are only two alleles for a single gene. Few examples of multiple alleles in animals are,
(i) ABO blood group in man
(ii) Coat colour in rabbits
(iii) Eye colour in Drosophila
ABO Blood groups in man
ABO blood groups were discovered by the biologist Karl Landsteiner in the year 1901. It is controlled by the gene I. There are three alleles for the gene I, namely I^A, I^B and i. These alleles determine the type of sugar polymers that protrude from the plasma membrane of red blood cells. The alleles I^A and I^B produce slightly different forms of sugar, whereas, i allele doesn’t produce any sugar polymer.
Human organisms are basically diploid and hence contain only any two of the above alleles. I^A and I^B alleles are dominant over the other i allele. The following gives you the idea about the genotype and its respective phenotype.
If the genotype is I^AI^B, the respective blood group type is AB.
If the genotype is I^Ai or I^AI^A, the respective blood group type is A.
If the genotype is I^Bi or I^BI^B, the respective blood group type is B.
If the genotype is ii, the respective blood group type is O.

Note: Often, multiple alleles and polygenes are two terms which are confused. When three or more alleles govern a single phenotype, it is known as multiple alleles. Whereas, when three or more genes govern a single phenotype, it is known as polygenes or polygenic inheritance. Example of polygenic inheritance is skin colour in man.