Question

For the ABO blood grouping in human beings, the ratio of different genotypes and phenotypes is:

• A. 7: 4
• B. 4: 7
• C. 6: 4
• D. 6: 5

Answer 1

The correct option is C 6: 4

ABO blood groups of humans are controlled by three alleles: $I^A,I^B$ and i. This is an example of multiple allelism as a single gene has more than two alleles.

The number of different genotypes for multiple alleles is calculated by the formula n(n+1)/2, where n is the number of alleles.

Thus for ABO blood groups in humans, the number of possible genotypes are 3(3+1)/2 = 6.

The number of phenotypes for multiple alleles can be calculated using the formula (n+1), where n is the number of alleles.

Thus for ABO blood groups, the number of phenotypes would be 3+1=4.

Hence the ratio of different genotypes to phenotypes is 6: 4.

$\begin{array}{cc}\text{Phenotype}& \text{Genotype}\\ \text{A}& I^A{I}^A\text{or}{I}^{A}i\\ \text{B}& I^B{I}^B\text{or}{I}^{B}i\\ \text{AB}& I^A{I}^B\\ \text{O}& \text{ii}\end{array}$