Question

The probability of having a girl child with blood group O when the parents have blood group A and B is
A. 0%
B. At least 50%
C. At most 25%
D. Exactly 75%

Answer 1

Hint: The blood grouping in humans decided by the ABO blood grouping system. The surface of our red blood cells may have specific proteins called antigens: A and B.
The presence or absence of these determine the blood type of an individual.

Complete answer: In the ABO blood grouping system, the blood type is decided by the gene I. This particular gene has three alleles (different forms of the same gene) IA, IB, and IO.
There are four possible types of phenotypes: A, B, AB, and O blood type.
A person with A blood type may have two genotypes i.e. IAIA or IAIO; a person with B blood type may have two genotypes i.e. IBIB or IBIO; a person with AB blood type can have only one possible genotype i.e. IAIB, and lastly, a person having O blood type would have IOIO genotype.
Now, according to the question since the offspring has O blood type this implies that both parents must have one IO allele, and hence they have heterozygous genotypes.
Working out the Mendelian cross we can determine the ratio:

		Parent 2
		IA	IO
Parent 1	IB	IAIB	IBIO
	IO	IAIO	IOIO

Each square represents a 25% chance of the offspring having either A, B, AB, or O type blood.

So, the correct answer is option C. At least 25%.

Note: The ABO blood grouping is also coupled with another, i.e. the Rh blood grouping. Just like the antigens A and B, it is decided by the presence or absence of a protein Rh factor (Rhesus factor) on the surface of red blood cells. If the Rh factor is present then the person is Rh +ve and if it is absent then the person is said to be Rh -ve.