A man whose blood group is heterozygous B is married to a woman whose blood up is also heterozygous B Which of…

Analyze the question

Both parents are heterozygous blood group B, so each has the genotype I^B i (or BO). We need to find which statement is correct about their four children.

Concept

ABO blood groups are controlled by three alleles: I^A, I^B and i (O). I^B is dominant over i. Group B can be I^B I^B or I^B i. Group O is only ii.

Solution

Cross: I^B i × I^B i

| | I^B | i | |---|---|---| | I^B | I^B I^B (B) | I^B i (B) | | i | I^B i (B) | ii (O) |

• Genotype ratio: 1 I^B I^B : 2 I^B i : 1 ii
• Phenotype ratio: 3 group B : 1 group O

For four offspring, the expected ratio is 3 group B : 1 group O. That matches the idea "three of them belong to blood group B" (and one to group O).

Checking the options

• A. Three of them group B: matches the expected 3:1 ratio. ✔
• B. Three group O: wrong, since O is the minority (1/4).
• C. None group O: not guaranteed, as each child has a 25% chance of being O.
• D. Two group B: does not match the expected ratio.

Note: Strictly, Mendelian ratios are probabilities, so each child is independent and the actual outcome could differ.

Answer: A

Exam tip: Heterozygous × heterozygous always gives a 3:1 phenotypic ratio for a dominant/recessive pair. With four children, think "3 dominant : 1 recessive." Remember that O blood needs ii, so both parents must carry the i allele.