A cross between two parents produced four offspring's with blood groups AB, BB, BO and AO. what is the blood group of their parents?

Analysis: We need to find the parental genotypes that can produce all four offspring: AB, BB, BO and AO.

Concept: Each parent passes only one allele to each child. A and B are codominant, and both are dominant over O.

Step-by-step reasoning:

1. BB offspring: Both parents must each carry a B allele.
2. AB offspring: One parent gives A and the other gives B. Since both parents carry B, one of them must also carry A.
3. AO offspring: An O allele must come from one parent, so one parent carries O.
4. BO offspring: This confirms that one parent carries both B and O.

So one parent is AB (carries A and B) and the other is BO (carries B and O).

Check with a Punnett square (AB × BO):

| | B | O | |---|---|---| | A | AB | AO | | B | BB | BO |

This gives exactly AB, AO, BB and BO, which matches the four offspring.

Why the others fail:

• A (AA × BO): No parent can give a second B, so BB is impossible.
• B (BB × AO): AB and BO would be possible, but BB is impossible because the AO parent has no B, and the offspring would never include AB and BO together with BB.
• D (AB × AO): No parent can give two B alleles, so BB is impossible. It would also produce AA, which is not among the offspring.

Answer: C. AB and BO

Exam tip: For blood group questions, first look at the homozygous offspring (like BB). It tells you which allele both parents must carry. Then use the other offspring to fill in the remaining alleles.