Answer to Question #316934 in Genetics for Nhlonipho Manganyi

Question

There are three alleles for the gene that determines the ABO blood grouping system in

humans namely IA

, IB and IO. IA and IB are co-dominant and both are dominant over IO.

Two babies were born in a hospital on the same day (these were the only babies born

in the hospital on that day). One set of parents suspected that the babies were

swopped in the hospital nursery and immediately ordered blood tests to determine their blood

groups. Of the parents, Mr Suspicious was blood group A and Mrs Suspicious was blood group B.

The baby given to them was blood group O. Mr Convinced was blood group A and Mrs Convinced

blood group AB. The baby given to them was blood group B.

Did a mix-up occur? (Give a reason for your answer) What are the genotypes of the parents and

their babies? Show all the possible genotypes and phenotypes of children that can be produced by

Mr and Mrs Suspicious, and the expected ratios in which they will occur (include a Punnett Square)

10 marks

Answer 1

Let's assume that the genotypes of Mrs and Mr Suspicious are I^BI⁰ and I^AI⁰ respectively.

The cross will look like this:

P: I^BI⁰ (♀, blood group B) + I^AI⁰ (♂, blood group A)

↓

F1:

In this case a mix-up didn't occur, since parents with the genotypes I^BI⁰ and I^AI⁰ can indeed have a child with the genotype I⁰I⁰, who also has blood group O.

But it should be noted that if one of the parents was a monozygote, that is, the genotype of Mrs would be I^BI^B or the genotype of Mr would be I^AI^A, then they could not have a child with the genotype I⁰I⁰, and then there a mix-up would be occur.