Question

(a) List the three different allelic forms of gene '$I$' in humans. Explain the different phenotypic expressions, controlled by these three forms.
(b) A woman with blood group '$A$' marries a man with blood group '$O$'. Discuss the possibilities of the inheritance of the blood groups in the following starting with 'yes' or 'no' for each:
(i) They produce children with blood group "A" only.
(ii) They produce children some with '$O$' blood group and some with '$A$' blood group.

Answer 1

Genotype	Phenotype
I${}^A$I${}^A$, I${}^A$I${}^O$	A
I${}^B$I${}^B$, I${}^B$I${}^O$	B
I${}^A$I${}^B$	AB
I${}^O$I${}^O$	O

a) Gene 'I' which controls the blood group formation in humans exists in three different allelic forms: I${}^A$, I${}^B$ and I${}^O$. The different combinations of the three alleles produces 4 different phenotypes which are A, B, AB and O.

b) i) No, they do not produce children with blood group 'A' only.

ii) Yes, they produce children some with blood group 'A' and some with 'O'.

This is illustrated as follows:

A woman with blood group 'A' can have genotypes-

I${}^A$I${}^A$, I${}^A$I${}^O$

A man with blood group 'O' can have genotype- I${}^O$I${}^O$

Thus there are two possible crosses:

Cross I

I${}^A$I${}^A$ X I${}^O$I${}^O$

I${}^A$I${}^O$, I${}^A$I${}^O$, I${}^A$I${}^O$, I${}^A$I${}^O$

Cross II

I${}^A$I${}^O$ X I${}^O$I${}^O$

I${}^A$I${}^O$, I${}^A$I${}^O$, I${}^O$I${}^O$, I${}^O$I${}^O$

This is evident from the two possible crosses that a child born to such parents can have blood groups A or O.