Can a child have blood group '$O$' if his parents have blood group '$A$' and '$B$'? Explain.

(N/A) Yes,a child can have blood group '$O$' if both parents are heterozygous for their respective blood groups.
Blood group inheritance is controlled by three alleles: $I^A$,$I^B$,and $i$.
- $A$ person with blood group '$A$' can have the genotype $I^A I^A$ (homozygous) or $I^A i$ (heterozygous).
- $A$ person with blood group '$B$' can have the genotype $I^B I^B$ (homozygous) or $I^B i$ (heterozygous).
If both parents are heterozygous,i.e.,the father has genotype $I^A i$ and the mother has genotype $I^B i$,the cross is as follows:
Parents: $I^A i \times I^B i$
Gametes: $(I^A, i) \times (I^B, i)$
Offspring genotypes: $I^A I^B$ $(AB)$,$I^A i$ $(A)$,$I^B i$ $(B)$,$ii$ $(O)$.
Thus,there is a $25\%$ probability that the child will have blood group '$O$' (genotype $ii$).