Question

A man with blood group $A$ marries a woman with blood group $O$ and their daughter has blood group $O$.

Answer 1

The give information is not enough to tell us which of the traits - blood group $A$ or $O$is dominant. In blood heredity, blood Type $A$ is always dominant and blood Type $O$ is always recessive. Here, father's Blood group can be $AA$ (homozygous) or $AO$ (heterozygous) genotypically, where as that of mother can be $OA$ or $OO$. For daughter to be born with blood group $O$, she must receive $O$ type gene one each from father and mother. For this father must have heterozygous $AO$ blood group and mother must have homozygous blood group $OO$.