A man with blood group A marries a woman with blood group O and their daughter has group O. Is this information enough to tell you which of the traits-blood group A or O- is dominant?Why or why not?

Given information is not sufficient to tell which of the blood is dominant because blood group is determined by multiple alleles ( I${}^A$, I${}^B$ and I ) and each individual have two alleles for blood group. These alleles are segregated during gamete formation and a new combination of alleles after fertilisation determines blood group of offspring.

Hence, a man with blood group A might have genotype I${}^A$I${}^A$, I${}^A$i while a woman with blood group has genotype 'ii'. As their daughter has blood group O, she has genotype ii. This is possible only if the father has genotype I${}^A$i. This is because allele I${}^A$ is dominant over 'i'.