A man with a blood group "A" marries a women with a blood group "O" and their daughter has a blood group "O". Is this information enough to tell u which of the traits -blood group A OR O IS DOMINANT. Please explain
No it is not enough.
This information is insufficient to determine the dominant blood group. During cross breeding, even recessive traits can express themselves phenotypically. For example, if a heterozygous male AO mates with OO female, then there is a probability of the child inheriting OO genotype. So, you cannot determine the dominant blood group in this case.
The ABO blood type is controlled by a single gene with three alleles: i, IA, and IB. The IA allele gives type A, IB gives type B, and i gives type O. As both IA and IB are dominant over i, only ii people have type O blood. Individuals with IAIA or IAi have type A blood, and individuals with IBIB or IBi have type B. A type A and a type B couple can also have a type O child if they are both heterozygous (IBi,IAi).
So in case of a heterozygous male with IAi genotype marrying a ii genotype female, the child has a probability of inheriting ii phenotype, which results in O blood group.
Parents IAi (Father) ii (Mother)
Gametes IA and i i
So daughter could be IAi i.e. blood group A or ii i.e blood group O.
The A blood group is actually dominant over O, but here we cannot determine the dominancy based on the given information.