Question

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

Answer 1

Dear Student,
The information is not enough to decide which blood group is dominant. But universally blood group A is dominant whereas blood group O is recessive.

According to universal law of blood group

If a man has blood group A it may be homozygous (AA) OR heterozygous (AO) as pairs. So, as A is dominating and mother has only OO and blood group O is recessive, then the daughter may have either of blood group O or A.

if A from father crosses with O of mother then blood group of daughter = A(dominating)

but if O from father crosses with O of mother then blood group of daughter = O (as recessive blood group would express only in homozygous condition).
Regards