If the mother has type O positive blood and father AB baby A negative where does come from if both parents are positive?

The mother has type O positive blood, which means she has the O antigen on her red blood cells and the Rh factor (D antigen) on her red blood cells. The father has AB positive blood, which means he has both the A and B antigens on his red blood cells and the Rh factor on his red blood cells.

The baby has A negative blood, which means he has the A antigen on his red blood cells but does not have the Rh factor on his red blood cells.

This is possible if the mother is heterozygous for the Rh factor (Dd) and the father is homozygous dominant for the Rh factor (DD). In this case, the mother would pass on either a D allele or a d allele to the baby, while the father would always pass on a D allele. If the mother passes on a d allele and the father passes on a D allele, the baby will have A negative blood.

Therefore, the baby's A negative blood type could have come from the parents if the mother is heterozygous for the Rh factor (Dd) and the father is homozygous dominant for the Rh factor (DD).