Can an offspring have negative blood group if parents hav positive ones? my blood group is -A.my mum's blood group is+O while my dad's is +A.is this possible?
First let's look at the AO main grouping. Yes, it is possible for one parent with O and the other with A to have a child of group A. If your dad is heterozygous ie carries O recessively, then it is also possible for your parents to have a child of blood group O. Half his sperm will carry A and the other half O, and there is then a 50 percent chance of the child being A and a 50 percent chance of O. But if he is homozygous ie only carries A, then your parents can only have children of blood group A. Your mother is homozygous, like all group O people, ie only carries O.
Now the rhesus factor. It's more complicated than it looks. When most people talk about pos and neg, they are talking about rhesus D. Many people who are pos for D are neg for rhesus E, like me. But mostly only D is significant, so that's what I'll explain.
Yes, it is possible for your parents to have a child with a neg rhesus factor, but only if they both carry the coding for neg recessively. In other words, only if they are both heterozygous. This just means carrying something different that you can pass on, as well as carrying the characteristic that you yourself have. If just one parent is homozygous and only has their own blood group in the genetic coding, then it's not possible. It looks as if both of your parents, while positive in the phenotype, are neg and pos in the genotype and can therefore pass on either pos or neg. Since they also carry pos, you could have a sister or brother who is rh pos like your parents. The reason that your parents are heterozygous is that they got neg from one of their own parents and pos from the other. Unlike myself. I am rhesus pos but because I got pos from both my parents, I can only ever pass on pos.