How can I have blood group O- if both my mother and father have O+?

Answer

Best,

This is perfectly possible: the human blood group is generally defined on the basis of two different blood group genes, namely the ABO gene, for which your parents and yourself, all three are homozygous for the “O” allele. This allele is recessive to alleles A and B. So to have blood group “O” you must be “homozygous” for the allele “O”, which means that you have 2 copies of that allele.

The “positive” and “negative” do not refer to the ABO gene, but to another gene called the Rhesus factor. This gene displays two alleles, namely the allele “positive” (+) and the allele “negative” (-). However, the (+) allele is dominant over the (-) allele. This means that it is enough that you have one copy of the (+) allele to have a (+) blood group, but you must have two copies of the (-) allele to show blood group (-). Since you have blood group (-), this means that you have two copies of the allele (-) (so you are homozygous for the allele “negative”). This also means that you got both a (-) allele from your father and a (-) allele from your mother. This is now easy to understand if both your parents are heterozygous, i.e. if they both have 1 allele (+) and 1 allele (-). In that case, they are indeed both positive, because they each have one copy of the dominant (+) allele. But both can also pass on the recessive (-) allele, which is what happened in your case.

Hopefully this explanation is clear.

Kind regards,
Thierry Backeljau