I have a basic understanding of genetics, and I'm really puzzled by this. My grandma's blood type is A (I don't know if it's heterozygous or homozygous) and my grandpa is AB. Yet my mother is type O.
Grandma says that my mom is 100% my grandpa's child, so how did it happen? I have a few hypotheses regarding this. Either my grandma is lying, which I find to be really implausible, or there's some genetical anomalies either in my mom or my grandparents.
I suspect that grandma is heterozygous, having the IAi genes for blood type, and grandpa has a weird case of IAIBi alleles, coming either from a failed disjunction which caused both alleles for A and B to be present in one sperm cell, and the i allele to be present in the egg, resulting in the IAIBi genotype. Then, when grandpa had my mom, both i alleles got combined and out came my mom with O type.
My other hypothesis is that mom is heterozygous, either IAi or IBi, but the dominant allele underwent a deletion (or something else) and was inactivated, and as such only the recessive allele remained, giving her type O blood.
Could I get some insight on how this happened? I can't seem to find anything related to it on the internet.