I dont understand the answer to this questionWatch
In the grandfather and the grandson, the Y acts as what is referred to as a "sleeping partner" ie it has no effect on the phenotype; but because the mother and the grandmother in this example had the abnormal G antigen phenotype, they must have been homozygous for the abnormal allele.
One condition inherited in this way is haemophilia, in which there is a deficiency of Factor VIII in the blood clotting mechanism leading to a bleeding tendency. It ran in the ancestors of the British royal family. Because of the explanation above, haemophilia is 8 times more common in men than in women (because females need to have both X chromosomes with the abnormal allele (homozygous), which is of course less likely.