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aTacoParty t1_ix5ftfp wrote

Great question. This is a special case since the hemophilia A gene is only on the X chromosome and not on the Y chromosome. Since XY males will only have one copy of the X chromosome, they only need to inherit one mutant allele to develop disease. The Y chromosome doesn't have a copy of that gene to make up for the mutant one. In XX females, they have two copies of the gene and thus will need two copies of the recessive disease allele to develop disease (just like genes on any other chromosome).

All chromosomes except sex chromosomes are symmetrical so this is a special case that makes XY males more susceptible to certain genetic diseases.