In humans, hemophilia (OMIM 306700) is an X-linked recessive disorder that affec
ID: 23231 • Letter: I
Question
In humans, hemophilia (OMIM 306700) is an X-linked recessive disorder that affects the gene for factor VIII protein, which is essential for blood clotting. The dominant and recessive alleles for the factor VIII gene are represented by H and h. Albinism is an autosomal recessive condition that results from mutation of the gene producing tyrosinase, an enzyme in the melanin synthesis pathway. A and a represent the tyrosinase alleles. A healthy woman named Clara (II-2), whose father (I-1) has hemophilia and whose brother (II-1) has albinism, is married to a healthy man named Charles (II-3), whose parents are healthy. Charles's brother (II-5) has hemophilia, and his sister (II-4) has albinism. The pedigree is shown. What are the genotypes of the four parents (I-1 to I-4) in this pedigree? I-1 is HYaa; 1-2 is H Aa; 1-3 is HY Aa; 1-4 is hhAa I-1 is HY Aa; 1-2 is H Aa; 1-3 is HY Aa; 1-4 is HhAa I-1 is HY Aa; 1-2 is HAa; 1-3 is HY AA; 1-4 is HhAa I-1 is hY Aa; 1-2 is HAa; 1-3 is HY Aa; 1-4 is HhAaExplanation / Answer
Answer is choice D. Charles's siblings have albinism and hemophilia, which means that parents I-3 and I-4 must be heterozygous for albinism (Aa). Also the mother (I-4) is a carrier of hemophilia: Hh since one of her children has it. The dad (I-3) does not have it so he is HY. This leaves us with choices B and D. Focus on I-1. Since he has hemophilia, he must have the recessive allele: hY. This leads us to choice D.