MCAT Question A Day - 3/20/14 - Answer!

The pedigree in the following diagram represents the inheritance of deaf-mutism over three generations within a family. (An arrow on a circle is a male, and a cross below a circle is a female; affected individuals have blackened circles). What kind of trait is deaf-mutism?

A. Autosomal dominant

B. Autosomal recessive

C. Sex-linked dominant

D. Sex-linked recessive

The correct answer is (B). The question requires a solid foundation in Mendelian genetics and a familiarity with the interpretation of pedigrees. By process of elimination, the trait must be an autosomal recessive trait. The affected female in the second generation (third from the fight on the second line) could not have an autosomal dominant trait if neither parent has the allele (both parents are unaffected). Similarly, the same affected female could not result from such a cross if the trait is either sex-linked dominant or sex-linked recessive (the father would also have to be affected in either case). Only an autosomal recessive trait successfully works its way through the pedigree in the pattern presented.

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