Human Genetics Problem Set

Problem 1 Tutorial: Inheritance of an X-linked recessive trait

Red-green color blindness is X-linked in humans. If a male is red-green color blind, and both parents have normal color vision, which of the male's grandparents is most likely to be red-green color blind?
If both parents have normal vision, the mother of the affected male must be heterozygous for the X-linked, recessive alleles for red-green color blindness. The father of the affected male could not have been the source of the red-green color blind allele since fathers can only pass X-linked traits to their daughters, and Y chromosomes to their sons.
The two possible pedigrees for inheritance from a maternal grandparent are shown in the pedigree charts labeled A and B.

Which of the mother's parents, the maternal grandmother (pedigree chart B) or maternal grandfather (pedigree chart A), is more likely to both be
1. red-green color blind,
2. the source of the allele inherited by their grandson?
Males with only a single X chromosome are more commonly affected by X-linked, recessive traits than are females with two X chromosomes. Why?

A male need only inherit the recessive allele from a heterozygous female carrier. A female would need to inherit the recessive allele from both parents, an affected father and a carrier (or affected) mother.

To review how to trace the inheritance of the human X chromosome see problem 7, problem 9, and problem 10 of the Sex-Linked Inheritance Problem Set in Mendelian Genetics.

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University of Arizona
Revised: November 5, 1998
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