Human Genetics Problem Set

Problem 8 Tutorial: X-linked traits

A human male carrying an allele for a trait on the X chromosome is:
A. heterozygous
B. homozygous
C. hemizygous
D. monozygous
E. holozygous
Sex-determination in humans
Sex-determination in humans is dependent on the sex chromosomes. Females are XX. Males are XY. Since males have only a single copy of the X chromosome, they have only a single allele for any gene on the X chromosome.

Males are said to be "hemizygous" for any X-chromosome genes, meaning that there are only half ("hemi") as many alleles as normally present for a diploid individual. As a consequence, if males (but not females) inherit an X-linked recessive trait from their mothers, they display the recessive phenotype.

The Biology Project
University of Arizona
Revised: November 5, 1998
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