Hemizygous

Hemizygous refers to the presence of only one allele for a characteristic, as opposed to the usual two. This is often the case for genes on the X chromosome in males, who have only one X chromosome and are therefore hemizygous for the genes it carries.

Definition

In genetics, a hemizygous organism has only one allele for a particular trait or gene, rather than the usual two. This can occur in several ways, but most commonly it is the result of the organism having only one copy of a chromosome, or a deletion of a region of a chromosome that contains the gene in question. In humans, males are hemizygous for most genes on the X chromosome, because they have only one X chromosome.

Hemizygosity in Humans

In humans, males are hemizygous for the genes on the X chromosome because they have one X and one Y chromosome (XY sex-determination system). Females, on the other hand, have two X chromosomes and are therefore not hemizygous for these genes (XX sex-determination system). This has important implications for the inheritance of traits and diseases linked to the X chromosome.

Implications of Hemizygosity

The implications of hemizygosity can be significant, particularly in the case of X-linked recessive disorders. Because males have only one X chromosome, they will express the trait or disorder if they inherit one recessive allele. Females, however, must inherit two recessive alleles to express the trait or disorder. This is why X-linked recessive disorders are more common in males than in females.

Examples of Hemizygous Conditions

Some examples of conditions that can result from hemizygosity include Hemophilia and Duchenne muscular dystrophy, both of which are X-linked recessive disorders. In these cases, a male who inherits the recessive allele from his mother will have the disorder, while a female must inherit the recessive allele from both parents to have the disorder.

See Also

• Allele
• Genetics
• X-linked recessive inheritance
• Y-linked inheritance

Hemizygous Resources
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