What does it mean when genes are described as hemizygous?

When genes are described as hemizygous, it specifically means that only one copy of a gene is present in an organism where two copies are typically expected, such as in diploid species. This situation is commonly observed for genes located on the X chromosome in males of species with XY sex determination, where males possess one X and one Y chromosome, resulting in the presence of only one copy of any X-linked genes.

In this context, hemizygosity highlights that there is no second allele for a comparative effect, distinguishing it from situations where two identical (homozygous) or two different alleles (heterozygous) might be present. It also underscores the potential impact on phenotype, as a recessive mutation in a hemizygous state will be expressed more readily compared to a gene where two copies are available to mask the recessive effect.

The concept is therefore central in understanding genetic expression and inheritance patterns, especially in relation to sex-linked traits.