In terms of X-linked traits, which group is referred to as hemizygous?

Hemizygous refers to the condition of having only one copy of a gene, which is commonly seen in males for X-linked traits. Males have one X chromosome and one Y chromosome (XY system), meaning they have only one allele for genes located on the X chromosome, making them hemizygous for those traits. This contrasts with females, who have two X chromosomes (XX system) and therefore have two copies of the genes, allowing for the presence of dominant and recessive alleles which can impact the expression of traits in different ways.

Understanding hemizygosity helps clarify why X-linked traits are often expressed differently in males compared to females, as any recessive alleles on the X chromosome in males will be phenotypically expressed due to the lack of a corresponding allele on the Y chromosome. In contrast, females may be carriers of those traits without expressing them if they have a dominant allele on one of their X chromosomes that masks the effect of the recessive one.