What characterizes a sex-linked recessive disorder?

A sex-linked recessive disorder is characterized by the fact that it is caused by mutations in genes located on the sex chromosomes, particularly the X chromosome. For a disorder that is recessive, an individual must have two copies of the recessive allele to express the trait. In the case of males, who have one X and one Y chromosome, they only require one copy of the recessive allele on their single X chromosome to express the disorder. Therefore, males can exhibit the disorder if they inherit the recessive allele from their mother, who holds two X chromosomes and may pass on one of her recessive alleles.

For females, who have two X chromosomes, they would need to inherit the recessive allele from both parents to exhibit the disorder. Thus, although females can also have the disorder, it is more frequently observed in males due to their hemizygous condition (having only one X chromosome).

The emphasis on needing one (for males) or two (for females) copies of the recessive allele correctly highlights why individuals can exhibit such disorders based on the genetic makeup tied to sex-linked inheritance. This understanding of inheritance patterns is crucial in veterinary medicine, as many hereditary disorders can present similarly in animal populations.