Final answer:
Sex-linked genes are typically more expressed in males, as they have only one X chromosome and are hemizygous for X-linked traits; however, in birds, sex-linked traits are more likely to appear in females because they are the sex with non-homologous sex chromosomes.
Step-by-step explanation:
Sex-linked genes are more likely to be expressed in males than females due to the presence of only one X chromosome in males. This makes them hemizygous for genes on the X chromosome, which means they express sex-linked traits with just one copy of the allele. In contrast, females have two X chromosomes, so a recessive allele on one X chromosome is often masked by a dominant allele on the other, unless the female inherits two recessive alleles for the trait, making her homozygous recessive. However, in birds, it is the female that has the non-homologous sex chromosomes and is therefore more likely to express sex-linked traits. Moreover, sex-linked disorders such as red-green color blindness and hemophilia are more frequently observed in males because they only need one recessive mutant X allele to be affected. Carrier females may pass on the trait to their offspring without expressing it themselves.