Final answer:
A carrier of an X-linked recessive trait who expresses the phenotype is a hemizygote, which refers specifically to males with only one X chromosome. Females can be carriers of X-linked recessive conditions without expressing the phenotype, while males with the condition are hemizygotes and express the phenotype.
Step-by-step explanation:
A carrier of an X-linked recessive trait who expresses the phenotype is called a hemizygote. The term 'hemizygote' applies specifically to males, who have only one X chromosome. In contrast, females have two X chromosomes.
For X-linked recessive diseases, males either have the disease or are genotypically normal; they cannot be carriers. Therefore, when they have the disease, they are hemizygotes. On the other hand, females can be carriers (heterozygous) without expressing the phenotype, as they possess two X chromosomes, one of which may carry a normal allele that masks the effect of the recessive allele.
It is important to distinguish this from autosomal recessive conditions, where an individual must be homozygous for the recessive allele to express the phenotype. For example, in conditions like sickle cell anemia, carriers are heterozygous and do not display symptoms but can pass on the allele to offspring. However, in the case of X-linked recessive traits, females can be carriers without showing symptoms, while males will express the phenotype if they have the recessive allele because they do not have a second X chromosome with a potentially normal allele.