A single recessive allele can cause a recessive trait if it is located on

Human males have only one X chromosome. If a male inherits an X chromosome that carries a mutated gene for a particular trait or disorder, he will develop the trait or disorder because he does not have a second, normal copy of the gene to compensate for the mutated copy.

Females have two X chromosomes. If they inherit a mutated copy of a recessive allele on one X chromosome, they may have a normal (dominant) allele of the gene on their other X chromosome that can compensate for the mutated copy. Females who carry a mutated gene on only one of their X chromosomes are called “carriers” because they carry the mutation, but they do not usually develop the trait or disorder associated with the mutation.

In case of autosomes, both male and female have two chromosomes in each homologous pair. When an individual has a single copy of a recessive allele (heterozygous), the dominant allele on the other chromosome will mask the expression of the recessive trait. In order to express a recessive trait, an individual must inherit two copies of the recessive allele (one from each parent).