Where are X-linked genes located?

X-linked genes are specifically located on the X chromosome, which is one of the sex chromosomes in humans. Males typically have one X chromosome and one Y chromosome, while females have two X chromosomes. This unique positioning is significant because it leads to specific inheritance patterns, particularly affecting males more than females when it comes to X-linked recessive traits. For instance, if a gene associated with a condition is located on the X chromosome and a male inherits that X chromosome with the recessive trait, he will express that trait because he does not have a second X chromosome to potentially mask that allele.

In contrast, the other options represent locations for genes that are not X-linked. The Y chromosome is distinct from the X chromosome and carries genes related primarily to male sex determination and spermatogenesis. Chromosome 21 is one of the autosomes and does not contain genes linked to the X chromosome inheritance patterns. Mitochondrial DNA is inherited maternally and is involved in energy production within cells, not related to the X-linked inheritance seen with genes on the X chromosome.