X-linked

X-linked describes genes or traits located on the X chromosome. Because females have two X chromosomes (XX) and males have one (XY), X-linked traits often show specific inheritance patterns: for example, an X-linked recessive trait can be masked in females (who have a second X) but will express in males who carry the allele (since males have only one X).

• X-linked recessive traits tend to appear much more frequently in males than females. A male has no "backup" normal allele on a second X, so a single recessive allele on his X will cause the trait. Classic examples: red-green color blindness and hemophilia A affect males predominantly, while females are usually carriers unless they inherit two affected X alleles.
• Fathers cannot pass X-linked traits to their sons (because a father gives his son a Y chromosome, not an X). However, an affected father will pass the X-linked allele to all of his daughters. Thus, in X-linked recessive cases, an affected father makes all his daughters carriers (if the mother provides a normal X).
• When the mother is a carrier of an X-linked recessive trait, each son has a 50% chance of receiving the affected X (and being affected), and each daughter has a 50% chance of receiving the affected X (and thus becoming a carrier). This is why X-linked recessive conditions often skip generations through carrier females.

• In pedigree analysis, a telltale sign of X-linked recessive inheritance is that the trait affects mostly males and often skips a generation (transmitted through carrier females). For instance, an affected maternal grandfather may have carrier daughters who then have affected sons.
• If a question notes that an affected father transmits the trait to all his daughters but none of his sons, it points to an X-linked pattern (X-linked dominant if daughters are affected, or X-linked recessive if daughters are carriers).
• Problems may ask about probabilities: e.g., "If a mother is an asymptomatic carrier of an X-linked disorder and the father is unaffected, what is the chance of their son having the disease?" (Answer: 50% for a son, since the son must inherit the mother's X). Recognize that scenario as X-linked inheritance.