3.18B - Sex-linked inheritance

3.18B — Higher: sex-linked inheritance

Biology-only Higher extension: 3.18B

A sex-linked gene is located on a sex chromosome. The pattern below is X-linked recessive inheritance, illustrated by haemophilia, a disorder that affects blood clotting. The inheritance explanation depends on the location of the allele on X, not on the sex of the person alone.

Let $X^H$ carry the dominant allele for unaffected clotting and $X^h$ carry the recessive haemophilia allele. The Y chromosome is written simply as $Y$ because it does not carry the corresponding allele for this X-linked gene.

A male has only one X chromosome in this model, so a recessive allele on that X has no corresponding dominant allele to mask it. A female with one $X^h$ can have a dominant $X^H$ on her other X and is therefore usually an unaffected carrier. This is why X-linked recessive disorders are more common in males, although females can be affected if they inherit $X^h$ from both parents.

Worked example: carrier mother and unaffected father

Cross a carrier mother, $X^H X^h$, with an unaffected father, $X^H Y$.

One of the four equally likely cells is an affected male, so the probability is 25% of all children. If the question asks only about sons, one of the two male outcomes is affected, so the probability is 50% of sons. The two statements use different denominators and are both correct for this cross.