X-linked recessive inheritance
If a gene is on the X chromosome, it mostly affects men: they have only one X, so a recessive allele shows straight away. Women with one affected allele are usually unaffected carriers.
Rules
- Men XY: one Xᵃ is enough → affected
- Women XᴬXᵃ: unaffected but carriers
- Sons always get their X from the mother, daughters one X from each parent
- Carrier × unaffected man: 50% of sons affected, 50% of daughters carriers
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
Duchenne muscular dystrophy is inherited X-linked recessive. The mother is unaffected, not a carrier (XᴬXᴬ), the father affected (XᵃY). What is the probability that a son is affected, a daughter is affected, and a child (sex not yet known) is affected?
Son affected in %
Daughter affected in %
Child affected in %
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
Red-green colour blindness is inherited X-linked recessive. The mother is an unaffected carrier (XᴬXᵃ), the father unaffected (XᴬY). What is the probability that a son is affected, a daughter is affected, and a child (sex not yet known) is affected?
Worked solution
- 1.Sons get the mother’s X and the father’s Y: affected if they inherit Xᵃ → 50%
- 2.Daughters get one X from the mother and the father’s X (Xᴬ): affected only with two Xᵃ → 0%
- 3.Child: half sons, half daughters → (50% + 0%) / 2 = 25%
Typical mistake: Applying “son affected” to all children. With a carrier mother 50% of sons are affected, but only 25% of all children.
Common questions
Why are men affected more often?
They have only one X chromosome. There is no second allele to make up for the recessive one.
Can an affected father pass the condition to his son?
No, the son gets the father’s Y chromosome. But all daughters of an affected father are at least carriers.