Read X-Linked Risk Through the Family Tree
For each generation, follow the X chromosome—not the disease name. An affected father gives his X to daughters but his Y to sons; a carrier mother gives either X to each child with equal probability.
X-Linked Recessive Inheritance is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Draw the first generation
Use Xᴴ for the usual allele and Xʰ for the recessive haemophilia allele (the same logic works for the colour-blindness alleles). An affected father is XʰY. Every daughter inherits his Xʰ; every son inherits his Y, never his Xʰ. If the mother is assumed non-carrier (XᴴXᴴ), all daughters are XᴴXʰ carriers and all sons are XᴴY unaffected. These are genotype predictions, not a claim that normal appearance alone proves non-carrier status.
Follow the carrier daughter to her sons
A carrier daughter XᴴXʰ passes Xʰ to half of her children on average. Conditional on a child being a son, he receives Y from his father and has a 1/2 chance of receiving Xʰ from her. That gives the keyed 1/2 risk for an affected daughter's son in the sample question. The original woman's lack of family history is used as a non-carrier assumption in this school-level cross; it is not genetic proof.
Multiply when several sons must all be affected
In the NSEB PYQ, an affected man's normal daughter is an obligate carrier because she received his affected X but does not show the recessive phenotype. With an unaffected husband, each of her sons has a 1/2 affected risk. Given that the couple has four sons, the chance all four are affected is (1/2)⁴ = 1/16. Do not insert another 1/2 for being male: the question already conditions on four sons.
X-linked recessive probabilities
Track the X chromosome through each generation. A carrier mother passes the allele to half her children; an affected son must receive it from his mother, while a daughter of an unaffected father can be a carrier but is not affected. Apply each transmission probability separately.
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- ✓Affected father: affected X to every daughter, Y to every son.
- ✓With a non-carrier mother, those daughters are carriers and sons are unaffected.
- ✓A carrier mother gives an affected X to each son with probability 1/2.
- ✓Given four sons, all four affected: (1/2)⁴ = 1/16.
- ✓State the non-carrier assumption; normal appearance or no family history alone does not establish it.
Do not pass an X-linked allele directly from father to son. And distinguish 'risk for a son' from 'risk for a child of unspecified sex': the extra sex-probability factor only belongs in the latter.
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