What is sex-linked inheritance? Explain the inheritance of haemophilia with a cross between a carrier woman and a normal man.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define sex-linked inheritance. Inheritance of genes carried on a sex chromosome, usually the X; haemophilia and colour blindness are X-linked recessive.
2 · Explain why males are affected more. Males have one X, so one faulty allele shows; females need two and are usually carriers.
3 · Set up the cross. Carrier woman XXʰ × normal man XY; eggs X and Xʰ; sperm X and Y.
4 · Draw the cross. [Diagram: Punnett square with XX, XXʰ, XY, XʰY]
5 · State the result. Daughters: half normal, half carriers. Sons: half normal, half haemophilic.
6 · Give the pedigree evidence. Queen Victoria was a carrier, and many of her male descendants had haemophilia.
7 · Link to real life. Genetic counsellors use pedigrees to tell families the risk for each child.
8 · Close in one line. So an X-linked recessive trait passes from carrier mothers to half their sons.
Scoring tip: Mark the X carrying the allele clearly (Xʰ). Write separate results for sons and daughters.
