Sex determination and X-linked inheritance – Punnett squares and pedigrees

What it shows

This simulation covers sex determination and X-linked inheritance for ages 14–18. Students see that the father's sperm decides the sex of a child, follow haemophilia and red–green colour blindness through Punnett squares with carrier females and affected males, and compare simulated children with expected fractions. A pedigree builder tests autosomal and X-linked hypotheses and marks people who must be carriers. Model: Mendelian inheritance with full penetrance and no new mutations, a 1 : 1 sex ratio, and pedigrees without marriages between relatives.

How to use

Use the tabs to switch screens. In Sex determination, set Children per family and add families. In X-linked cross, choose a Trait and the Mother and Father genotypes, then press +1 child, +10 children or +100 children. In Pedigree, pick an Example pedigree, tap a person and use Affected ↔ unaffected, Male ↔ female, Add partner, Add child or Remove; tap a hypothesis to show its genotypes.

Parameters you can change

• Starting screen Sex determination, X-linked cross, Pedigree
• Trait Haemophilia A (X-linked recessive), Red–green colour blindness (X-linked recessive), Cystic fibrosis (autosomal recessive, for comparison)
• Mother's genotype Not a carrier, Carrier (heterozygous), Affected (homozygous recessive)
• Father's genotype Unaffected, Carrier (autosomal trait only), Affected
• Children per family (Sex determination screen) 1–8 children
• Example pedigree Family 1, Family 2, Family 3, Blank: one couple

Questions to explore

1. A carrier mother and an unaffected father have a son. What is his chance of haemophilia, and why is it different for a daughter?
2. Why are X-linked recessive traits much more common in males than in females?
3. Which kind of family in a pedigree rules out X-linked recessive inheritance but not autosomal recessive inheritance?