Investigate the inheritance patterns including but not limited to: sex-linkage, codominance, incomplete dominance, multiple alleles

What this dot point is asking

NESA wants you to explain sex-linked (X-linked) inheritance patterns and use Punnett squares to predict offspring probabilities for X-linked traits. This dot point sits within a broader cluster of non-Mendelian inheritance patterns (sex-linkage, codominance, incomplete dominance, multiple alleles). This page focuses on sex-linkage.

The answer

Why sex-linked matters

In mammals (including humans), biological sex is determined by the sex chromosomes. Females are XX; males are XY. Most genes on the X chromosome have no equivalent on the much shorter Y chromosome.

This produces an asymmetry. Females have two copies of each X-linked gene. Males have only one copy. So a recessive allele on the X chromosome shows up immediately in a male (he has no second X to mask it), but only in females who are homozygous for the recessive allele.

The result: X-linked recessive disorders are far more common in males than in females. Classic examples include haemophilia, colour blindness, and Duchenne muscular dystrophy.

Notation

Use superscripts on the X chromosome to show the allele.

• IMATH_0 = dominant (normal/unaffected) allele.
• IMATH_1 = recessive (affected) allele.
• Y = no allele (Y is irrelevant for X-linked traits).

Female genotypes can be $X^H X^H$ (unaffected, homozygous), $X^H X^h$ (unaffected carrier), or $X^h X^h$ (affected).
Male genotypes can be $X^H Y$ (unaffected) or $X^h Y$ (affected).

Why no male carriers

A male only has one X chromosome. He either has the recessive allele (and is affected) or he doesn't (and is unaffected). There is no carrier state for males in X-linked recessive inheritance.

Standard carrier-mother cross

Carrier mother ($X^H X^h$) × unaffected father ($X^H Y$).

• Daughters: 50% unaffected non-carrier, 50% unaffected carrier. No affected daughters.
• Sons: 50% unaffected, 50% affected.

The two key statistics from this cross.

• 50% of sons are affected.
• 50% of daughters are carriers (none are affected).

Affected father, unaffected mother

Affected father ($X^h Y$) × unaffected non-carrier mother ($X^H X^H$).

• All daughters are carriers ($X^H X^h$).
• All sons are unaffected ($X^H Y$).

This is why an affected man cannot pass the X-linked allele to his sons (he passes Y to sons, not his X). All his daughters become carriers, however.

Worked example: haemophilia

A carrier mother and an affected father have children. Predict offspring outcomes.

Mother $X^H X^h$ × Father $X^h Y$.

Daughters: 50% carrier, 50% affected.
Sons: 50% unaffected, 50% affected.

This is the only standard cross that produces affected daughters in X-linked recessive inheritance.

Common sex-linkage traps

Wrong notation. Always write the allele as a superscript on the X. Writing just "H" or "h" without the X is a 1-mark deduction because it hides the sex-linkage.

Forgetting the Y has no allele. The Y chromosome does not carry the X-linked gene. Males are NEVER carriers of X-linked traits.

Wrong denominators. "50% of sons are affected" is different from "25% of all children are affected sons." Read the question.

Confusing X-linked dominant and X-linked recessive. Most exam questions focus on X-linked recessive. X-linked dominant is rare; if a question says dominant, the pattern flips.

In one sentence

X-linked inheritance is governed by alleles on the X chromosome, with X-linked recessive disorders far more common in males because males have only one X (no second copy to mask the recessive allele), and the standard carrier-mother cross produces 50% affected sons and 50% carrier daughters.