Why are males more often affected by X-linked recessive disorders?

Males have only one X chromosome. A single recessive allele on their X chromosome is enough to express the trait, since there's no second X to mask it.

A female heterozygous for an X-linked recessive trait (X^A X^a). She doesn't express the trait but can pass the recessive allele to half her children.

Can a daughter be affected?

Yes, but only if she inherits the recessive allele on both X chromosomes (X^a X^a). This requires a carrier (or affected) mother and an affected father, which is uncommon.

What is the inheritance pattern of haemophilia?

X-linked recessive. Famous example: Queen Victoria was a carrier; haemophilia spread through European royalty via her daughters.

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NSW · HSCBiology

Sex-linked (X-linked) inheritance calculator

Pick the mother's and father's genotypes; get the Punnett square and the probability of affected sons, affected daughters, and carrier daughters.

Inputs

Mother genotypeFather genotype

Assumes the trait is X-linked recessive (the standard NESA case, e.g. haemophilia, red-green colour blindness, Duchenne muscular dystrophy).

Result

Punnett square

	X^A	Y
X^A	X^A X^A	X^A Y
X^a	X^a X^A	X^a Y ●

● = affected.

Probabilities (of any one offspring)

Affected son: 50%
Affected daughter: 0%
Carrier daughter: 50%

How this calculator works

The calculator treats the X-linked locus correctly: mothers contribute an X allele, fathers contribute either their X or a Y. Affected sons have X^a Y; affected daughters have X^a X^a; carrier daughters have one X^A and one X^a.

Common questions

Why are males more often affected by X-linked recessive disorders?: Males have only one X chromosome. A single recessive allele on their X chromosome is enough to express the trait, since there's no second X to mask it.
What is a carrier?: A female heterozygous for an X-linked recessive trait (X^A X^a). She doesn't express the trait but can pass the recessive allele to half her children.
Can a daughter be affected?: Yes, but only if she inherits the recessive allele on both X chromosomes (X^a X^a). This requires a carrier (or affected) mother and an affected father, which is uncommon.
What is the inheritance pattern of haemophilia?: X-linked recessive. Famous example: Queen Victoria was a carrier; haemophilia spread through European royalty via her daughters.