Sex-linked Inheritance

Why are far more men colour-blind than women? Genes on the X chromosome are inherited differently: sons get their only X from their mother, so a single recessive allele is enough.

GeneticsX-linkedColour blindness

Inputs

Input examples

Xᵃ carries the recessive allele. A woman needs two copies to show the trait (one copy makes her a carrier); a man needs only one, on his single X.

Parents

Mother

Father

The trait

Genes on the X Chromosome

Women have two X chromosomes, men an X and a Y. The Y carries very few genes, so for most X-linked genes a man has just one copy. If that copy carries a recessive variant, he shows the trait — there is no second X to cover for it.

That is why red-green colour blindness affects about 1 in 12 men but only 1 in 200 women, and why haemophilia was famous in the royal families of Europe: carrier mothers pass it to half of their sons, while fathers never pass it to their sons at all.

sonsX from mother only

carrier mum50% of sons affected

affected dadall daughters carriers

dad → sonnever passed on

Punnett Square (X and Y)

mother × father

Result

Enter the values above

Key Relations

Sons

A son gets the Y from his father, so only the mother’s X decides.

Example:

Daughters

A daughter needs a recessive X from both parents to show the trait; with one she is a carrier.

Per Child

Half of the children are sons and half are daughters.

Sources & Verification

References

T. H. Morgan, “Sex limited inheritance in Drosophila”, Science 32, 120–122 (1910).