X-linked recessive

X-linked recessive is a pattern of inheritance involving a recessive allele that is carried on an X chromosome. Sex-linked recessive diseases include Duchenne muscular dystrophy, haemophilia and Red-Green colour blindness.

The pedigree below is typical of an X-linked recessive pattern of inheritance:

Pedigree 7
Pedigree 7

There are multiple hallmarks of an X-linked recessive pattern of inheritance, notably:

  • Such disorders mainly affect males, since males need inherit only one copy of the allele (on their single X chromosome), while females need two copies.

  • Male-to-male transmission is impossible because only the Y chromosome is passed down the paternal-son line.

  • Affected males are often born to unaffected parents. All affected males are related through their mothers.

  • Affected females require both an affected father and a mother who carries the allele on one of her X chromosomes.

  • The trait or disease is typically passed from an affected grandfather, through his carrier daughters, to half of his grandsons.