Genes on the sex chromosomes
Sex linkage occurs when the inheritance of an allele is associated with a sex chromosome. The sex-linked examples considered here involve genes carried on the X chromosome.
Females have two X chromosomes, whereas males have one X chromosome and one Y chromosome.
| Sex | Sex chromosomes | Copies of an X-linked gene |
|---|---|---|
| Female | XX | Two |
| Male | XY | One |
Because a male has only one X chromosome, a recessive allele carried on that chromosome can be expressed without a corresponding dominant allele on another X chromosome.
A female has two copies of an X-linked gene. She can therefore have two dominant alleles, one dominant and one recessive allele, or two recessive alleles.
A heterozygous female carrying a recessive X-linked allele can be a carrier: she has the recessive allele but does not express the recessive phenotype.
Males cannot be carriers for a recessive X-linked condition in this pattern: the single allele present on their X chromosome is expressed. A father also does not pass an X-linked allele to his sons because sons receive his Y chromosome.