sex linkage The location of a gene on a sex chromosome. Such a gene is usually unrelated to primary or secondary sexual characters and is located on the X-chromosome. The heterogametic sex (e.g. a male mammal or female bird) receives the X-chromosome and therefore all its sex-linked genes from the homogametic parent, and passes them on to offspring of the homogametic sex. Since there is no partner to the X-chromosome in the heterogametic sex, recessive genes cannot be masked by their dominant alleles and so are expressed in the phenotype. It is for this reason that male humans manifest a larger number of recessive genes (e.g. red-green colour blindness, and haemophilia) than females.

sex linkage The tendency for certain inherited characteristics to occur far more frequently in one sex than the other. For example, red–green colour blindness and haemophilia affect men more often than women. This is because the genes governing normal colour vision and blood clotting occur on the X sex chromosome. Women have two X chromosomes. If one carries an abnormal allele it is likely that its effects will be masked by a normal allele on the other X chromosome. However, men only have one X chromosome and any abnormal alleles therefore will not be masked. See also carrier.