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Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.
In mammals, the female is the homogametic sex, with two X chromosomes (XX), while the male is heterogametic, with one X and one Y chromosome (XY). Genes on the X or Y chromosome are called sex-linked.
In birds, the opposite is true: the male is the homogametic sex, having two Z chromosomes (ZZ), and the female (hen) is heterogametic, having one Z and one W chromosome (ZW).
X-linked recessive traits are expressed in all heterogametics, but are only expressed in those homogametics that are homozygous for the recessive allele. For example, an X-linked recessive allele in humans causes hemophilia. Hemophilia is much more common in males than females because males are hemizygous - they only have one copy of the gene in question - and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population. Tsarevich Alexei of Russia was the most famous sufferer of X-linked hemophilia.
The incidence of recessive X-linked phenotypes in females is the square of that in males (squaring a proportion less than one gives an outcome closer to 0 than the original). If 1 in 20 males in a human population are red-green color blind, then 1 in 400 females in the population are expected to be color-blind (1/20)*(1/20). (The term 'color-blind' is not completely accurate. There are degrees of weakness in color vision and it is now called 'color vision deficiency')
X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele. There are a few Y-linked traits; these are inherited from the father.
In classical genetics, a reciprocal cross is performed to test if a trait is sex-linked.
|Illustration of some X-linked heredity outcomes (A) the affected father has one X-linked dominant allele, the mother is homozygous for the recessive allele: only daughters (all) will be affected. (B) the affected mother is heterozygous with one copy of the X-linked dominant allele: both daughters and sons will have 50% probability to be affected. (C) the heterozygous mother is called "carrier" because she has one copy of the recessive allele: sons will have 50% probability to be affected, 50% of unaffected daughters will become carriers like their mother.|
Each child of a mother affected with an X-linked dominant trait has a 1/2 chance of inheriting the mutation and thus being affected with the disorder. All female children of an affected father will be affected (daughters possess their fathers' X-chromosome). No male children of an affected father will be affected (sons do not inherit their fathers' X-chromosome).
Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder. All males possessing an X-linked recessive mutation will be affected. (Males have a single X-chromosome and therefore have only one copy of X-linked genes.) All offspring of a carrier female have a 1 in 2 chance of inheriting the mutation. All female children of an affected father will be carriers. (Daughters possess their father's X-chromosome.) No male children of an affected father will be affected. (Sons do not inherit their father's X-chromosome.)