Sex linkage

Sex linkage is the patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). They are characteristically different from the autosomal forms of dominance and recessiveness as they are different depending on the sex of the individual. Sex linkage is the patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). They are characteristically different from the autosomal forms of dominance and recessiveness as they are different depending on the sex of the individual. Since humans have several times as many genes on the female X chromosome than on the male Y chromosome, X-linked traits are much more common than Y-linked traits. Additionally, there are more X-linked recessive conditions than X-linked dominant, and X-linked recessive conditions affect males much more commonly, due to males only having the one X chromosome required for the condition to present. In humans, X-linked traits are inherited from a carrier or affected mother or from an affected father. In X-linked recessive conditions, a son born to an unaffected father and a carrier mother has a 50% chance of inheriting the mother's X chromosome carrying the mutant allele and presenting with the condition. A daughter on the other hand has a 50% chance of being a carrier, however a fraction of carriers may display a milder (or even full) form of the condition due to their body's normal X-inactivation process preferably inactivating a certain parent's X chromosome (the father's in this case), a phenomenon known as skewed X-inactivation. If the condition is dominant, or if the father is also affected, the daughter has a 50% chance of being affected, with an additional 50% chance of being a carrier in the second case. A son born to an affected father and a non-carrier mother will always be unaffected due to not inheriting the father's X chromosome. A daughter on the other hand will always be a carrier (some of which may present with symptoms due to aforementioned skewed X-inactivation), unless the condition is dominant, in which case she will always be affected. There are a few Y-linked traits; these are inherited by sons from their father and are always expressed. The incidence of X-linked recessive conditions in females is the square of that in males: for example, 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 inheritance patterns are different in animals which use different sex-determination systems. In the ZW sex-determination system used by birds, the mammalian pattern is reversed, since the male is the homogametic sex (ZZ) and the female is heterogametic (ZW). In classical genetics, a mating experiment called a reciprocal cross is performed to test if an animal's trait is sex-linked.