Write a brief note on sex-linkage.
Answers
Sex linkage is the phenotypic expression of an allele related to the allosome (sex chromosome) of the individual. In autosomal chromosomes both sexes have the same probability of existing (see Fisher's principle), but since humans have many more genes on the female X chromosome than on the male Y chromosome, these are much more common than Y-linked traits.
In mammals, the female is homogametic, with two X chromosomes (XX), while the male is the heterogametic sex, with one X and one Y chromosome (XY). Genes on the X or Y chromosome are called sex-linked. In ZW sex-determination system used by birds the opposite is true: the male is the homogametic sex (ZZ), and the female is heterogametic (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 haemophilia, which is much more common in males than females because males are hemizygous 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.
The incidence of recessive X-linked phenotypes 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 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. Each daughter born to a affected father and a non-carrier mother will be a carrier. There are a few Y-linked traits; these are inherited by sons from their father.
Sex linkage
The phenomenon of inheritance of sex-linked genes is called sex linkage.