Sex linkage

Sex linkage is a term in genetics. It applies to alleles (forms of a gene) on the X chromosome.

In mammals, which have an XX/XY system of sex determination, the X chromosome has many more genes than the Y chromosome. This explains the inheritance of such conditions as colour blindness and haemophilia. They are caused by recessive alleles, and usually do not show the condition in females. This is because those females carrying the mutant are most likely to be heterozygous for the allele.Because the frequency of recessive X-linked phenotypes in females is the square of that in males. If 1 in 20 males in a human population are green colour blind, then 1 in 400 females in the population are expected to be colour blind (1/20)*(1/20). Since the mutant alleles are recessive, it takes two 'doses' to make the effect show in the females. However, if the mutant allele is carried by a male, the condition will develop, because his X chromosome has the mutant allele, but his Y chromosome has no allele at that locus.

 Related terms 
Sex-linked characters controlled by genes on sex chromosomes are not the only kind of sex-related characters.Zirkle, Conrad 1946. The discovery of sex-influenced, sex limited and sex-linked heredity. In Ashley Montagu M.F. (ed) Studies in the history of science and learning offered in homage to George Sarton on the occasion of his sixtieth birthday. New York: Schuman, p167194.

 Sex-influenced traits 
Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body.King R.C; Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press.  Even in a homozygous female the condition may not be expressed fully. Example: baldness in humans.

 Sex-limited traits 
These are characters only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes. Examples: female sterility in Drosophila; and polymorphism in insects, especially in relation to mimicry. Closely linked genes on autosomes called "supergenes" are often responsible for the latter.Ford E.B. 1965. Genetic polymorphism. p17-25. MIT Press 1965.

 Notes 

Category:Chromosomes