What is Sex linkage?
'''Sex linkage''' is a term in [[genetics]]. It applies to [[allele]]s (forms of a [[gene]]) on the [[X chromosome]]. In [[mammal]]s, which have an XX/XY system of [[sex determination]], the [[X chromosome]] has many more genes than the [[Y chromosome]]. This explains the [[Heredity|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.<ref>Because the [[frequency]] of recessive X-linked [[phenotype]]s 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.</ref> However, if the [[Mutation|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.<ref>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, p167{{ndash}}194.</ref> === Sex-influenced traits === Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body.<ref name=King>King R.C; Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics''. 7th ed, Oxford University Press. {{ISBN|0-19-530761-5}}</ref> 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 g