Sex linkage
The sex chromosome Genetic structures that usually occur in functional pairs in the nucleus of cells (except gametes and bacteria)., although mainly responsible for determining sex, carry some geneThe basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. that code for a number of body functions.
These genes are described as sex-linked.
The other 22 pairs of chromosomes have the same genes in the same position on both chromosomes.
The difference in size between the X chromosome and the shorter Y chromosome means some genes appear only on the X chromosome.
The effect of sex-linked genes is more obvious in males because the X chromosome can contain genes the Y does not.
A recessive An allele that will only show in the phenotype if there is no dominant allele present (i.e. when two recessive alleles are present). allele on the X chromosome will show in the phenotypeThe visible characteristics of an organism which occur as a result of its genes. as there is no dominant allele present to override it.
A female will have another allele on her other X chromosome which, if dominant An allele that will override a recessive allele., would mask the effect of a recessive allele.