In mammals, sex is determined by a pair of chromosomes that have the same appearance in the female (XX) and differ in appearance in the male (XY). These are the sex chromosomes, and many animals have sex determined by a similar system: eg, in the silk moth "male" is ZZ and "female" is ZW.
For the mammalian X/Y system, these chromosomes pair during meiosis, but due to differing shapes, there is a segment which is non-homologous and so does not pair. It is this segment that leads to X or Y linkage.
An example of a human sex linked characteristic is haemopilia, a form of excessive bleeding caused by reduced clotting of blood. This disorder is linked to the X chromosome, thus it occurs on the differential segment of the X chromosome. It is also a recessive trait. Below are shown the nuclei of several individuals showing only the sex chromosomes, and displaying all the genotypes which could be expected in a diploid adult.
- display the disorder, as they are homozygous for the recessive allele,or
- be carriers of the trait (ie. possess the "h" allele without displaying haemophilia) because they possess two X chromosomes one of which has a compensating "H" allele, or
- not display or carry the disorder, as they are homozygous dominant.
During meiosis, we know that homologous chromosomes are divided into haploid gametes. Thus males produce gametes with either the X or the Y chromosome present. Female gametes possess only the X chromosome. This means that male offspring must inherit the Y chromosome from the male parent. It is for this reason that an X-linked chararcteristic cannot be transmitted from father to son.
It is common for an X-linked characteristic to skip generations, usually remaining in the family in a carrier (heterozygous individual).
The next generation, where the carrier is crossed with a normal male, faces the possibility of producing male offspring which display the trait.
Knox et al., pages 161-165.
Lecture notes and Web lecture notes for Lecture 32.