MoBio Sex Chromosomes Chapter 1

Sex chromosomes determine the sex of an organism. A human somatic cell has two sex chromosomes: XY in male and XX in female. A human germ cell has one sex chromosome: X or Y in a sperm and X in an egg. When an X-sperm is combined with an egg, the resulting zygote (fertilized egg) will contain two X chromosomes. A person developed from the XX-zygote will have the characteristics of a female. Combination of a Y-sperm and an egg will produce a male.


Figure 1-C-4. Sex determination.

The SRY Gene

Normally, a woman has two X chromosomes (XX) and a man has one X and one Y (XY). However, both male and female characteristics can sometimes be found in one individual, and it is possible to have XY women and XX men. Analysis of such individuals has revealed some of the genes involved in sex determination, including one called sex-determining region Y gene (SRY), which is important for testis formation.

Mutations in the SRY gene give rise to XY women with gonadal dysgenesis (Swyer syndrome); translocation of part of the Y chromosome containing this gene to the X chromosome causes XX male syndrome.