In mammals and many insects there are one pair of chromosomes that are not identical, the sex chromosomes (X, Y). Females have two identical XX chromosomes and males have an X and a Y. All the other chromosomes are identical pairs and are called autosomes.

Genes that are one sex chromosome but not the other are said to be sex linked, or X-linked.

The Y chromosome carries relatively few genes other than those that determine maleness. The X chromosome carries many genes that have nothing to do with femaleness. For example:

color blindness, blood clotting, some structural proteins in muscles.

Most cases of color blindness, blood clotting, and certain types of muscular dystrophy occur in males! Why?

E.g. Eye Color in Fruit Flies

Drosophilia "Fruit Fly" Red eyes are the normal phenotype Sometimes get males with white eyes	F1 are all red eyed. Xw+ means has red gene on X Xw means no red gene on X
F2 1/2 are all males are white eyed.

Sex-linked traits are far more frequent in males and typically skip generations. Two good Human examples are red-green color blindness and Hemophilia.

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Key terms:

Sex Chromosomes: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexChromosomes.html