In Chapter 3, we learned several basic principles of heredity that Mendel discovered from his crosses among pea plants. A major extension of these Mendelian principles is the pattern of inheritance exhibited by sex-linked characteristics, characteristics determined by genes located on the sex chromosomes. Genes on the X chromosome determine X-linked characteristics; those on the Y chromosome determine Y-linked characteristics. Because the Y chromosome of many organisms contains little genetic information, most sex-linked characteristics are X linked. Males and females differ in their sex chromosomes, so the pattern of inheritance for sex-linked characteristics differs from that exhibited by genes located on autosomes.