The XY chromosome pair determines sex--XX is a female and XY is a male. However, the X chromosome is slightly longer than the Y and contains more genes along its length. Therefore, there are traits known as sex-linked and most of these are found along the X chromosome. An example is red-green colorblindness and another is Hemophilia(bleeder's disease). Since males only have one X chromosome which is passed down from mother to son and if they get an X with the disease gene, then they will express a disease phenotype. Since females have two X chromosomes, if one is normal and the other contains the recessive gene for the disease, they will be carriers and express a normal phenotype. There are also approximately 3 dozen Y-linked genes. These only affect males and relate to male sexual characteristics as well as infertility. These are only passed from father to son.