X-linked traits are traits that are passed on from parents to offspring on the X chromosome (the chromosome related to gender). A common X-linked trait is colorblindness. The genes needed to distinguish red from green are on the X-chromosome. A female with one defective and one normal X-chromosome has normal vision. However, a male with a defective color vision gene on his X-chromosome, is colorblind. There are no genes for normal color vision on the Y-chromosome to cover for the defective X-chromosome. One way of tracing a trait through generations is a pedigree chart. Here is a colorblindness pedigree chart.
A horizontal line joining a circle and square means that the couple had children.