If a gene for a trait is on one of the sex chromosomes, the trait is called a sex-linked trait.
Because females get two X chromosomes, they get two copies of every gene that’s located on the X chromosome, whereas males only get one. The genes on the X chromosome aren’t about being female — they’re genes for important traits, including some that affect muscle and nerve development, vision, and blood clotting.If a girl inherits a defective copy of one of these genes from one of her parents, she can still have normal physiology if she gets a normal copy of the gene from her other parent. But boys don’t have that extra insurance. If they get a defective copy of a gene on their X chromosome, they may show abnormal physiology. When you look at the inheritance of these traits in families, you see that they show up far more often in males than in females. A famous example is the disease hemophilia, which spread through the royal families of Europe. Another example is red-green color blindness.
If a gene for a trait is on the X chromosome, the trait is called an X-linked trait. Because males only get one X chromosome, they’re more likely to show abnormalities in these traits.
So far, scientists have only identified about three dozen genes on the Y chromosome. The traits pass from fathers to sons. The most famous gene on the Y chromosome is the one that turns on maleness. This gene activates in a male fetus at about six weeks of life. It makes a protein that acts as a master switch, turning on many genes on other chromosomes that all work together to build the male reproductive system and male characteristics. Without a Y chromosome, and without this gene, a fetus develops into a female.If a gene for a trait is on the Y chromosome, the trait is called a Y-linked trait. Y-linked traits pass from fathers to their sons and include the gene that turns on male development in humans.
