Boys vs. Girls: Sex-Linked Inheritance
The inheritance of almost all our genes is exactly the same for males and females. When it comes to the genes on the sex chromosomes, though, sometimes one sex has an advantage over the other. That’s because females get two X chromosomes, while males get an X and a Y. The X chromosome is a long chromosome with lots of genes on it, while the Y chromosome is very short and has far fewer genes.
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.