There are over 1,000 X-linked genes, including the genes for red-green color blindness, hemophilia, male pattern baldness, and body fat distribution. And if you remember your high school genetics, males have one “X” and one “Y” chromosome, and females have two “X” chromosomes.
Since females have two “X” chromosomes, only one of the X chromosomes will be expressed in any given cell. The determination of which one is expressed is random. Tortoiseshell and calico cats (all females) are the perfect example- they have a gene for orange fur on one of their X chromosomes, and a gene for black fur on the other. Their random coat patterns are due to the random expression of X-chromosomes; areas where the fur is black express the X-chromosome with the black fur gene, and areas where the fur is orange express the X-chromosome with the orange fur gene.
OK, so the cats look pretty awesome. But it doesn’t stop there. Researchers are working with calico cats to try to understand how X-chromosomes are inactivated, in an attempt to figure out a way to turn certain genes on or off in a way that isn’t random. How cool would it be if genes linked to obesity or other diseases could be selectively silenced without altering a person’s DNA? Or if X-chromosome linked disorders could be silenced in a way that they wouldn’t be passed down to our offspring?