Inside the X-(Chromosome) Files
In honor of Mother’s Day, let’s take a closer look at the “female” sex chromosome: The X.
Not to stir up a battle of the sexes, but the X chromosome (females have two of them, while males have one) is five times larger than the Y chromosome, and has 10 times the number of genes. That means it carries more traits — and diseases — than the Y chromosome.
The Y chromosome is important in determining a person’s biological gender. But it has much less of a say in someone’s genetic makeup, since the X chromosome likely has more than 1,000 genes whereas the Y chromosome has fewer than 80. And some scientists believe that over subsequent generations, the Y chromosome may lose even more genes.
A mother always passes an X chromosome onto her offspring, but a father can pass on either an X or a Y chromosome. If he passes on his X chromosome, their baby will be genetically female. If the father passes on his Y chromosome, though, their baby will be genetically male.
A person’s gender influences whether or not they develop certain disorders or conditions. While most disorders linked to the Y chromosome have to do with male development and fertility, a broader range of disorders are linked to the X chromosome. Genes on the X chromosome can be recessive or dominant, so X-linked recessive genes are expressed in females only if there is one on each of her X chromosomes. But because men don’t have two X chromosomes from which to draw traits, having just one copy of a recessive gene is sufficient for a male to express a recessive X-linked trait or disorder.
Here are some common X-linked traits and disorders:
Red-green color blindness is a recessive trait linked to the X chromosome. People who have it can see normally, but can’t distinguish shades of red and green. Among those with Northern European ancestry, it occurs in about 1 in 12 males but only 1 in 200 females.
Tetrachromatism is the one X-linked trait that is seemingly a superpower. Whereas people with normal color vision have three types of cone cells, each conferring the ability to differentiates approximately 100 shades, allowing us to perceive about a million colors, some people have four types of cone cells, which may increase the number of possible combinations to a staggering 100 million. Because the gene to distinguish color is present on the X chromosome, it’s been speculated that women could possibly have distinct versions of the gene that would give them more cones and hence tetrachromatism. Just a few women with this trait have been identified, including one who is, unsurprisingly, a painter.
Hemophilia A and B are blood disorders linked to the X chromosome. Hemophilia A is caused by a deficiency of a clotting factor called Factor VIII. As a result, people with the disorder may bleed heavily, without stopping, even from small wounds. They also bruise easily and are at risk of internal bleedings. About 1 in 4500 males have the disorder at birth. Hemophilia B, due to the deficiency of another clotting factor — Factor IX. About one in 20,000 males may have this disorder at birth. Some female carriers of the gene can show mild signs, such as bruising easily.
Klinefelter Syndrome occurs when a male receives one or more extra copies of the X chromosome. This extra material can prevent testes from functioning normally and can reduce testosterone levels, sometimes leading to delayed puberty, breast enlargement or infertility. Those with the disorder can also suffer from learning disabilities and delayed speech.
Triple X Syndrome occurs when a female receives an extra copy of the X chromosome. Females with the syndrome tend to be tall and have learning problems. Other possible symptoms include weak muscle tone and unusually curved pinky fingers.
Duchenne Muscular Dystrophy is a form of muscular dystrophy that occurs mostly in males — about 1 in every 3500 at birth. It is associated with a mutation of a gene on the X chromosome, the DMD gene, which encodes dystrophin, the muscle protein.
With new technologies that allow us to learn more about our genes every day, geneticists are bound to uncover new mysteries of X-linked traits.