Eli5: why do woman get less of some mutation like color blindness?


If one X chromosome is healthy and one is mutated, why does the healthy take control?

In: 8

Many of the issues are actually on the Y chromosome. No Y, no problem. This is not always why. But it is sometimes the answer.

When it comes to X-linked disorders, it helps to think of the faulty/ mutated gene as recessive to a healthy X. So for women, since they have 2 Xs, they won’t get colorblindness unless they get saddled with 2 bad copies. And since males only have a single X, without a backup X, colorblindness is easier to get.

Sometimes you only need one good gene out of two. If you have even one gene for brown eyes, you get brown eyes regardless of the other gene. So if you wanted to not have brown eyes, you’d need the gene for brown eyes to be “missing” on both chromosomes.

If the gene is on the long part of the X chromosome, then women get two shots at having the one gene they need, which can compensate for not having it on the other chromosomes. Men only get one chance, because the gene they would have gotten from their father doesn’t exist on the Y chromosome.

There’s something called dominant and recessive genes. A simplified case is eye color. Blue is recessive and brown is dominant. If you have two brown eye genes, your eyes are brown. If you have two blue eye genes, your eyes are blue. If you have one of each, your eyes are brown. This means two brown-eyed parents can have a blue-eyed child, assuming each has a recessive blue eye gene and they both pass it to the child. But two blue-eyed parents can never have a brown-eyed child as they each have to have two blue eye genes and therefore always pass one each to the child. (Reality has eyes a little more complicated than this especially as there are other colors like green and hazel, but this is the basic idea.)

Colorblindness is a recessive gene, but there’s an interesting foible: it’s on a sex gene, specifically the X, and the Y gene does not have a corresponding dominant gene. So a woman can only have colorblindness if she gets a colorblindness gene from each parent, while a man will be colorblind if he gets a single colorblindness gene from his mother. And it has to specifically be from the mother, as he’s always getting the Y from his father.

If a man is colorblind, he will always pass along that X gene to any daughters he has, but never to his sons. If a woman is colorblind, she will always pass along a colorblind gene to any children she has, which means 100% of her sons will be colorblind, regardless of whether the father is, but her daughters will be colorblind only if the father also was. If a woman is a “carrier” for colorblindness (one colorblind X and one non-colorblind X), it’s 50/50 to pass along the colorblind X to each child, meaning half of her sons on average would be colorblind, and her daughters would be colorblind half the time if and only if the father was also colorblind and carriers the other half, and would be carriers half the time if he wasn’t and fully non-colorblind the other half.

The X chromosome has genes for all kinds of things, including things that you wouldn’t think were related to sex.

You might know that Queen Victoria was a hemophilia carrier. The gene that tells the body how to make the stuff that gets your blood to clot is on the X chromosome. If you have at least one copy of that gene, your body can make that stuff, and you don’t have hemophilia.

Queen Victoria had two X chromosomes, one with a normal copy of the gene, one with a mutated copy, so she didn’t have hemophilia. When her egg cells formed, they got one of her two X chromosomes. Any of her daughters would have gotten another X chromosome as well, from Albert. They might get a normal or defective X chromosome from her, but they also had a normal one from him,so they didn’t have hemophilia. Her sons got one X chromosome from her and a Y chromosome from him. They had a 50% chance of getting the good X chromosome, and a 50% chance of getting the bad one. If they inherited her defective X chromosome, they didn’t have a backup copy of the gene, so their bodies couldn’t produce the stuff that gets blood to clot.

Victoria’s daughter Alexandra got the bad X chromosome from her mother, then passed it on to her son, the Tsarevich Alexei of Russia, who had hemophilia. Then Rasputin claimed to be able to help Alexei. He gained Alexandra’s trust, which was unpopular with the Russian people, and it was a factor in the revolution.

Note: this applies to organisms that use the XY system of determining sex. There are other systems as well. Birds use the ZW system, where male birds get two Z chromosomes and females get one Z and one W. In those species, the females would be more likely to get genetic disorders than the males.