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Sex-Linked Genes

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Sex linked genes are carried on the X and Y chromosomes, also known as the sex chromosomes. If a gene is carried on the Y chromosome, only men can inherit it. Recessive genes carried on the X chromosome are always expressed for men, who only have one X chromosome, but must be homozygous to be expressed in women.

One of the most common complications to introduce to Mendel's laws is the idea of sex linked traits. Now sex linked genes are those genes that are found on the x chromosome. Now that's a significant difference from the standard way of doing genetics which is talking about the autosome or chromosomes because unlike the autosomes, the other non-sex chromosomes, males only get one of those x chromosomes. Some common examples of tricks that are used during tests or in class that are sex linked are hemophilia and color blindness. Now because they're on the x chromosome, when you write them instead of putting just the letter big H for having normal blood clotting and little h for having hemophilia, you actually put the x to represent the x chromosome and typically as a super script floating up high like an exponent. You put the capital H for normal or the lower case version for hemophilia.

Now males again only get one x chromosome. So you can't say they're homizygous or heterozygous or anything like that because they don't have two of the same or two different things. Instead, some people will call that hemizygous because it has half the normal number of chromosomes and hemi means half. Males sorry females will actually get two x chromosomes. So they can be homozygous dominant with x big h x big h or heterozygous x big h x little h, or homozygous recessive x little h x little h.

Now what would some of the problems that would involve sex linked traits look like? Here's a standard kind of question. A normal man marries a woman who is a carrier for hemophilia. Predict their offspring. So the first thing you have to do is figure out alright, a normal man. What does that mean? Well, that means that he does not have the hemophilia allele. So he has the normal blood clotting allele.

The woman, she is normal but she's a carrier. So that means she has one normal and one hemophilia allele. So she is heterozygous for this trait. So now that I've got their genoptypes figured out, let me plug in them in to this punnett square here. So I put the guy's genes up here. Hers right there. And now it's time for them to have some babies.

So let me put this daughter here is xx and gets a big h here and a big h there from mummy and daddy. x from mummy with a big h and daddy's daddy's y. What does this mean? Son. xx we have another daughter. Little h from mummy and big h from daddy. Remember you always put the dominant allele first. It doesn't matter who you get it from. You always put the dominant first. So we'll see that the daughters are both normal. Here mummy gives the x that she has. Daddy gives y that he has and that and now we have our two sons. And what you'll see with sex linked traits, the way to spot them is that you'll see an equal expression of the traits in male versus female. Here this daughter is normal. This daughter is normal even though she's a carrier, they both have normal blood clotting. So 100 percent of the females and I'll use the symbol for female are normal whereas this guy is normal. So 50 percent of the males are normal and this guy here however, he's a hemophiliac. 50 percent of the males are hemophiliacs. So that's the easy way to do it. So of course, some bio teachers like me like to sneak in stealthy little sex link examples. So let me show how that could happen.

In fruit flies white eyes are recessive to red. So we have a white eyed male fruit fly mating with a red eye female to produce 24 white males, 28 white females, 27 red males and 23 red females. Can this be sex linked? Well obviously based on this video it can. But if you just saw these normal results you might say, well, it's affecting the males and females the same. It's just somebody's homozygous recessive crossing with somebody who's heterozygous and if they gave you no other information you'd be right. That's the simplest answer and often the simplest answer is the right one. However, there is a way to make the sex link and usually what will happen is they'll talk about another generation. So let me show you how you could make this sex linked.

So, we have our white eyed male. I'm going to use because red is the dominant trait I'm going to use big r for red, little r for white. So he's little r. Now, she's red we know that. Now if she was homozygous dominant all of her offsprong are going to be red because that's what homozygous dominant people do. So let's make her heterozygous. She's got this little one right there. So let's take a look.

So, this daughter here is big r little r. This daughter here is little r little r. This son here is big r, nothing, so red, and this son, sorry that's the son and this is the son, is x little r. So we have as we see predicted by our punnet square half of our female offspring have red eyes and we see that. Half of our male offspring have red eyes and we see that, half have white and half have white.

Now let's take this daughter here ooh fly incest and mate it with this son here and we'll see x big r, x little r, red female. x big r, x little r red female. 100 percent of our females are red. Oops. White male, white male. So we see ultimately in the second generation finally we see that unequal distribution of the effects in the genders that equals sex linked traits.