Solving Genetics Problems III: Sex Linkage


Some special operations are necessary when solving a problem about a gene which is carried by the X or Y chromosome. These genes are called sex linked. A gene on the X chromosome is X linked; one on the Y chromosome is Y linked.

Because males only receive one X chromosome, the pattern of inheritance of X linked genes is different from that of autosomal genes. For one thing, since males have only one X chromosome, they have only one allele for any X linked gene. For example, red-green colorblindness is caused by an X linked recessive allele. If a male receives one colorblindness allele, he will be colorblind, since he has no second X chromosome to provide a dominant allele to mask it. A female won't be colorblind unless she inherits two copies of the colorblind allele-one on the X she gets from her mother, and the other on the X she gets from her father. This is why X linked recessive traits like colorblindness and hemophilia are so much more common in males than in females. The male only has to get "hit" once to express the trait; the female has to get "hit" twice.

Another oddity about the inheritance of X linked traits is that baby boys always inherit their X linked traits from their mothers. Since a male baby gets his Y chromosome from his father, he must get his X chromosome from his mother. So all X linked traits in a male are inherited from his mother. Our colorblind male above got the allele from his mom.

Y linked traits are the easiest of traits to understand. It's just too bad that there aren't more of them. Since Y chromosomes are always passed from father to son, and never from father to daughter or mother to either son or daughter, and since the father only has one Y, everything on his Y chromosome goes to his sons and nothing on his Y goes to his daughters. This is rather a pointless observation, at least in humans, since the only gene we know of on the human Y chromosome is the male determining gene.

Unlike problems involving unusual dominance relationships or epistasis, sex linkage has to be monitored all the way through the problem. It's a good idea to use a kind of notation that reminds you at every step that you are dealing with a gene carried on the X chromosome. Actually, it's more accurate to say that you must keep track of the Y chromosome at every step.

Here's an example:

Remember John and Elizabeth? They both have normal color vision. Elvis, their blue-eyed baby boy, is colorblind. If John and Elizabeth go on to have several more children, what phenotypic ratio do you predict for this trait (colorblindness)?

First, here are our givens:

Even though this is a sex linkage problem, we still follow the same set of rules for solving it, and we still use those Five Rules to figure out genotypes.

In this case, we need to pay attention to that exception for Rule #1. Baby Elvis is colorblind. He's male, so he has only one X chromosome. That X came from Elizabeth, since the only place he could get a Y was from John. So Elizabeth is a heterozygote-she's hiding a recessive colorblindness allele. John, like Elizabeth, is not colorblind, but of course he can't be hiding anything, since he has only one X chromosome, and thus only one allele for this gene. Obviously, the one he's got is the dominant normal color vision allele.

So here is our mating:

Notice that John's genotype includes the Y. We don't want to lose track of that chromosome. Remember that each "C" is carried by an X, so these genotypes represent Elizabeth's XX and John's XY. We will treat the Y symbol just as we would treat an allele.

Next step is to figure out the gametes:

Once again, keep track of that Y chromosome. 1/2 of John's sperm will have no X chromosome in them; they will have the Y instead. If you stop tracking the Y, all of John's and Elizabeth's children will be female, and while I don't have any problem with that in principle, it would be a real problem if it went on globally!

Now we can create our Punnett Square:

Next, the genotypic ratio. None of the interior boxes of the square have the same genotype, so the ratio is 1 CC : 1 Cc : 1 CY : 1 cY.

Now things get just a bit tricky. If we consider the phenotypic ratio simply in terms of the colorblindness trait, only one of our four genotypes produces colorblindness (cY). So it would seem that our phenotypic ratio should be 3 Normal Color Vision : 1 Colorblind. However, while this isn't strictly incorrect, it isn't correct either. Or rather, it is incomplete. Look again at the genotypic ratio. Note that every single colorblind child they have will be a boy. Even if they were to have a hundred children (!), there would be no colorblind girls. So the gender of the child is really significant in this phenotypic ratio. You should therefore include gender as part of the phenotype if you are doing a problem involving sex linkage. It won't always make a difference, but it often does. Our correct, complete phenotypic ratio is 2 Normal Vision Daughters : 1 Normal Vision Son : 1 Colorblind Son. Or you could simply express it: All daughters will have normal vision; sons will be 1 Normal Vision : 1 Colorblind.

Sex Influenced and Sex Limited Traits

This section is for your enlightenment only. There are some traits which are affected by gender, but whose genes are not on the X or Y chromosome; they are controlled by genes carried by autosomes. These fall into two categories: sex influenced and sex limited.

Sex influenced traits are expressed differently in males than they are in females. For instance, singing voice is sex influenced. The genotype which causes males to have deep bass voices is the same genotype that causes females to have high soprano voices. The genotype that causes males to have high tenor voices causes females to have deep contralto voices. This answers a question which has puzzled choir members since the invention of choral music: Why is it that true tenors and true contraltos are the rarest of all of the singing voices?

One rather notorious trait is often blamed on a sex linked gene, but is actually sex influenced. The allele for Pattern Baldness is highly affected by hormones. In the presence of high levels of testosterone, this allele has a very powerful effect, and behaves like a dominant allele. If testosterone levels are low, the same allele is very weak in its effect, and behaves like a recessive. So in males, whose testosterone levels are constantly high once they pass puberty, baldness is dominant; in females, whose testosterone levels are always low, baldness is recessive. Even a homozygous bald woman may only experience a bit of thinning on top, about in the area where many men develop a bald spot.

Sex limited traits, like sex influenced traits, are autosomal. They are not carried on the X or Y chromosome. However, these traits can only be expressed in one gender. They are often genes which affect the primary and secondary sexual features of the organism. For instance, genes which affect how much milk a mother will produce when she is nursing a baby are present in both genders, but are never expressed in the male, since males don't produce milk. Duh.


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Updated 25 September 2004