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Exceptions to Independent Assortment: Sex Linked and Sex Limited Traits

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  1. 0:53 Sex Determination
  2. 2:42 Carriers and Disadvantageous Genes
  3. 4:55 Sex-Linked Traits
  4. 7:20 Sex-Limited Traits
  5. 8:23 Lesson Summary
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Taught by

Greg Chin

More men are color blind compared women. But often, not every brother, cousin or uncle in a family tree is color blind. Why not? How can genetics explain this seemingly complex inheritance pattern?

So as Adrian has been performing his flying hamster crosses, he's noticed that some of the hamsters have horns, and unfortunately, they keep headbutting him when he's not looking. Finally, he gets fed up with it and he decides he's going to create an experimental strain without any horns. But as he's looking through the hamsters, he realizes that all the males in all of his strains - it doesn't matter which phenotype he was studying - have horns and none of the females have horns.

In his previous experiments with coat color and ear size, all of the genotypes and phenotypes in those cases were evenly distributed among the sexes. Now he's wondering what could be going on with the horn thing.

Sex Determination

Adrian is puzzled so he walks down the hall again to have a discussion with his friend, Ben, to try to figure out what could be going on with this horn gene. Ben tells Adrian that you really have to be careful about exceptions to scientific rules and that, as he pointed out earlier, that there are lots of exceptions to Mendel's laws.

Ben explains that sex is determined in many organisms by sex chromosomes. The genotype of the sex chromosomes determines the sex of that individual. How the sex chromosomes determine the sex of an organism varies from species to species.

The sex of humans is determined by the presence or absence of a Y chromosome
Y Chromosome Determines Sex

In humans, the presence or absence of the Y chromosome determines the sex of the individual. For instance, an XY individual is male and an XX individual is female.

If we draw out a Punnett square, we can actually see how the presence or absence of the X chromosome is determining the sexes. If we have XX (mother) and XY (father), we can see that we're going to get 50% XX individuals, which will be female, and 50% XY individuals that will be male.

It's interesting to note that the father is the one that determines the sex of the children. The only sex chromosomes that the mother can contribute are X's so whether or not the child gets an X or Y from the father is what determines the sex of the child.

And it would probably have helped out Anne Boleyn a little bit if they had known a lot more about genetics back in her time so she could have avoided her husband Henry VIII's wrath that she wasn't producing a male heir to the English throne.

Carriers and Disadvantageous Genes

Since males only have one X chromosome, the sex chromosomes present a unique situation compared to the rest of the chromosomes in the genome. The chromosomes in the genome that are not sex chromosomes are called autosomes, and up until now, we've only been considering genes that are on the autosomes.

In the case of the autosomes, there are always two copies of the gene - one on each of the two homologs. And since there are two copies, a dominant allele can mask the phenotype of a recessive allele. This is a major reason why people can be carriers for disadvantageous genes.

Consider a disease like sickle cell anemia. It's caused by the presence of two recessive alleles. In this disease, red blood cells can become sickle-shaped and lodge in small blood vessels. So you can imagine this produces a whole host of problems for someone that's suffering from this disease. Let's examine the genetics of this disease a little bit.

Let's represent the disease allele with a lowercase 'sca' since it's the recessive allele. Let's represent the 'normal' gene, or the non-disease gene, with an uppercase 'SCA' because it's the dominant allele. If we have an individual that's 'sca'/'sca', that individual is going to exhibit the disease. In contrast, if I have 'SCA'/'SCA', that individual does not exhibit the disease phenotype because the 'SCA' allele is the non-disease allele.

Interestingly though, the individual that is heterozygous for the two alleles - 'SCA'/'sca' - also does not exhibit the disease phenotype because the dominant allele can mask the phenotype of the recessive allele. We refer to this person as a carrier because this person does not exhibit a phenotype but carries the ability to pass on that phenotype to his or her progeny.

Let's now try to apply this concept of recessive disadvantageous traits to the sex chromosomes of humans.

Sex-Linked Traits

Since there are two X chromosomes in a female, a disadvantageous recessive phenotype can be masked in a heterozygote. This means that the recessive traits on the X chromosome are less likely to be observed in a female.

However, males only have one X chromosome, so that means that whatever is on the X chromosome is going to get expressed because it's the only allele that can be expressed. That means that males have a higher chance of displaying a recessive X-linked trait because there is no way to mask the recessive phenotype.

A recessive gene that is located on the X chromosome produces a sex-linked trait because a recessive trait will preferentially be found in males compared to females.

A classic example of a sex-linked trait is color blindness. The color blindness gene is located on the X chromosome, so men are more likely to be color blind than women.

Punnett square for the colorblindness example
Punnett Square for Colorblindness

Now to understand how this is working a little better, let's return to our Punnett squares. If we represent the color blindness allele as 'cb' and the normal allele as 'CB', we can track the genotypes and phenotypes in our Punnett square. Let's consider the children of a woman who is heterozygous for the color blind gene and a man who isn't color blind.

How many girls are going to be color blind? If we look at our Punnett square, we can see that both of these girls have at least one big 'CB' allele. So none of the daughters are going to be color blind because each of them has at least one good color blindness gene. How many of the boys will be color blind though? Well, half of the sons are going to be color blind because half of them are going to receive the recessive 'cb' allele from their mother. So notice that the children of this carrier mother had the same chance of getting the color blindness allele; however, since girls have two X chromosomes, a second good copy of the color blindness gene can mask that phenotype. Since the sons have only one X chromosome, half of the sons end up exhibiting the color blindness phenotype.

Sex-Limited Traits

So Adrian rushes back to the lab now with new perceptive on chromosomal-based gene inheritance. Since he never sees females with antlers, he speculates that maybe this is similar, but not quite the same as the color blindness example that Ben was telling him about.

Since only males have horns, he hypothesizes that maybe the gene is on the Y chromosome. If the gene was only on the Y chromosome, that could explain why only males exhibit the trait. Note the in the case of a sex-linked trait, like color blindness, it is linked to the males but it's not exclusive to the males. In this case we have a trait that is only expressed in one of the sexes; this is a sex-limited trait. Note though that this is an overly-simplistic example where the sex-limited gene is on the Y chromosome, but sex-limited genes are usually found on an autosome, they're just only expressed in one of the two sexes.

Lesson Summary

In summary, we've learned that sex chromosomes are the chromosomes which determine the sex of the organism. Autosomes are the chromosomes in the genome which are not sex chromosomes. A carrier is an individual heterozygous for a recessive phenotype. A sex-linked trait is the phenotype produced by a recessive gene that is located on the X chromosome. A sex limited trait is a trait which is only expressed in one of the sexes.

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