Sex-Linked Traits on the X and Y Chromosomes

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  • 0:01 Sex Chromosomes Carry…
  • 1:13 How Sex-Linked Genes Work
  • 4:01 Sex-Linked Disorders
  • 4:43 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The sex chromosome you inherit from your parents determines whether you are male or female. But many genes are carried on these chromosomes as well, and the inheritance of this genetic information works a bit differently.

Sex Chromosomes Carry Many Genes

I have a twin brother, and people often ask me if we're 'identical,' which when you think about it, isn't possible. Even if we looked, talked, and acted exactly the same, we couldn't be identical twins because identical twins have the same DNA.

And no matter how similar we may look on the outside, there's a major difference between us on the inside: our sex chromosomes. These are the chromosomes that make us either male or female. Because he's male, he has an X and a Y chromosome, and because I'm female, I have two X chromosomes. That may seem like a small difference on paper, but biologically it makes us very different indeed!

Sex chromosomes do more than this, though. They carry a lot of information that has nothing to do with biological sexual identity. For example, eye color in fruit flies is determined by genes carried on the sex chromosomes. These genes that are carried on sex chromosomes are called sex-linked genes, because they are 'linked' to these chromosomes.

Don't confuse these with linked genes, though, because these are different: they're genes that are inherited together because they are on the same chromosome. So think of one as being linked to the chromosome itself, while the other is linked to other genes.

How Sex-Linked Genes Work

Sex-linked genes are interesting because they work a bit differently in terms of genetic inheritance. And because X chromosomes carry more genetic information than Y chromosomes, when we talk about sex-linked genes we're usually referring to the genes on the Xs.

Females have their two X chromosomes, one of which they get from Mom and the other of which they get from Dad. They have a 50-50 chance of getting either of Mom's two X chromosomes, but Dad only has one that he can pass on. Males, on the other hand, have that same 50-50 chance of getting either of Mom's X chromosomes, but will get Dad's Y chromosome instead of his X.

Let's look at an example to see how this type of inheritance pattern works. Let's say that purple fingernails are a sex-linked trait. Those with the dominant allele, which is just a gene variation, have beautiful purple fingernails. We can represent this allele with a capital letter P, and show it as a superscript on the X chromosome letter like this: X^P. Those with the recessive allele, represented by the lower case letter p, have plain old white fingernails, and their chromosomes would look like this: X^p.

Because the allele is carried on the X chromosome, if you are a male and your mother is homozygous dominant, meaning that she has the dominant allele on both X chromosomes ('homo' means 'same'), you will also have the dominant allele on your one X chromosome (and have purple fingernails). We would write out your chromosomes like this: X^PY.

If you are a female, you will also have purple fingernails because no matter what your father gives you, that dominant allele from your mother is, well, dominant! If your father had the dominant trait, your chromosomes would look like this: X^PX^P; if he had the recessive trait, your chromosomes would instead look like this: X^PX^p. Either way, your fingernails are as purple as can be.

But let's say that you are male and that your mother is heterozygous, meaning that she has both a dominant and a recessive allele, one on each chromosome ('hetero' means 'different'). You now have a 50-50 chance of inheriting the recessive allele and ending up with white fingernails since that would be your only X chromosome and the only allele for this trait. If you did inherit the recessive allele, your chromosomes would look like this: X^pY.

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