Back To CourseLife Science: Middle School
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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.
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.
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.
As a female, though, in order to have white fingernails both your mother and father would need to pass down recessive alleles. This is because it only takes one dominant allele to 'hide' that recessive allele. So your mother would either need to be heterozygous (and pass on the recessive allele instead of the dominant one) or homozygous recessive, having two recessive alleles. Your father would also need to have that recessive allele on his one X chromosome so that you end up with two recessive alleles and plain white fingernails.
As you can see, males are more likely to be affected by the alleles passed down on sex chromosomes because they only get one X. So, as a male, if you inherit a recessive trait there's no chance of the dominant trait taking over because it's just not there to do so.
This means sex-linked disorders are more common in males than females. This includes disorders such as hemophilia, red-green colorblindness, and Duchenne muscular dystrophy. Because these are sex-linked and recessive, they are less likely to 'show' in females: if they inherit a dominant allele, the recessive one will stay 'hidden.' A female would have to inherit two recessive alleles in order for the disorder to be prevalent.
Sex-linked genes are important because they are the chromosomes that make us either male or female. But they also carry a lot of other important genetic information that has nothing to do with maleness or femaleness.
However, the genetic information carried on these chromosomes works a little differently in terms of inheritance. Males get their X chromosome from Mom and their Y chromosome from Dad, while females get an X chromosome from each parent. On these chromosomes are alleles, or gene variations, which determine certain genetic traits. These could be as simple as eye color in fruit flies or as complex as blood-clotting in humans.
X chromosomes carry far more genes than Y chromosomes, and males are more likely to be affected by sex-linked disorders because they only have that one X. If a disorder such as hemophilia is recessive and sex-linked, and the male inherits this recessive allele on his X chromosome, he has no chance of a dominant X chromosome allele 'covering' it up. Females, on the other hand, may inherit one recessive allele from either parent, but if they also inherit a dominant allele then it essentially 'hides' that recessive allele and the disorder associated with it.
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Back To CourseLife Science: Middle School
35 chapters | 241 lessons