ShmoopTube

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And here's your Shmoop du jour, brought to you by hemophiliacs. [red blood cells travelling quickly]

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They're a never-ending buffet for vampires everywhere. [man in a hospital bed and vampire appears]

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Okay, here's our question…

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Hemophilia is an X-linked, recessive trait. A woman who is XHXh has

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a son with a father who is XHY What is the probability that their son will

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have hemophilia? And here are our potential answers…[mumbling]

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Okay, let's break it down… Hemophilia is an X-linked, recessive trait.

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…What does that mean? Unfortunately, it has nothing to do with the [Two men dressed in X-men costumes]

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X-Men. Though that sounds like a good idea for the next sequel…X-Men: Recessive Trait. [Girl watching X-Men: Recessive Trait in an empty cinema]

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We'd watch it...just us? Anyway, X-linked means that the gene encoding

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the trait is on the X chromosome. And remember that a recessive allele will

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not show its phenotype if it's paired with a dominant allele.

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So if hemophilia is X-linked and recessive, the picture looks a little more like this…

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But what makes a sex-linked gene special, compared to all of the other genes out there? [Chromosome and gene stood together]

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Well, besides the fact that all genes are special in their parents' eyes…

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Sex-linked genes are exactly what the name says: sex-linked.

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So when a mother makes her eggs, half of them will carry the diseased chromosome. [Woman with two chromosomes]

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Now let’s take look at the father. He can either pass down a dominant allele

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linked to the X chromosome, which would guarantee a healthy female offspring…

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Or he could pass down a Y chromosome, guaranteeing a male offspring. [Man passing down a Y chromosome]

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But since the hemophilia gene is on the X chromosome, a male offspring only has one [Boy opening a wrapped gift]

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chance to get it right? And that chance comes from…?

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Bingo. Mommy dearest. So the chances he’ll get the recessive gene

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from his mom are… B, fifty percent.

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That’s what makes sex-linked genes special. Because male offspring are haploid for the [Parents stood with their son]

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X chromosome, all traits on the X chromosome, which come from the mother, are expressed. [Woman with an X chromosome]

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Because of this, most X-linked disorders are more common in males than females.

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Which could also explain why more of the X-Men are male than female but...don't quote us [X-Men characters in a park]

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on that.

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