X-linked Recessive Inheritance

by Georgina Cornwall, PhD

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    00:00 Here’s a question. Just to make it a little bit more interesting as we dig a little deeper. If a grandfather had hemophilia and the grandmother wasn’t a carrier or anything, could that hemophilia appear in descendants of his sons? Try to wrap your head around that. I know it looks like one of those exam questions, word problems and such. But let’s take a look at it. First of all, draw yourself out upon its square. Let’s put the grandfather across the top and the grandmother down the side.

    00:45 We call the grandfather as affected. So he’s X, little h, and a Y. The grandmother is not affected and not a carrier. So ,she has X big H, X big H. Go ahead and take a moment to fill it out.

    01:01 Let’s see what we come up with. See if you can answer the question. Would his grandsons be affected? Alright, so as we fill this out, we noticed that all of his sons would not even be carrying the recessive allele.

    01:20 Thus, it’s not possible even for him to pass on the disorder to any of his male progeny or grandsons.

    01:31 Of course, the grandsons could perhaps pick up an allele elsewhere and pass it on in future generations.

    01:38 But in his grandsons, we’re not going to see it travel through the paternal line. It has to go through a female carrier. We just examined this cross. You can see clearly that all females will be carriers and all males are unaffected. What about a different case? Now, we have a male parent that is not a carrier or not affected and we have a female that is a carrier. We can see that we have a one-to-one-to-one ratio. I challenge you now before we reveal what the wording is all the way over on the right-hand side. Take a moment to see if you can name these different phenotypes.

    02:30 For example, what is the first one in the top left-hand corner? We’ll start with that as a noncarrier female and you take it from there. Now, that you’ve taken a moment with that, we see that we have a 25% chance of having a carrier female. We have a 25% chance of having a normal male.

    02:57 We also have a 25% chance of having an affected male. If you got those right, you’ve got the verbiage down. Now, you have X-linked inheritance or recessive inheritance covered. How then, let’s consider how a female could be affected by X-linked recessive disorder. We clearly just covered one of them, so we’ll let that one go right away. First of all, they could be homozygous.

    03:33 However, being homozygous for a trait that’s recessive is very, very unlikely. So we would have to be generally compound heterozygote. The two different mutant alleles in the same sort of locus came from different origin. They’re slightly different mutations. That is the mutation for hemophilia A or whatever it is has probably arisen multiple times and those two come together. To be truly homozygous, most likely it’s consanguinity. Now, here’s another situation to complicate things a bit. Let’s say you have a female who has two X’s. Some of the X’s or one X is going to be made into a Barr body.

    04:28 What if, let’s take a look at the upper image here, early in the development the split is fairly equal which we would expect, 50/50. Some have this X. Some have that X. As cells divide and proliferate, we’ll see basically a 50/50 balance. However, on occasion, we’ll see that an individual does not have an equal spread because maybe the allele wasn’t favorable for cell division, the mutant allele.

    05:02 So, we see a skewed X inactivation because the progeny of one cell was more successful with one copy of the X than it was with the other copy. Our cells somehow magically, we don’t really know yet, are capable of doing this sort of checks and balances to make sure things are okay.

    05:23 We can have a manifesting heterozygote resulting from unbalanced expression.

    05:31 We might see, again, variance in how dominant or recessive looks.

    About the Lecture

    The lecture X-linked Recessive Inheritance by Georgina Cornwall, PhD is from the course Single-Gene Disorders.

    Author of lecture X-linked Recessive Inheritance

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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