In this video I describe some specific genetic disorders that are known to have negative consequences for intellectual development including Down Syndrome, Fragile X Syndrome, Williams Syndrome, and Phenylketonuria.
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Introduction to Williams Syndrome: https://www.youtube.com/watch?v=M6n4z…
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Video Transcript
Hi, I’m Michael Corayer and this is Psych Exam Review. In the previous video we looked at the heritability of intelligence and we saw that genes are an important part of the explanation for why it is that people have different IQ scores. So in this video I want to look at some specific examples of gene interactions that are known to influence cognitive ability.
Now you’ll notice that all of these examples involve impairment, so they’re associated with intellectual disability. That’s because we have a better understanding of impairments to intelligence than we do for why it is that some people have exceptionally high IQ, but maybe that’s something we’ll get to understand more clearly in the future. So we’ll start by looking at Down Syndrome. So Down Syndrome occurs in about one in 1000 births and it involves an extra copy of the 21st chromosome. So this is called trisomy. So the person has three of the 21st chromosome rather than the usual pair that most people would have.
So we can say that Down syndrome is genetic and it’s associated with intellectual disability but when we say it’s genetic it’s not inherited. And so this is an important distinction because we say it’s genetic because it’s directly related to the genes. It’s directly related to this extra copy of the 21st chromosome but it’s not inherited because it doesn’t get passed down through generations. It’s not the case that one of your parents has Down syndrome and they pass it on to you. Each time this extra copy occurs it’s a new mutation because people who have Down Syndrome generally are infertile. So they’re not able to have children, so it’s not something to get to passed on through generations. It’s not something that you inherit from your parents. When it occurs it’s a new mutation so it is genetic but it’s not inherited.
Ok, so now let’s look at it an example of an intellectual impairment that is inherited then this brings us to what’s called Fragile X Syndrome. And so what happens in Fragile X Syndrome is that certain triplets of base pairs in the genetic code get repeated and these repeated triplets accumulate over time throughout generations. And if you build up enough of these repeated triplets of base pairs then you end up with this Fragile x Syndrome. And this is associated with intellectual disability.
Now you might recall that males are XY and females are XX, and so this means that males are going to be more likely to have this Fragile X Syndrome because if they have these repeated triplets accumulating on their X chromosome and they only have one X chromosome, then they end up with Fragile X. But females have two X chromosomes so for them to have Fragile X it needs to occur on both of the X chromosomes that they have. So in males this occurs in about 1 in 5,000 births. It’s half as common in females; only about 1 in 10,000 females. And this is an example that we’ll come back to when we talk about gender and IQ because we’ll see that actually the two normal curves for IQ are slightly different if we look at males and females. So we’ll talk about that in more detail in a future video but Fragile X is one thing that’s going to be associated with that. Where we have this cause of intellectual disability that’s actually the most common inherited cause, right? Again, it’s not as common as Down Syndrome but I’m saying most common inherited because this is passed down through generations, these repeated triplets build up through successive generations. So it’s the most common inherited cause of intellectual disability and it’s much more common in males. Ok, so that’s Fragile X Syndrome.
Next we’ll look at a really interesting example and this is Williams Syndrome. And so Williams Syndrome involves the deletion of some genes on the seventh chromosome. So some genes are missing and this is the cause of Williams Syndrome and this leads to IQs below 70. So usually we have some intellectual disability here, generally IQs can be below 70 and people with Williams Syndrome are going to have difficulty with everyday tasks like telling left from right or tying their shoes. Yet the strange thing about Williams Syndrome is that those same people show gifts in the area of language and music. And so this is a good way of questioning our view of intelligence as being sort of this general cognitive ability because we have these high ability in areas of language and music accompanied by disability in other areas. Now this occurs in about 1 in 10,000 births so it’s not very common and then that means maybe it doesn’t tell us about intelligence development in most people.
But it does raise some questions about the nature of intelligence and how we go about defining it. I’ll post a video link in the description where you can see some children with Williams Syndrome; you can see some of these gifts in language and music that they have, as well as the disabilities that they also have to deal with.
Ok, so that’s Williams Syndrome and the last example that we’ll look at is one that I mentioned in a previous video and this is phenylketonuria or PKU. And I bring up PKU again because it’s a good demonstration of a single gene influencing cognitive development. So we have, if you didn’t see the previous video on PKU, the idea of PKU is people have this single gene mutation and it prevents them from making this enzyme called phenylalanine hydroxylase and so what phenylalanine hydroxylase does is it breaks down this substance called phenylalanine and phenylalanine is common in lots of foods.
So you eat meats or cheeses or artificial sweeteners that contain some phenylalanine and this phenylalanine hydroxylase breaks it down, converts it into a substance called tyrosine and then it’s harmless. But if you have this particular gene mutation then you don’t produce the enzyme phenylalanine hydroxylase and so you can’t break down the phenylalanine and what happens is this phenylalanine accumulates in your body and it’s actually toxic to neurons.
So what happens is the phenylalanine starts damaging your brain and hindering your development and you end up with cognitive impairments as a result of this. Now the reason I think this is a good example to think about is that we have something, it appears to be purely genetic, we have the single gene that we know is involved; we can screen for PKU in infants, but we can prevent the impairment if we choose the correct environment. So by intervening and making sure that people with PKU don’t consume foods with phenylalanine in them, and you may have noticed some warnings on foods that contain things like artificial sweet, artificial sweeteners that contain phenylalanine, and if you avoid those foods and you maintain this strict diet then you’re able to avoid the cognitive impairments that would normally result from PKU.
So even though we have this single gene that seems to be associated with this cognitive impairment it’s still dependent upon the environment. And so we can think about the complexity of intelligence just based on this simple example. We have the single gene but depending on the environment will influence how much it impairs cognitive ability or not. And then we can think about intelligence overall as being hundreds, perhaps thousands of genes, all of which are interacting with one another and also interacting with different factors from the environment in influencing the development of intelligence. So we can see just how complex of a problem it is to understand why it is that people’s IQs vary.
Ok, so these are some specific examples of gene interactions, I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!
