What then are the most common sex chromosome aneuploidies? I thought we’d take a look at some of those.
Although there are others, these ones are certainly the most common and actually come up fairly frequently.
There are four conditions that we’ll consider here and compare in some detail. I won’t go into too, too much
detail because it’s actually most important that you understand that all of these individuals are fairly normal
in appearance. The only things that really show up, there are some. We’ll look at a table.
But they’re mostly normal-appearing individuals. These individuals wouldn’t necessarily show up
with a disorder as such until the puberty years when a female hasn’t reached the menstrual cycle
by the age of sixteen or in reproductive years when a male perhaps is azoospermic. He doesn’t produce
sperm or produces less sperm than is required for fertilization. The first syndrome is Klinefelter syndrome.
It’s an XXY, so 47 chromosomes. There’s an extra X. It doesn’t turn out to be too much of a problem
because the extra X gets made into a Barr body and it all works out fine. Then we could have a trisomy X,
so an individual with three X chromosomes. Again, a little bit more of an issue because there’s extra stuff
but two of them will become Barr bodies. We’ve talked about already the options for multiple nondisjunctions,
nondisjunction in the sperm and nondisjunction in the egg. We could have a lot of extra X's.
Then we consider 47, XYY. In 47, XYY, it used to be called the supermale because you’ve got extra Y chromosome.
In general, individuals with the extra Y tended to be more angry. They did some prison studies
and found out that a lot of these very angry individuals had XYY, 47, XYY. So, they were supermales and that’s why.
But it turns out there are also some other feature or developmental things. These children tend to have
more acne and such. Perhaps it’s more of an environmental thing. They’re made more fun of in school.
So, they become more angry and combative. It turns out the studies were really inconclusive.
So, we’ve since dropped the title supermale. The other very viable situation is XO, so 45 chromosomes.
We’ve got an X and then nothing else. Again in this situation, the ovaries can’t be maintained very well.
We usually see infertility. So Turner syndrome, you could see pretty much any of these individuals
and not know again that anything was awry genetically. Here’s a great table that you can find
in your downloadable materials that summarizes some of these differences. I don’t think that it’s important
for you to memorize everything specifically but do recognize that everyone is fairly normal in presentation.
You'll notice that Klinefelters are infertile as are Turner syndrome individuals are infertile. But otherwise,
basically everything is normal. They make some mentions of educational capacities and IQ tests.
I’m not sure that those are the most conclusive things, so I definitely wouldn’t take those two seriously.
But in short, we have lots of different chromosomal aneuploidies that could develop. We’ve highlighted
the main ones that you would be responsible for. Finally, we should consider the incidence of these
sex chromosome abnormalities. As I mentioned earlier, they are much more common than autosomal
abnormalities because they can make it to term because of dosage compensation and such.
So, we see as much as 1 in 300 males. I think it’s surprising how frequent these chromosomal abnormalities are.
One in three hundred males could have a chromosomal abnormality as far as sex chromosomes.
Then 1 in 650 females could have one of these abnormalities. Now, I’m giving you statistics here
just so that you can recognize that it’s a fairly regular occurrence relative to some of the other disorders.
I don’t want you to get wrapped up on the actual numbers because numbers are reported differently
by different authors all over the place. So please, really just take it from the terms of it’s very frequent
or it’s very infrequent, common abnormalities versus rare abnormalities. In wrapping up our discussion
of sex chromosome abnormalities, I like you to remember that we can have a number of different
aneuploidies or polyploidies in the sex chromosomes. Then also, when we consider structural abnormalities,
most of the structural abnormalities are about sexual development and a lot of different genes other than SRY.
But SRY is the main one, can come into play in the sexual development especially in male
sexual development and the default sex is female. On that note, I’d love to wrap up our
chromosomal abnormalities. I look forward to seeing you in our next lectures. Thank you.