Now, I’m going to move into talking about some less common situations but Down syndrome gives us
a good position to look at them from. This only happens in about 2% of Down syndrome cases.
But it can happen in other chromosomal or genetic disorders also. We may see mosaicism show up,
chromosomal mosaicism, where some cells have three chromosomes and some cells only have two.
Now, how could this really happen? We’ve certainly seen it happen with the sex chromosomes.
It’s normal with the X chromosome that one would form a Barr body. We’ve certainly discussed the idea
of mosaicism of X chromosome linked characteristics. We’ll dig more into that in the future.
But here, we have a situation where something happens early in development. One of the cells is maybe
able to rid itself of the additional chromosome. We’re not quite sure how that happens. But some cells
are able to do that, or trisomy corrections, or you could have something go on like a Robertsonian translocation.
I mentioned that in an earlier lecture. So, now it’s time to dig into precisely what a Robertsonian translocation is.
Even though it’s fairly rare as a cause of Down syndrome, it’s a great opportunity to bring it up.
So, what is a Robertsonian translocation? To start with, it’s two acrocentric chromosomes that may have
some kind of translocation event between them such that the two long arms come together
and the two short arms come together. Depending on how this works out in cell division as chromosomes
segregate from each other, you could end up with some cells that have a balanced situation
where they have all of the genes that would have been on both chromosomes. You could have
some cells that come out that are not balanced as in they don’t have all of the products. So, in this way,
we might again end up with a mosaic of Down syndrome but also we could see this happen
during oogenesis or spermatogenesis. We can even see that some patients with Down syndrome
could have, by chromosome number count, one less or actually a normal number of 46 chromosomes.
So, Robertsonian translocation is something that you should understand and how it could lend itself
to having the same chromosome number, although they won’t match up and pair properly.
So 45 or 46 is possible and mosaicism is possible. Another way that we could potentially end up
with Down syndrome and again, not that it’s common, in fact, it’s fairly uncommon but we could have
a partial trisomy. In this case, we could have a variable expression of Down syndrome because we have
a partial additional chromosome 21 rather than having three complete chromosome 21s.
Again, all of these are examples using Down syndrome that could happen to any number of different chromosomes.
You could have partial trisomy of multiple other chromosomes including ones that are not classic,
ones that make it to term. Some of the other chromosomes other than 18, 21, and 13 could be viable
with partial trisomies. As I mentioned, only three autosomal polyploidies are viable.
But Down syndrome is the only one that is viable and survivable into middle ages.
Edwards syndrome and Patau syndrome, you should certainly know is trisomy 18 and trisomy 13 respectively.
However, we’re not going to cover the details because the details of the manifestations, how it displays clinically
are fairly similar and they’re much more severe. Because they don’t survive generally more than a year,
obviously the chromosomal dosage is far too much and we’re not going to examine those.
You do need to know again respectively that they’re trisomy 13 and 18 but not so much of the details.
On that note, we’ve covered our full chromosome polyploidies. In the next lecture, we’re going to move on
and look at some of those partial chromosome abnormalities. So I look forward to seeing you then.