Random question(s).

[conuly]

Down's Syndrome is a type of trisomy - they have an extra copy of chromosome 21.

http://en.wikipedia.org/wiki/Aneuploidy

1. If somebody with some form of aneuploidy has a child, is it heritable at that point?
2. Sometimes, closely related species seem to have vastly different amounts of chromosomes. Do we know why this can happen? I mean, this isn't one of those slow development things that makes sense to me. How do extra sets of chromosomes appear (or spare sets disappear)?

Comments

[redbird]

I think (or I think biologists think) that one way related species have different numbers of chromosomes is that a chromosome/chromosome pair splits, or two chromosomes/pairs combine. That's things like humans with 46 and chimps with 48.

You sometimes get vastly different numbers of chromosomes in plants because they can double the entire set of chromosomes, so instead of (say) 20 chromosomes, a mutant will have 40. It's called tetraploidy or, more generally, polyploidy.

I am now going to go see if I can find an answer to your first question, because I hadn't thought about it and it seems interesting and potentially important.

[conuly]

Is that also why some of our chromosomes are HUGE and the rest are much smaller?

[steorra]

1. From elsewhere on Wikipedia; I only partly understand what I'm quoting:

Genetic origins of Down Syndrome:
There has been reported some cases of Down syndrome parents having trisomy 21 children. In these cases (all from mothers), the ovaries were trisomy 21, leading to a secondary nondisjunction during gametogenesis and a gamete with an extra chromosome 21. Such Down syndrome trisomies are indistinguishable from Down syndrome trisomies created through meiotic nondisjunction.

Turner Syndrome
Women with Turner syndrome are almost universally infertile. While some women with Turner syndrome have successfully become pregnant and carried their pregnancies to term, this is very rare and is generally limited to those women whose karyotypes are not 45,X. Even when such pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner Syndrome or Down Syndrome.

Triple X Syndrome
Triple X syndrome is not inherited, but usually occurs as an event during the formation of reproductive cells (ovum and sperm). [...] Most women with Triple X have normal sexual development. Some experience an early onset of menstruation. (Second portion included to indicate that it seems like non-inheritance isn't a result of infertility.) Similarly for XYY Syndrome

From the abstract of an article turned up by googling for {Down syndrome reproduction}:
A new case of pregnancy in a 29-year-old woman with trisomy 21 is described. She gave birth to a male infant, chromosomally and phenotypically normal, who died the day after delivery due to prematurity. Thirty pregnancies of 26 affected mothers, including the present woman, resulted in 10 children with Down syndrome, 18 children (1 set of twins) without Down syndrome, and 3 spontaneous abortions.

[steorra]

Argh, I included things in quote tags, and they did not show up as expected.

[conuly]

Thankies :)

[leora.livejournal.com]

I can answer the first part, as I looked it up before. (At least, in part.)

Women with Down Syndrome are less likely to be fertile than women without, but some can and do have children. The children can have either the typical number of chromosomes or Down Syndrome. I believe the rate is 50-50 for each type. I am not positive about the fertility levels of men with Down Syndrome, as I looked this up a while ago, but I suspect they too would have roughly even odds of children with Down Syndrome and children without.

[leora.livejournal.com]

As you might guess, that assumes the other parent does not have Down Syndrome. The odds would increase for a Down Syndrome child if both parents had Down Syndrome, although I don't know what the odds of a viable pregnancy occurring would be given a decreased fertility rate. And I'm not sure if four copies is viable at all (no clue at all), so that might cause 25% of the conceptions to not take, which would cause a drastic reduction in whatever rate was present. But if four copies is viable, then it wouldn't be as much of a problem. But I've never heard of it... of course, many things happen that I haven't heard of. I'd look up further details but I'm rather tired today, so random speculation is all you get from me. :P

[ncp.livejournal.com]

I'm an unemployed high school biology teacher, so this is more information than you need to know ('cause otherwise my brain will atrophy).

Do you remember mitosis and meiosis from high school biology? You remember that we have two copies of each chromosome, one from mom, one from dad. Meiosis is the process that creates gametes, which only have ONE copy of each chromosome. In order for meiosis to occur properly, the two copies of each chromosome have to find each other and line up next to each other so they can be sorted into gametes. If this doesn't happen properly, you have what is known as non-disjunction, which can lead to aneuploidy (after fertilization).

SO in a child with aneuploidy, there are an improper number of chromosomes, and they don't line up properly. In the case of Down's Syndrome, which is trisomy (3 copies of chromosome 21), theoretically, you can have a 50% chance of a functional gamete and a 50% chance of a gamete with 2 copies of chromosome 21.

In the case of aneuploidy with sex chromosomes, the sex organs are usually not mature anyway, so even if theoretically there could be viable gametes, meiosis probably wouldn't be occurring anyway.

[conuly]

'cause otherwise my brain will atrophy

What, worrying about pottying and naps and how cute your kid is isn't enough? :P

Also, thank you, that's a good explanation :)

[diatryma.livejournal.com]

On chromosomes: http://diatryma.livejournal.com/236788.html

That's from a while back, and it's mostly to do with plants. With all organisms, chromosomes sometimes merge-- I think that's what we think happened between humans and whatever ape is closest, and between horses and donkeys. I might be wrong on that, though. The genes don't disappear, but they do squidge into new places.