Down Syndrome: Non disjunction at anaphase 1 or 2?

[Dark Horse]

In down syndrome, does non-disjunction occur at anaphase 1 or 2? what is the difference in the end result of non-disjunction at anaphase 1 and at anaphase 2?

Thanks

[Russ]

Trisomy 21 is an extra chromosome, so can happen from a non disjunction during either round of replication. Based on gametes produced non disjunction at 1 would be more likely to cause the trisomy though.

In the first round, you'll get NO normal cells produced. In the second round you'll get 2 normal cells, 2 abnormal cells produced.
Draw it out, it helps so so much.

[stonecold]

It can also be due to a translocation of chromosome 21, and in rarer cases, non-disjunction of chromosome 21 chromatids in mitosis, particularly in early development.

[shinny]

non-disjunction of chromosome 21 chromatids in mitosis, particularly in early development.

Just to clarify, that'll produce what's known as mosaicism as obviously only cells produced from that original defective cell will have the trisomy. So the person will show some of the signs of Down Syndrome really depending on how early in development (and hence, a greater proportion of cells affected) the mutation occurred.

[Russ]

If you want a meta-clarification () it will also lead to a milder phenotype due to less cell lines being affected

[Dark Horse]

Thanks all, but if non-disjunction occurs in anaphase 1, then what happens in anaphase 2? After non-disjunction in anaphase 1, the two homologous pairs will be present in one daughter cell and no chromosomes of that particular number (eg. in this case  chromosome 21) in the other. Then in anaphase 2, will the two homologous pairs both split into 4 chromatids, or will only one chromosome split? I'm sorry if my logic is completely wrong (which it most likely is), but im still struggling to exaclty understand how anaphase 1 and 2 produce different daughter cells after meiosis. Any further clarification wuld be much appreciated    

[Russ]

The crappy ASCII schematics on wiki are actually pretty decent and explain this much simpler than I would

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

[Dark Horse]

Thanks so much Russ! The wiki schematic was great!

[yuuhei]

wow.. lol
sure does xD