Trisomy 21 occurs via Nondisjunction in which stage?

[OrangeMed]

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Hey guys,

I drew out the nondisjunction in both meiotic stages and it seems like nondisjunction in either of the meiotic stages (MI and MII) can yield a zygote with trisomy 21. Same for Klinefelter's syndrome.
Is one meiotic nondisjunction favored over the other for particular aneuploidies or can these genetic defects result from either nondisjunctions?

Thanks!

 

[IslandStyle808]

I am going to try and recall my physiology from so long ago. Usually mitosis I happens far before fertilization. So for years and years the egg is stuck at mitosis I and this leads to the potential of having an aneuploidy. So the vast majority of the time these aneuploidies happen at mitosis II. If you look at the data on the subject, the risk of a chromosomal abnormality of the baby increases with mother's age. The one that is most highly prevalent is trisomy 21 (far above the rest). As so why, I am not sure.

Hope this helps.

 

[victorias]

If non disjunction happens in meiosis I, you end up with 2 x (n+1) and 2 x (n-1)

If non disjunction happens in meiosis II, you end up with n, n, n+1 and n-1

Trisomy 21 is cause by having an extra chromosome (n+1) which can be seen with non disjunction in both meiosis I and meiosis II. The chance of having gametes with an extra chromosome (n+1) is higher if non disjunction happens in meiosis I where 2 gametes have n+1 chromosomes. But it is also possible if non disjunction were to occur in meiosis II which would result in 1 gamete with (n+1) chromosomes.

 

[OrangeMed]

Thanks guys!