Trisomy 21

[jdoc04]

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Okay, so its 3 copies of chromosome 21.

Is it caused by nondisjunction during:
Anapahse I --> Homologous chromosomes
Anaphase II --> sister chromatids

or either???? I'm guessing both

thanks

 

[biocmp]

I'm going to guess Anaphase I for a couple of reasons.

It's referred to as three copies of a gene. I'm assuming this means three distinct copies (coming from the homologous chromosomes and syngamy of the nuclei), and thus it could only be anaphase I.

If it were anaphase II, you would still only have two copies of a genome, but an extra backup for one of your copies (sister chromatids, not homologous chromosomes).

This is one of the harder questions I've seen on here...

 

[jdoc04]

I was reading wiki and it seems to infer both I & II but does not go into detail. I had genetics a while back and don't have the book anymore.

I drew it out and it seems if:

ND occurs Ana-I w/ normal A-II --> 1 homologous chromosome in each gamete instead of 1 sister chromatid in each gamete.

ND occur A--II preceded by a normal A-I --> 1 gamete with 2 homologous chromosomes and 1 with 0.

Anybody know?? For those of you with destroyer refer to 249. I am questioning E as the answer b/c of the wording in B.

 

[Meatwad]

I haven't read up on the specifics in a long time, but as far as I remember, most (>75%) is ND during meiosis I, and some (<25%) caused by ND in meiosis II.

 

[drteeth]

its both...