nondisjunction

[joonkimdds]

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destroyer says
if it occurs, one cell gets both and the other gets none.

I thought it means
XX + XX --> XXX + X instead of XX + XX
so one is called trisomy, another one is called monosomy.

This means the other one gets at least 1 X, not none.
I am confused~

 

[Shinpe]

you have to homologous chromosomes for each chromosome number right? Destroyer is talking about nondisjunction happening at only one of them (which is usually what would happen, the probability of nondisjunction happening at both is the square of the probability of it happening at one and highly unlikely). So then from one chromosome, one cell would get all (2 chromatids) and one cell would get none while the other chromosome separates normally (1+1) so that's why you end up with 3+1.

 

[UCB05]

destroyer says
if it occurs, one cell gets both and the other gets none.

I thought it means
XX + XX --> XXX + X instead of XX + XX
so one is called trisomy, another one is called monosomy.

This means the other one gets at least 1 X, not none.
I am confused~

It means:
XX + X'X' --> XXX'X' + 0
in meiosis 1. "XX" means sister chromatids of the same chromosome.

or
XX --> XX + 0
in meiosis 2 or somatic cells

In either case, it means one daughter cell gets both the chromosomes or chromatids, and the other gets shafted.

The example you give doesn't really happen. It would mean one set of sister chromatids remains intact, and the other pair breaks in the same step. You're kinda mixing the two phases of meiosis together. Nondisjuction means that either the homologous chromosomes in meiosis 1 or the sister chromatids in meiosis 2 do not properly separate

 

[joonkimdds]

Thank you guys~

 

[Cofo]

A haploid cell, which is a gamete (sperm or egg) will contain two chromatids. These two chromatids will replicate during Interphase cycle of the cell cycle, so now you have two chromosomes in one diploid cell (gamete). However, gametes are supposed to be haploid (contain one copy of each chromosome 21)
But because chromosomes or chromatids didn't separate in anaphase 1 or anaphase 2 respectively, and now one gamete has TWO copies of chromosome 21, the gamete is diploid. <---- this is nondisjuction.
When this diploid gamete fuses with a normal gamete (a haploid gemete that contains only ONE copy of chromosome 21)...a zygote forms with THREE copies of chromosome 21 (two from one gamete, and one from a regular gamete). This is trisomy 21.

I'm studying for the DAT, and I'll be taking it in December. Good luck in August!