2+ Year Member
May 25, 2016
Medical Student
we know that most trisomies or monosomies occur due to nondisjunction in either Meisosis I or meisois II. How can we know whether the nondisjunction occurred in meiosis I or meiosis II of gametogenesis?


5+ Year Member
Jun 11, 2014
Resident [Any Field]
Let's say we have a parent cell "AC." During normal meiosis I the cell becomes AACC and divides into two daughter cells, AA and CC.
Now we have two cells AA and CC that will divide during meiosis II. What is the end result? 4 daughter cells, A, A, and C, C.

Let's say we have nondisjunction in meiosis I. (Occurs in Down's syndrome)
The parent cell AC -> AACC does not become AA and CC. Rather, it now becomes AACC after meiosis I.
The daughter cells will now be AC and AC instead of A, A, C, C.

Let's say we have nondisjunction in meiosis II.
The parent cell AC -> AACC divides correctly to form AA and CC (because there is no problem with meiosis I.)
Now, since there is nondisjunction in meiosis II, the AA and CC will become, for example, AA and C, C.

Please let me know if this makes sense or not. Correct me if i'm wrong! :)
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2+ Year Member
May 5, 2016
Resident [Any Field]
Someone asked this in class and professor explained that you cannot tell because of crossing over and just normal mixing. However, just as a review I looked this up:

1. 23rd chromosome does not do much crossing over especially in males (occurs in p1)
2. First division in meiosis occurs to homologous chromosomes (mom twins get divorced from dad twins)
3. Second division occurs to sister chromatids (twins split)
4. Nondisjunction in meiosis 1 leaves a mom and dad pair in one cell in the additional chromatid condition (heterozygous)
5. Nondisjunction in meiosis 2 leaves twins in one cell in the additional chromatid condition (homozygous)

The above explains niche conditions where you can have person with Kleinfelters and express x-recessive disorders expressed in neither parent (m2 nondisjunction). I don't expect to see such complicated questions show up on step 1. I was much happier not knowing all this additional info because complicated.

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