One cell's extra chromosome is its partner's missing one
- The idea
- Predict
- See it
- Why
- Check
The idea
“Nondisjunction adds or deletes a chromosome, and the other cells come out normal.”
Trisomy sounds like a chromosome was added, and monosomy like one was deleted. You rarely see the cell the extra came from.
The short version
The mistake: Nondisjunction adds or deletes a chromosome, and the other cells come out normal.
What's actually true: Nondisjunction sends a chromosome to the wrong cell, so one cell's extra copy is the one its partner cell is missing.
- In nondisjunction, homologs in meiosis I or sister chromatids in meiosis II go to the same pole. Nothing is copied or broken. One cell gets both.
- A meiosis I error gives two n + 1 and two n - 1 gametes. A meiosis II error gives one of each and two normal ones.
- An n + 1 gamete plus a normal one makes a 2n + 1 zygote, a trisomy such as trisomy 21.
On the exam: Nondisjunction misplaces chromosomes. Every n + 1 cell has an n - 1 partner from the same division.