In Meiosis, two divisions take a cell with two sets of chromosomes (2n) to four cells with one set (n): meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids. Usually the separations are perfect. When they are not, the cells made have the wrong number of chromosomes.
Key idea
Nondisjunction is a failure of chromosomes to separate. If a homologous pair fails to separate in meiosis I, all four cells are abnormal: two have an extra chromosome ($n + 1$) and two are missing one ($n - 1$). If sister chromatids fail to separate in meiosis II, two cells are abnormal ($n + 1$ and $n - 1$) and two are normal ($n$).
Why it works
Trace one pair. In meiosis I the pair should split, one homolog to each cell. If both go to one cell, that cell has two copies and its sister cell has none, and meiosis II then passes the error to all four products. In meiosis II only the cell where the chromatids stayed together is affected; the other cell from meiosis I divides normally. When an $n + 1$ gamete combines with a normal gamete, the new cell has $2n + 1$ chromosomes (a trisomy); an $n - 1$ gamete gives $2n - 1$ (a monosomy). Such a change in number is called aneuploidy.
Worked examples
Worked example: Counting chromosomes
A fruit fly has 2n = 8. One pair fails to separate in meiosis I. How many chromosomes are in each of the four cells made?
- Normally each cell gets n = 4.
- In meiosis I, both homologs of one pair go to the same cell.
- Those two cells (after meiosis II) have 4 + 1 = 5; the other two have 4 − 1 = 3.
Answer: Two cells with 5 chromosomes and two with 3.
Worked example: Which division went wrong?
Of four cells from one meiosis in a plant with 2n = 14, two have 7 chromosomes, one has 8 and one has 6. In which division did nondisjunction happen?
- n = 7, so two cells are normal.
- One is n + 1 and one is n − 1.
- Only a meiosis II error leaves two normal cells.
Answer: Meiosis II.
Common mistake
Thinking a meiosis II error affects all four cells. A meiosis I error spreads to all four, because both cells of meiosis I are already wrong. A meiosis II error happens in only one of the two cells, so the other cell's two products are normal. Check by drawing the tree: 1 → 2 → 4, and mark where the error is.
Sometimes every chromosome fails to separate, giving gametes with two full sets (2n). In plants, this is one route to polyploidy, which you will meet in speciation: many crop plants, such as wheat, are polyploid.