Meiosis is usually highly accurate, but errors in chromosome separation can create gametes with abnormal chromosome numbers. These nondisjunction events are a major source of aneuploidy and associated developmental disorders.

What nondisjunction is

Nondisjunction is an error of chromosome segregation during meiosis that causes some gametes to receive too many chromosomes and others too few, rather than being properly haploid.

Nondisjunction matters because meiosis is meant to halve chromosome number; when it fails, it directly violates the AP focus statement: it “produces gametes that are not haploid.”

Where nondisjunction happens in meiosis

Nondisjunction in meiosis I

In meiosis I, homologous chromosome pairs are supposed to separate to opposite poles.

• What fails: homologous chromosomes stay together (do not disjoin) at anaphase I

• Immediate result: one daughter cell gets both homologs; the other gets none for that chromosome

• After meiosis II: produces two gametes with an extra chromosome and two gametes missing that chromosome (all four gametes abnormal for that chromosome)

Common mechanistic contributors (high-level):

• improper spindle attachment to kinetochores

• failure to release cohesin at the right time

• disrupted alignment at the metaphase plate

Nondisjunction in meiosis II

In meiosis II, sister chromatids are supposed to separate.

• What fails: sister chromatids stay together at anaphase II

• After meiosis II: produces two normal gametes and two abnormal gametes (one with an extra chromatid/chromosome, one missing it)

Because meiosis II nondisjunction affects only one of the two meiosis I products, fewer gametes are abnormal than in meiosis I nondisjunction (for a single event).

This diagram compares nondisjunction in meiosis I versus meiosis II and shows the resulting gamete chromosome counts. It visually reinforces why meiosis I nondisjunction makes all four gametes abnormal (two $n+1$ and two $n-1$), while meiosis II nondisjunction yields two normal gametes plus one $n+1$ and one $n-1$. Source

Chromosome-number outcomes: aneuploidy

Gametes produced by nondisjunction are not haploid for the affected chromosome.

Aneuploid gametes can form aneuploid zygotes after fertilisation:

This figure illustrates how a nondisjunction event can produce an $n+1$ gamete that, after fertilisation with a normal $n$ gamete, forms a $2n+1$ (trisomy) zygote. It uses trisomy 21 as an example, making the abstract $n+1 \rightarrow 2n+1$ relationship biologically concrete. Source

• $n+1$ gamete + normal $n$ gamete → $2n+1$ (trisomy) zygote

• $n-1$ gamete + normal $n$ gamete → $2n-1$ (monosomy) zygote

Many aneuploid conceptions do not develop to birth because chromosome imbalance often disrupts gene dosage across many pathways.

Biological significance and key AP takeaways

• Nondisjunction is a meiotic segregation error, not a mutation within a gene’s DNA sequence.

• It changes chromosome number, which can alter gene dosage (the number of copies of many genes at once).

• The AP-essential idea is causal: failure of separation (homologs in meiosis I or sister chromatids in meiosis II) → gametes that are not haploid.

• Distinguish the two stages by outcomes:

  • Meiosis I nondisjunction: all four gametes abnormal (for that chromosome)

  • Meiosis II nondisjunction: two gametes normal, two abnormal