In AP Bio, nondisjunction is the failure of homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) to separate properly, producing gametes that are no longer haploid (1n) and carry an abnormal number of chromosomes.
Meiosis is supposed to be tidy. In meiosis I, homologous chromosomes split apart, and in meiosis II, sister chromatids split apart, so every gamete ends up with exactly one copy of each chromosome (a haploid, or 1n, set). Nondisjunction is when that split fails. A pair of homologs or a pair of sister chromatids sticks together and both go into the same cell.
The result is gametes with the wrong chromosome count. One gamete ends up with an extra chromosome (n+1) and another ends up missing one (n-1). The CED ties this directly to EK 5.2.A.1: correct separation in meiosis I and II is what guarantees a haploid set, and "when incorrect separation occurs (nondisjunction), gametes are no longer haploid." When one of these abnormal gametes fertilizes a normal one, the zygote ends up with an unusual chromosome number, which is what you see on a karyotype.
Nondisjunction lives in Unit 5: Heredity, mainly in topics 5.2 (Meiosis and Genetic Diversity) and 5.6 (Chromosomal Inheritance). It connects to learning objective AP Bio 5.2.A, which asks you to explain how meiosis generates genetic diversity. Nondisjunction is the flip side of that objective. Most of meiosis is built to sort chromosomes correctly, and nondisjunction is what happens when that sorting breaks. Understanding it shows you actually know what proper separation accomplishes, not just that it happens.
Keep studying AP Biology Unit 5
Trisomy 21 / Down Syndrome (Unit 5)
Trisomy 21 is the textbook outcome of nondisjunction. A gamete carrying two copies of chromosome 21 fuses with a normal gamete, giving the zygote three copies. If you can explain nondisjunction, you can explain where trisomy 21 comes from.
Anaphase I (Unit 5)
Anaphase I is exactly when homologous chromosomes are supposed to pull apart. Nondisjunction in meiosis I means they failed to separate here, so both homologs travel to the same pole. Nondisjunction in meiosis II is the same idea, but with sister chromatids.
Monosomy and Polyploidy (Unit 5)
These are the other chromosome-number errors. Monosomy (one copy) comes from the n-1 gamete nondisjunction produces, while polyploidy (whole extra sets) usually involves failures across all chromosomes. Same general theme of chromosomes ending up in the wrong count.
Genetic Diversity and Crossing Over (Unit 5)
Crossing over (EK 5.2.A.2) is meiosis working as intended to boost diversity. Nondisjunction is meiosis malfunctioning. Both show up in the same chromosome dance, which is why exam questions often pair them.
Nondisjunction shows up most often in MCQs built around a karyotype. A stem describes an individual with an abnormal chromosome number, often three copies of chromosome 21, and asks what cellular process explains it. The answer points to nondisjunction during gamete formation. Harder versions push you to pinpoint the stage and parent. For example, a child with Klinefelter syndrome (47, XXY) whose extra X came from the mother means nondisjunction happened in the mother's meiosis. You may also need to decide whether it occurred in meiosis I (homologs failing to separate) or meiosis II (sister chromatids failing). On the FRQ side, nondisjunction connects to released questions about meiosis I segregation, including the 2024 LRFRQ on how crossing over helps homologous chromosomes align and segregate during the first division. If alignment and segregation go wrong, nondisjunction is the consequence.
Both happen during meiosis, but they do opposite jobs. Crossing over is a normal process that swaps DNA between homologous chromosomes to increase genetic diversity (EK 5.2.A.2). Nondisjunction is an error where chromosomes fail to separate, producing gametes with the wrong number of chromosomes. One adds variation on purpose, the other adds problems by accident.
Nondisjunction is the failure of chromosomes to separate during meiosis, producing gametes that are not haploid.
It can happen in meiosis I (homologous chromosomes fail to separate) or meiosis II (sister chromatids fail to separate).
Each nondisjunction event makes one gamete with an extra chromosome (n+1) and one missing a chromosome (n-1).
Trisomy 21 (Down syndrome), monosomy, and Klinefelter syndrome (47, XXY) all trace back to nondisjunction.
On the AP exam, karyotype questions showing an abnormal chromosome number almost always point to nondisjunction as the cause.
It's when homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) fail to separate properly. Per EK 5.2.A.1, this makes gametes that are no longer haploid, so they carry an abnormal number of chromosomes.
No. It can occur in meiosis I, where homologous chromosomes fail to separate, or in meiosis II, where sister chromatids fail to separate. Exam questions sometimes ask you to figure out which division was involved based on the specific chromosome error.
Crossing over is a normal process that swaps DNA between homologous chromosomes to increase genetic diversity. Nondisjunction is an error where chromosomes don't separate, giving gametes the wrong chromosome count. One is meiosis working right, the other is meiosis going wrong.
If chromosome 21 fails to separate during meiosis, a gamete ends up with two copies of it. When that gamete is fertilized by a normal gamete, the zygote gets three copies of chromosome 21, which is trisomy 21, or Down syndrome.
Meiosis I nondisjunction means homologous chromosomes (the two different versions of a chromosome) failed to separate. Meiosis II nondisjunction means identical sister chromatids failed to separate. For a case like Klinefelter syndrome (47, XXY), figuring out which X copies are involved tells you the division and even the parent where the error occurred.