---
title: "Aneuploidy — AP Biology Definition & Exam Guide"
description: "Aneuploidy is having an abnormal chromosome number from nondisjunction. Learn how it links to mutations, phenotype, and natural selection on the AP Bio exam."
canonical: "https://fiveable.me/ap-bio/key-terms/aneuploidy"
type: "key-term"
subject: "AP Biology"
unit: "Unit 6"
---

# Aneuploidy — AP Biology Definition & Exam Guide

## Definition

In AP Biology, aneuploidy is a condition where a cell or organism has an abnormal chromosome number (an extra or missing chromosome), usually caused by nondisjunction during meiosis. It's a chromosomal-level mutation covered in Topic 6.7.

## What It Is

Aneuploidy means an organism ends up with the wrong number of [chromosomes](/ap-bio/unit-5/meiosis/study-guide/FC0aTuODYikjJuhlBO1Z "fv-autolink"), usually one too many or one too few. The most common cause is **[nondisjunction](/ap-bio/key-terms/nondisjunction "fv-autolink")**, which is when chromosomes (or sister chromatids) fail to separate properly during meiosis. A gamete then carries an extra chromosome or is missing one. When that gamete fertilizes a normal one, the offspring has an abnormal chromosome count.

Think of normal human meiosis as splitting 46 chromosomes evenly into gametes of 23 each. If nondisjunction happens, you might get a gamete with 24 instead of 23. Fertilize a normal 23-chromosome gamete with that one, and the zygote has 47 chromosomes, an example of aneuploidy. The CED files this under [mutations](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt "fv-autolink") (Topic 6.7) because, like a point mutation or frameshift, it's a change in the genetic material that can alter phenotype. The difference is scale. Instead of swapping one nucleotide, aneuploidy adds or removes whole chromosomes' worth of genes at once.

## Why It Matters

Aneuploidy lives in **[Unit 6](/ap-bio/unit-6 "fv-autolink"): Gene Expression and Regulation**, specifically Topic 6.7 Mutations. It directly supports learning objective **[AP Bio](/ap-bio "fv-autolink") 6.7.B**, which asks you to explain how changes in genotype produce changes in phenotype. Essential knowledge **EK 6.7.B.2** is explicit: errors in mitosis or meiosis change phenotype, and nondisjunction-driven changes in chromosome number often cause developmental disorders.

It also touches **AP Bio 6.7.A** (types of mutation) because aneuploidy is a chromosomal mutation, and brushes up against **AP Bio 6.7.C** when changes in chromosome number become raw material for [natural selection](/ap-bio/unit-7/natural-selection/study-guide/Nc1t327OihZEnIVHHYtC "fv-autolink"). That last connection matters in plants, where whole-genome duplication can produce new species. The big-picture theme is that mutation, including the chromosomal kind, is the ultimate source of genetic variation.

## Connections

### [Chromosome Number (Unit 6)](/ap-bio/key-terms/chromosome-number)

Aneuploidy is defined entirely against the normal [chromosome number](/ap-bio/key-terms/chromosome-number "fv-autolink"). If 2n is the baseline, aneuploidy is 2n plus or minus one (or a few). Knowing what counts as normal is the only way to spot what's abnormal.

### Mutations as a source of variation (Unit 6, Topic 6.7)

Aneuploidy sits in the same family as point mutations and frameshifts under AP Bio 6.7.A. They differ in scale, but all are [genotype](/ap-bio/key-terms/genotype "fv-autolink") changes that can shift phenotype, and all feed the genetic variation that natural selection acts on.

### Polyploidy and speciation in plants (Units 6-7)

When a plant doubles its entire genome (2n = 14 becomes 2n = 28), that's polyploidy, not aneuploidy, but both come from chromosome-separation errors. Polyploidy can instantly create a new, reproductively isolated plant species, linking chromosome errors to evolution.

### Cystic Fibrosis vs. chromosomal disorders (Unit 6)

Cystic Fibrosis comes from a small deletion in one gene, while Down syndrome comes from an extra whole chromosome. Comparing them shows that disorders can arise at the nucleotide scale or the chromosome scale.

## On the AP Exam

On multiple choice, expect a setup that walks you through nondisjunction and asks for the result. A classic stem describes a human gamete with 24 chromosomes instead of 23, then asks what condition the offspring will have when it fuses with a normal gamete (answer: aneuploidy, a 47-chromosome zygote). Another asks you to pick an example of aneuploidy in humans, where Down syndrome (trisomy 21) is the go-to answer. You may also see a karyotype comparison (2n = 28 vs. 2n = 14 in related plant species) testing whether you can tell whole-genome doubling from a single-chromosome error. No released FRQ has used the word verbatim, but the underlying idea, that meiotic errors change phenotype, is exactly the kind of cause-and-effect reasoning EK 6.7.B.2 expects you to explain. You should be able to trace the chain: nondisjunction in meiosis, abnormal gamete, abnormal chromosome number, altered phenotype.

## aneuploidy vs Polyploidy

Aneuploidy is an off-by-a-little error: one or a few extra or missing chromosomes (like 47 instead of 46). Polyploidy is a whole-set error: an entire extra copy of the genome (like 3n or 4n). The CED groups both under chromosome-number changes from nondisjunction, but aneuploidy adds or drops individual chromosomes while polyploidy multiplies the whole set. Polyploidy is common and tolerated in plants; aneuploidy in humans usually causes developmental disorders.

## Key Takeaways

- Aneuploidy is having an abnormal number of chromosomes, typically one extra or one missing, not a whole extra genome.
- The usual cause is nondisjunction, when chromosomes or sister chromatids fail to separate during meiosis.
- In humans, aneuploidy often produces developmental disorders, with Down syndrome (trisomy 21) the textbook example.
- The CED classifies aneuploidy as a mutation under Topic 6.7 because it changes the genetic material and can alter phenotype (EK 6.7.B.2).
- Aneuploidy adds or removes single chromosomes, while polyploidy multiplies the entire chromosome set.

## FAQs

### What is aneuploidy in AP Biology?

Aneuploidy is a condition where a cell has an abnormal number of chromosomes, usually one extra or one missing, caused by nondisjunction during meiosis. It's covered in Topic 6.7 as a type of mutation that can change phenotype.

### Is aneuploidy the same as polyploidy?

No. Aneuploidy is an off-by-one (or a few) error, like 47 chromosomes instead of 46. Polyploidy is a whole-set error, like having three or four complete copies of the genome. Both come from nondisjunction, but they differ in scale.

### What causes aneuploidy?

Nondisjunction, which is the failure of chromosomes or sister chromatids to separate properly during meiosis (or mitosis). This produces a gamete with too many or too few chromosomes, and fertilization passes that abnormal count to the offspring.

### Is Down syndrome an example of aneuploidy?

Yes. Down syndrome is trisomy 21, meaning three copies of chromosome 21 instead of two. That's exactly the kind of single-chromosome abnormality AP Bio multiple choice uses as a classic aneuploidy example.

### How is aneuploidy different from a point mutation?

A point mutation swaps one nucleotide in a DNA sequence. Aneuploidy adds or removes an entire chromosome and all the genes on it. Both are mutations under Topic 6.7, but they act at completely different scales.

## Related Study Guides

- [6.7 Mutations](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt)

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