---
title: "Chromosome Number — AP Bio Definition & Exam Guide"
description: "Chromosome number is the total count of chromosomes in a cell, and changes to it from nondisjunction (like aneuploidy) cause new phenotypes tested in AP Bio Unit 6."
canonical: "https://fiveable.me/ap-bio/key-terms/chromosome-number"
type: "key-term"
subject: "AP Biology"
unit: "Unit 6"
---

# Chromosome Number — AP Bio Definition & Exam Guide

## Definition

Chromosome number is the total count of chromosomes in an organism's cells, and changes to it (from errors in meiosis like nondisjunction) can produce aneuploidy or polyploidy, which often cause new phenotypes and developmental disorders tested in AP Bio Unit 6.

## What It Is

Chromosome number is exactly what it sounds like: how many [chromosomes](/ap-bio/unit-5/meiosis/study-guide/FC0aTuODYikjJuhlBO1Z "fv-autolink") a cell has. Humans have 46 (a [diploid](/ap-bio/key-terms/diploid "fv-autolink"), or 2n, number), wheat has 42, a fruit fly has 8. It's a stable, species-specific trait, until something goes wrong during cell division.

In [AP Bio](/ap-bio "fv-autolink"), chromosome number matters most when it *changes*. Under EK 6.7.B.2, errors in mitosis or meiosis can shift the count and produce new phenotypes. The classic culprit is **nondisjunction**, when chromosomes fail to separate properly during meiosis, leaving a gamete with an extra or missing chromosome. This causes **aneuploidy** (a wrong number of one chromosome type, like trisomy 21) or larger-scale changes like **triploidy** (a whole extra set, 3n). These aren't point mutations in a single base; they're large-scale chromosomal mutations that reshuffle whole chunks of the genome at once.

## Why It Matters

This lives in [Unit 6](/ap-bio/unit-6 "fv-autolink") (Gene Expression and Regulation), specifically Topic 6.7 Mutations. It directly supports **AP Bio 6.7.B**, which asks you to explain how changes in [genotype](/ap-bio/key-terms/genotype "fv-autolink") produce changes in phenotype, and EK 6.7.B.2 calls out chromosome-number changes from nondisjunction as a source of new phenotypes and developmental disorders. It also feeds **AP Bio 6.7.C**: because chromosome-number changes create genetic variation, they give natural selection something to act on. The big-picture theme is variation. Mutations of every scale, from a single swapped base up to an entire extra chromosome set, are the raw material of evolution.

## Connections

### Karyotype (Unit 6)

A karyotype is the picture you use to count chromosome number. Lining up and photographing every [chromosome](/ap-bio/key-terms/chromosome "fv-autolink") lets you spot aneuploidy at a glance, like seeing three copies of chromosome 21 instead of two.

### Nondisjunction and Meiosis (Units 5-6)

Chromosome number changes because meiosis messed up. When homologs or [sister chromatids](/ap-bio/key-terms/sister-chromatids "fv-autolink") fail to separate, one gamete gets an extra chromosome and the other gets none, so this term is really a meiosis error showing its consequences.

### Genetic Variation and Natural Selection (Units 6-7)

Changing chromosome number is one way to generate [variation](/ap-bio/unit-6/mutations/study-guide/WIuGA11Yy2RsVq8JpSnt "fv-autolink"). In plants especially, polyploidy (extra whole sets) can instantly create reproductively isolated populations, which is why it shows up in speciation questions.

### [Frameshift Mutation (Unit 6)](/ap-bio/key-terms/frameshift-mutation)

Both alter the genotype, but at totally different scales. A frameshift shifts the reading frame within one gene by adding or deleting a base, while a chromosome-number change adds or removes entire chromosomes carrying thousands of genes.

## On the AP Exam

Expect chromosome number in karyotype-based MCQs. A typical stem shows you a plant with 2n = 28 next to a relative with 2n = 14 and asks which process explains the difference (the answer points to polyploidy or chromosome doubling and its evolutionary significance). On FRQs, you'll often work with the underlying machinery rather than the phrase itself. The 2026 Short FRQ Q4 asked about chromosome movement during Meiosis I and why chromosomes behave the way they do, which is exactly where nondisjunction sneaks in. Your job: connect a number change to a *cause* (an error in meiosis) and an *effect* (a new phenotype or a developmental disorder), and recognize that these changes feed genetic variation.

## Chromosome Number vs Point mutation

A point mutation changes a single nucleotide inside one gene, so the chromosome number stays the same. A chromosome-number change adds or removes entire chromosomes, affecting thousands of genes at once. Scale is the whole difference: one letter versus one (or more) whole chromosomes.

## Key Takeaways

- Chromosome number is the total count of chromosomes in a cell, and it's a stable, species-specific trait until an error in cell division changes it.
- Nondisjunction during meiosis is the main cause of chromosome-number changes, producing aneuploidy (a wrong count of one type) or triploidy (a whole extra set).
- Changes in chromosome number often produce new phenotypes and developmental disorders, which is the core of EK 6.7.B.2.
- You count and detect chromosome number using a karyotype.
- Chromosome-number changes are large-scale mutations that create genetic variation natural selection can act on (AP Bio 6.7.C).

## FAQs

### What is chromosome number in AP Bio?

It's the total number of chromosomes in an organism's cells, like 46 in humans. AP Bio cares most about what happens when that number changes through errors in meiosis, because those changes can create new phenotypes (EK 6.7.B.2).

### Is a change in chromosome number a mutation?

Yes. It's a large-scale mutation. Topic 6.7 groups it with point and frameshift mutations as a way that genotype changes lead to phenotype changes, but it operates on whole chromosomes instead of single bases.

### How is a chromosome-number change different from a point mutation?

A point mutation swaps one nucleotide inside a single gene and leaves chromosome number unchanged. A chromosome-number change adds or removes entire chromosomes, altering thousands of genes at once, usually because of nondisjunction in meiosis.

### What causes a change in chromosome number?

Nondisjunction, when chromosomes fail to separate during meiosis. This leaves one gamete with an extra chromosome and another with a missing one, producing aneuploidy (like trisomy 21) or whole extra sets called polyploidy.

### How does chromosome number connect to evolution?

Changes in chromosome number generate genetic variation, the raw material for natural selection (AP Bio 6.7.C). In plants, polyploidy can instantly create new, reproductively isolated species, which is why it appears in speciation questions.

## Related Study Guides

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

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