What is a chromosome count?

It is the number of chromosomes in the cells of an animal. Each species has its own number, and it is one of the simplest facts we can know about a genome.

The short version

A chromosome is a long piece of DNA wrapped up with proteins. Cells keep their DNA in chromosomes. A chromosome count tells you how many a species has. Humans have 46, in 23 pairs.

Two numbers, n and 2n

Most animals get one set of chromosomes from each parent. One set is the haploid number, written n. Both sets together are the diploid number, written 2n. For humans, n is 23 and 2n is 46.

This site shows n by default. You can switch any chart to 2n, which is just n times two.

Some animals break the rule. Male honeybees grow from unfertilized eggs and carry a single set.

Why people count them

A count is one of the first things we learn about a genome. It takes a microscope and a well-prepared slide.
It is a check for genome assemblies. If an assembly has many more pieces than the species has chromosomes, it is probably not finished.
A change in number can help split one lineage from another, so comparing counts across species tells us about how groups diverge.

A bigger number does not mean a more complex animal. The count tells you how the DNA is divided into pieces, not how much the animal can do.

How to read the chart

Each bar is one haploid number. Its height is the number of species that have it. When you open a species, its bar is amber. The dashed line is the median, the species in the middle when you line them all up.

Words you will see

Haploid (n): One set of chromosomes.
Diploid (2n): Two sets of chromosomes.
Polyploidy: Having more than two sets. It is common in plants and less common in animals.
Conflicting counts: Sources report different numbers for the same species. It can be a mistake. It can also be real, because counts sometimes vary between populations or between males and females. The species page lists every count and every source.
Curated count: The number we cleaned up for the charts. If a source reports a half value, we round it up.

Try it

For a class, pick five animals and ask students to guess which has the most chromosomes. Then look them up here.