How many chromosomes do different species have?

Chromosome counts vary widely among species. Fruit flies have 8 chromosomes in a typical body cell, cats have 38, horses have 64, and dogs have 78.

There is no standard chromosome count across life. Humans have 46 chromosomes, while chimpanzees have 48. The number tells us how an organism packages its DNA and says little about its complexity or gene count.

How many chromosomes do different species have?

Chromosome counts vary widely: fruit flies have 8 in a typical body cell, humans have 46, and dogs and chickens each have 78.^[1]

The table gives the usual diploid count, written as 2n, for body cells. It states exceptions for organisms whose life cycles make a single diploid number misleading.

The animal counts come from the Animal Chromosome Count Database, a curated collection covering about 18,000 species. The sex-system labels follow the Tree of Sex database. The banana and yeast rows use species-specific genome and reproductive-biology sources.^{[1][2][3][4][13][14]}

How many chromosomes do dogs have? The usual answer is 78, or 39 pairs. Cats have 38 chromosomes, horses have 64, and chimpanzees have 48. In birds such as chickens, males are usually ZZ and females ZW, the reverse of the heterogametic pattern familiar from XY mammals.^[2]

These counts describe reference karyotypes, and exceptions occur. Wild house mouse populations include chromosome races created by fusions, while many edible bananas are triploid cultivars with 33 chromosomes rather than the 22 found in wild diploid Musa acuminata.^[1][3]

Which animal has the most chromosomes?

The Atlas blue butterfly (Polyommatus atlantica) has the highest confirmed count among non-polyploid animals: 229 chromosome pairs, equivalent to 458 chromosomes in its diploid complement.^[5]

A chromosome-level genome published in 2025 replaced earlier microscope estimates of 224 to 226 pairs. The butterfly has 227 pairs of very small autosomes and four sex chromosomes. Most of those autosomes arose when 24 ancestral chromosomes repeatedly split over roughly three million years.^[5]

Among multicellular organisms, the adder's tongue fern Ophioglossum reticulatum has a reported maximum near 1,440 chromosomes. It is highly polyploid, and the Chromosome Counts Database lists several lower counts from other populations, so 1,440 describes a maximum cytotype rather than every plant.^[6]

Some single-celled ciliates use a different kind of genome architecture. The somatic macronucleus of Halteria grandinella contains about 23,000 gene-sized nanochromosomes, each present in many copies. This specialized, high-ploidy nucleus uses a different counting basis from the diploid complements of animals and plants.^[7]

Why do species have different numbers of chromosomes?

Species have different chromosome numbers because chromosomes can fuse, split, or multiply as genomes evolve. Fusion and fission change the number of DNA packages, while whole-genome duplication adds one or more complete chromosome sets.^[8]

Humans and chimpanzees show how a count can change with little loss of genetic material. Two chromosomes that remain separate in chimpanzees joined end to end in a human ancestor, forming human chromosome 2. The fusion helps explain why humans have 46 chromosomes while chimpanzees have 48.^[9]

Chromosome changes do not always create a new species immediately. An individual carrying a fusion can still produce some balanced eggs or sperm because the fused chromosome contains much of the same DNA as its two unfused counterparts. Fertility may be lower, which can help populations diverge over time, but a different count is not an automatic reproductive barrier.

Whole-genome duplication is especially common in plants. It can double a diploid set into a tetraploid set in one event. Later chromosome losses, fusions, and rearrangements can obscure that history, which is why the same chromosome total can arise by different evolutionary routes.^[8]

The Atlas blue butterfly is an extreme fission example. Human chromosome 2 is a clear fusion example. Both show why closely related species can keep similar genes while carrying them on different numbers of chromosomes.

Does having more chromosomes make an organism more complex?

No. More chromosomes do not make an organism more complex. Chromosome count measures how DNA is divided into physical units, while genome size measures base pairs and gene count measures annotated genes.

The human haploid genome contains about 3.055 billion base pairs on 23 chromosomes.^[12] GENCODE lists 19,442 human protein-coding genes.^[11] The ciliate Oxytricha trifallax has a somatic genome of only about 50 million base pairs, yet divides it among roughly 16,000 unique nanochromosomes encoding about 18,500 genes.^[10]

The Atlas blue butterfly offers a comparison under the usual animal definition. Its annotation contains 19,251 protein-coding genes across 229 chromosome pairs. A close relative with 23 chromosome pairs has 17,462. A tenfold difference in chromosome number produced only a modest difference in annotated gene count.^[5]

Genome size and gene count are also imperfect measures of biological complexity. Repetitive DNA can enlarge a genome without adding genes, and gene regulation can create different cell types and traits from similar gene sets.

Chromosome count is also separate from sequence similarity. Humans and chimpanzees differ by one chromosome pair despite being about 98.8% identical across directly aligned DNA. A multiple sequence alignment compares sequence letters; a karyotype counts physical chromosomes.

For the same reason, the size of the human genome, the length of its DNA, and the number of human genes answer three different questions.

Sources▼

1. The Animal Chromosome Count Database Data Diversity Lab · July 23, 2026. https://cromanpa94.github.io/ACC/
2. Tree of Sex: A database of sexual systems Scientific Data · 2014. https://www.nature.com/articles/sdata201415
3. A BAC end view of the Musa acuminata genome BMC Plant Biology · 2007. https://pmc.ncbi.nlm.nih.gov/articles/PMC1904220/
4. Saccharomyces cerevisiae genome assembly ASM205788v1 NCBI Datasets · July 23, 2026. https://www.ncbi.nlm.nih.gov/datasets/genome/GCA_002057885.1/
5. Constraints on chromosome evolution revealed by the 229 chromosome pairs of the Atlas blue butterfly Current Biology · 2025. https://doi.org/10.1016/j.cub.2025.08.032
6. Chromosome Counts Database: Ophioglossum reticulatum Chromosome Counts Database · July 23, 2026. https://ccdb.tau.ac.il/Pteridophytes/Ophioglossaceae/Ophioglossum/Ophioglossum%20reticulatum%20L./
7. The Compact Macronuclear Genome of the Ciliate Halteria grandinella: A Transcriptome-Like Genome with 23,000 Nanochromosomes mBio · 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC7858049/
8. The Evolution of Chromosome Numbers: Mechanistic Models and Experimental Approaches Genome Biology and Evolution · 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC7875004/
9. Origin of human chromosome 2: an ancestral telomere-telomere fusion Proceedings of the National Academy of Sciences · 1991. https://pubmed.ncbi.nlm.nih.gov/1924367/
10. The Oxytricha trifallax Macronuclear Genome: A Complex Eukaryotic Genome with 16,000 Tiny Chromosomes PLOS Biology · 2013. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001473
11. Human release statistics (v50) GENCODE · July 23, 2026. https://www.gencodegenes.org/human/stats.html
12. The complete sequence of a human genome Science · 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9186530/
13. The Biology of Musa L. (banana) Australian Office of the Gene Technology Regulator · 2023. https://www.ogtr.gov.au/resources/publications/biology-musa-l-banana
14. Convergent Evolution of Chromosomal Sex-Determining Regions in the Animal and Fungal Kingdoms PLOS Biology · 2004. https://pmc.ncbi.nlm.nih.gov/articles/PMC526376/