How many chromosomes do humans have?
46, arranged in 23 pairs. Chimpanzees, gorillas and orangutans all have 48 — and the reason humans came out one pair short is the most quietly remarkable thing about our genome: two of our ancestor's chromosomes fused end-to-end into what we now call chromosome 2. You can still see the join under a microscope.
Quick answer
46 chromosomes in every normal body cell. 23 pairs — 22 autosome pairs (numbered 1–22 by size) plus one pair of sex chromosomes (XX in females, XY in males). Sperm and egg cells carry just 23 single chromosomes each.
People with Down syndrome have 47, because chromosome 21 appears three times instead of twice (trisomy 21).
How does 46 compare?
tap a speciesChromosome count does not track complexity. A fern beats us 27 to 1.
Advertisement
AdSense Slot: content_top
Why 46 and not 48
Every other great ape has 48 chromosomes (24 pairs). The standard explanation is that two ancestral chromosomes in our lineage fused at their tips. The fusion site lives on human chromosome 2 at band 2q13-q14, and it carries two giveaway signatures: an inverted block of telomere repeats sitting in the middle of the chromosome (telomeres normally only appear at the ends), and a second, inactive centromere — a remnant of one of the two original chromosomes whose centre got switched off when the fusion happened.
The fusion site was first identified by IJdo, Baldini, Ward, Reeders & Wells in PNAS, 1991. A 2005 NHGRI sequencing analysis confirmed the inactive centromere. A 2022 study by Poszewiecka and colleagues in BMC Genomics pinned the timing at roughly 0.9 million years ago, with a confidence interval of 0.4 to 1.5 million years. Neanderthal and Denisovan genomes carry the same fusion, which is why our extinct cousins also had 46.
For anyone who studies genomes, chromosome 2 reads like a paragraph that's been pasted in from two earlier ones. It's hard to explain by any route other than two becoming one.
The 30 years when "48" was the answer
The right number had a slow road in. In 1923, Theophilus Painter, a Texas cytologist, looked at human testicular tissue under a microscope and concluded humans had 48 chromosomes. He was looking at clumped, overlapping spreads — the staining and squashing techniques of the day weren't up to a clean count — and he got it wrong by one pair. The figure was repeated in textbooks for the next three decades.
In late 1955, working at the Institute of Genetics in Lund, the Indonesian-born cytogeneticist Joe Hin Tjio and his Swedish colleague Albert Levan used a new combination — colchicine to arrest cells in mitosis and hypotonic shock to spread the chromosomes apart cleanly — and counted, repeatedly, 46. Their paper appeared in Hereditas in 1956. Even then, several textbooks kept printing 48 for years.
It is, as one geneticist later put it, a clean little case study of scientific groupthink: a number gets a respected source, everyone repeats it, nobody bothers to recount until someone with better technique does — and then the literature takes years to catch up.
What's on the 23 pairs
The 22 autosome pairs are numbered, roughly, from biggest to smallest. Chromosome 1 has the most DNA (about 8% of your genome); chromosome 21 is the smallest, which is part of why trisomy 21 (Down syndrome) is survivable when other trisomies are not. The 23rd pair carries the sex chromosomes:
- The X chromosome carries around 800 genes and is essential — you cannot live without at least one X.
- The Y chromosome is much smaller and carries only ~50–80 protein-coding genes, but one of them is SRY, which triggers male development in the embryo.
- Mothers always pass an X. Fathers pass either an X or a Y — which is why, biologically, the father's contribution decides chromosomal sex.
When the count is something other than 46
Most variations come from a single missed split during cell division (non-disjunction), giving an extra or missing chromosome. The seven below are the ones a clinical geneticist sees most:
| Condition | Karyotype | Total | Note |
|---|---|---|---|
| Down syndrome | Trisomy 21 | 47 | Extra copy of chr 21. Described by Lejeune, 1959. |
| Edwards syndrome | Trisomy 18 | 47 | Extra copy of chr 18. Rare, severe. |
| Patau syndrome | Trisomy 13 | 47 | Extra copy of chr 13. |
| Turner syndrome | 45,X | 45 | A single X, no second sex chromosome. |
| Klinefelter syndrome | 47,XXY | 47 | Extra X chromosome in males. |
| XYY syndrome | 47,XYY | 47 | Extra Y chromosome. |
| Triple X syndrome | 47,XXX | 47 | Extra X in females. |
Count doesn't equal complexity
Two facts that catch most people off guard. First: the Indian muntjac, a small deer, has just 6 or 7 chromosomes; the related Reeves's muntjac has 46 — same as a human. Same genus. Second: Ophioglossum reticulatum, an unassuming little fern, holds the world record at around 1,260 chromosomes. Whatever 46 means about us, it isn't a complexity score.
What sits on each of the 23 pairs
The 22 autosomes are numbered roughly from biggest to smallest, with the sex chromosomes (X and Y) tagged on. Every single one carries at least one gene worth knowing — a famous disease locus, an oncogene, a developmental switch, or a "first of its kind" achievement. Tap through:
Tour all 23 pairs
tap a numberOne headline gene and one story per chromosome. Every one earns its row.
Sizes from GRCh38 reference; gene counts from Ensembl (protein-coding, rounded).
Advertisement
AdSense Slot: content_mid
Frequently asked
How many chromosomes do humans have?
46 in every regular body cell, in 23 pairs. Egg and sperm cells carry 23 single chromosomes each.
How many chromosomes in Down syndrome?
47 — an extra copy of chromosome 21. The condition is called trisomy 21.
Why do chimps have 48 and we have 46?
Two ancestral chromosomes fused to form human chromosome 2, roughly 0.9 million years ago. Neanderthals and Denisovans had the fusion too.
Who first counted human chromosomes correctly?
Joe Hin Tjio and Albert Levan, working in Lund, published the count of 46 in Hereditas in 1956. Painter's 1923 figure of 48 had been the textbook answer for 30 years.
Do men and women have the same chromosomes?
Females have 22 autosome pairs plus XX (46 total); males have 22 autosome pairs plus XY (also 46). It's the 23rd pair that differs.
What chromosome decides if a baby is male?
The SRY gene on the Y chromosome. The father's sperm carries either X or Y; the mother always contributes an X.
Sources
- Tjio JH & Levan A (1956) — The chromosome number of man, Hereditas 42(1): 1–6. The corrected 46.
- Painter TS (1923) — chromosome count of 48, the original error.
- IJdo JW, Baldini A, Ward DC, Reeders ST, Wells RA (1991) — Origin of human chromosome 2: an ancestral telomere-telomere fusion, PNAS 88(20): 9051–5.
- NHGRI press release, 6 April 2005 — confirmation of inactive centromere on chromosome 2 from full sequencing.
- Poszewiecka B et al. (2022) — Revised time estimation of the ancestral human chromosome 2 fusion, BMC Genomics 23(616). Fusion dated to ~0.9 Mya.
- Lejeune J, Gautier M, Turpin R (1959) — Comptes Rendus de l'Académie des Sciences. First description of trisomy 21 in Down syndrome.
- MedlinePlus Genetics (medlineplus.gov) and Genome.gov chromosomes fact sheet — current clinical reference.
- Khandelwal S (1990) — survey of plant chromosome counts; Ophioglossum reticulatum at ~1,260, the highest known.
Advertisement
AdSense Slot: content_bottom
Related questions
Forty-six because two of our ancestor's chromosomes once merged into one. And even then, the science took thirty years to count them right.