Chromosome numbers by species
Every species has its own fixed number of chromosomes in its body cells, and exactly half that number in its gametes.
| Species | In a body cell (2n) | In a gamete (n) |
|---|---|---|
| Human | 46 | 23 |
| Chimpanzee | 48 | 24 |
| Dog | 78 | 39 |
| Cat | 38 | 19 |
| Horse | 64 | 32 |
| Cattle | 60 | 30 |
| Mouse | 40 | 20 |
| Fruit fly | 8 | 4 |
| Pea | 14 | 7 |
| Maize | 20 | 10 |
| Onion | 16 | 8 |
| Tomato | 24 | 12 |
Table 20.2 — chromosome numbers in some familiar species.
Three things the table shows
- The gamete number is always exactly half the body-cell number. That is meiosis, which you met in the chapter on cell division.
- The body-cell number is always even, because every chromosome is one of a homologous pair.
- The number is fixed for a species and is one of the things that makes it that species.
A bigger number does not mean a more complex organism
A dog has 78 chromosomes and a human 46. A chromosome is a container, not a unit of complexity — what matters is how many genes are on it and what they do, not how many separate threads the DNA is divided into.
Fixed, even, and halved in gametes
Each species has a fixed chromosome number in its body cells, always even, and its gametes carry half that number.