The big idea: Each species has its own number of chromosomes, and in body cells that number is always even.
The points to remember
- Species differ in their chromosome number: there is great diversity.
- Humans have 46 chromosomes; chimpanzees have 48.
- Body cells are diploid: written 2n.
- Chromosomes come in pairs, so a diploid number is always even.
- Gametes are haploid (one set, n), so they hold half: 23 in a human egg or sperm.
Remember it as: Two of every kind: diploid numbers are even.
Real example: you and a chimpanzee: Your body cells hold 46 chromosomes in 23 pairs. A chimpanzee's hold 48 in 24 pairs, even though the two species share most of their genes.
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The big idea: Chromosome numbers change over evolution: chromosomes can join, split, or whole sets can double.
The points to remember
- Fusion: two chromosomes join, so the number falls (human chromosome 2 matches chimpanzee 12 and 13).
- Fission: a chromosome splits in two, so the number rises.
- Polyploidy: whole sets double, for example when all chromosomes fail to separate in meiosis, or DNA is copied without the cell dividing.
- A change in chromosome number can lead to new species.
1
Fusion
Two join into one: 48 to 46 in the human line.
2
Fission
One splits into two: the number rises.
3
Polyploidy
Whole sets double: common in plants such as wheat.
Polyploidy, not an extra chromosome: One extra chromosome (a trisomy) is not polyploidy. Polyploidy doubles every chromosome set.
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How it is tested: Paper 1B gives a table of chromosome numbers: notice that they are all even. HL Paper 2 can ask how the numbers came to differ.
IB-style questionSuggest[1 mark]
A table gives the chromosome numbers of the chimpanzee (48), dog (78), rice (24) and potato (48). Suggest, giving a reason, whether gametes or body cells were used to obtain these numbers.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
The two numbers to know: Human 46, chimpanzee 48. Other numbers will be given to you.