IB Biology SLTopic 3 — Classifying Living DiversityPaper 1 & 2Core idea~9 min read
Chromosome Number
Every species has its own chromosome number, and it never varies between healthy individuals of that species. Understanding why comes down to one simple idea: what happens when two gametes fuse.
📚 What you need to know
Diploid (2n) cells contain two complete sets of chromosomes. Body cells are usually diploid.
Haploid (n) cells contain one complete set — half the number. Gametes are haploid.
At fertilisation, two haploid nuclei fuse to form a diploid zygote.
Every body cell descended from that zygote has the same chromosome number.
Chromosome number varies between species but is fixed within a species.
The diploid number is always even, because it must divide evenly into two sets.
Learn humans = 46 and chimpanzees = 48. No other numbers need memorising.
Chromosome number is not a measure of how complex or advanced a species is.
Diploid and haploid
A diploid cell has two of everything — one set of chromosomes from each parent. Written 2n. Almost all your body cells are like this.
A haploid cell has one set only. Written n. In animals the haploid cells are the gametes: the egg and the sperm.
Both gametes must carry the same number of chromosomes, otherwise the zygote would not have matching pairs and would not be viable.
Notice the logic runs backwards from the zygote. The zygote must end up diploid, so each gamete has to supply exactly one full set. That is the reason meiosis has to halve the number in the first place.
Chromosome number varies between species
The number of chromosomes a species has depends on what happened during its evolutionary history — chromosomes have fused, split and duplicated over long stretches of time.
Species
Diploid number (2n)
Haploid number (n)
Human (Homo sapiens)
46
23
Chimpanzee (Pan troglodytes)
48
24
Rice (Oryza sativa)
24
12
Horse threadworm (Parascaris equorum)
4
2
A worm with four chromosomes, a plant with twenty-four. Chromosome number tells you nothing about how “advanced” an organism is. It is a record of that species’ history, not a ranking.
Three numbers people confuse
When a question gives you a chromosome number, be very clear which one it means:
The number of chromosomes in a diploid cell — for humans, 46.
The number of pairs in a diploid cell — for humans, 23. This is the same as the haploid number.
The number of chromosomes in a haploid cell — for humans, 23.
Differences in chromosome number are one reason members of different species usually cannot breed together successfully: the chromosomes have no matching partners to pair with.
One exception worth knowing. Mature red blood cells have no nucleus, so they contain no chromosomes at all. They are still human cells — they just gave up their DNA to make room for haemoglobin.
Worked examples
WORKED EXAMPLE
A chimpanzee body cell contains 48 chromosomes. State the number of chromosomes in a chimpanzee sperm cell, and the number of pairs in a body cell. [2]
Step 1: a sperm cell is haploid48 ÷ 2 = 24 chromosomesStep 2: pairs in a diploid cellthe number of pairs is the same as the haploid number48 ÷ 2 = 24 pairs24 chromosomes in the sperm; 24 pairs in a body cell
WORKED EXAMPLE
A student claims a newly discovered species has a diploid chromosome number of 17. Explain why this is not possible. [2]
Step 1: what diploid means
A diploid cell contains two complete sets of chromosomes.
Step 2: test the number17 ÷ 2 = 8.5, which is not a whole numberThe diploid number must be even, so 17 cannot be a diploid numberan odd diploid number would mean at least one chromosome with no partner
WORKED EXAMPLE
Suggest why organisms of two different species with different chromosome numbers are usually unable to breed successfully. [2]
Step 1: what happens at fertilisation
The gametes carry different numbers of chromosomes, so the zygote would not have matching pairs.
Step 2: consequence
Chromosomes cannot pair up properly during cell division, so the offspring is not viable, or is sterile if it survives.
No matching pairs = no viable or fertile offspring
💡 Exam tip
Memorise only 46 (human) and 48 (chimpanzee) — other numbers are always given in the question.
Read carefully whether the question wants chromosomes or pairs. They are different numbers.
Use the symbols properly: 2n for diploid, n for haploid.
If asked why the diploid number is even, say it must divide by two to give a whole haploid number.
Never say a species is more advanced because it has more chromosomes.
Every body cell from one zygote has the same chromosome number — a common one-mark statement.
⚠ Common mix-up
Saying humans have 23 chromosomes. We have 46 chromosomes in 23 pairs.
Confusing haploid with “half a chromosome”. Haploid means half the number, not half of each one.
Assuming all gametes are haploid in every organism. Some unusual species have haploid stages elsewhere in the life cycle.
Linking chromosome number to complexity. A rice plant has more pairs than a chimpanzee.
Forgetting that red blood cells have none. It is the classic exception question.
Writing that fertilisation halves the number. Meiosis halves it; fertilisation doubles it back.
Up next: Karyograms (Skills) — how chromosomes are photographed, sorted and read, and what they reveal about how humans and chimpanzees diverged.
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