IB Biology HLClassifying Living DiversityPaper 1 & 2~10 min read
Chromosome Number
Humans have 46 chromosomes. Chimpanzees have 48. A horse threadworm manages on 4. There is no ladder here — the number tells you nothing about how advanced an organism is, but it does tell you a great deal about who can breed with whom.
📚 What you need to know
A diploid cell (2n) has two complete sets of chromosomes. Adult body cells are usually diploid.
A haploid cell (n) has one complete set — half the number in a body cell. These are usually gametes.
At fertilisation, the nuclei of two haploid gametes fuse to form a diploid zygote.
Both gametes must have the same number of chromosomes for the zygote to be viable.
Every body cell from that zygote has the same chromosome number. Red blood cells are an exception, since they have no nucleus.
Chromosome number varies between species but is the same within a species, apart from rare mutations.
The diploid number is always even, because it must divide by two to give a whole haploid number.
Chromosome number is not linked to complexity. You need to know human 46 and chimpanzee 48, and no others.
Diploid and haploid
Your chromosomes come in matching pairs — one of each pair from your mother, one from your father. Two complete sets means diploid, written 2n. One complete set means haploid, written n.
Gametes have to be haploid, and the reason is arithmetic. If a full set met a full set at fertilisation, the chromosome number would double every generation. Halving it first keeps it stable.
Trace the loop and the arithmetic holds: 2n halved to n, two n added back to 2n, then copied unchanged into every body cell.
Numbers across species
The number of chromosomes a species has depends on changes that happened during its evolution. Within a species it is fixed — every individual has the same number, apart from rare cases where a chromosome mutation has occurred.
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
Two things follow from that table.
The diploid number is always even. It has to divide by two to give a whole haploid number.
The number says nothing about how advanced an organism is. Rice has 24 and a chimpanzee has 48. A parasitic worm gets by on 4.
Be precise about which number you are giving. Three different numbers get confused constantly: the number of chromosomes in a diploid cell (46 in humans), the number of pairs in a diploid cell (23), and the number in a haploid cell (23). The last two are the same value, which is exactly why students mix them up. Always state which one you mean.
🧠
Only two numbers to memorise
Human 46, chimpanzee 48. Any other chromosome number will be given to you in the question, so do not waste effort learning them.
Why the number matters
Differences in chromosome number are one reason organisms from different species cannot breed together successfully. Two gametes with mismatched numbers either fail to produce a viable zygote, or produce one carrying an uneven set of chromosomes that causes trouble later.
That is the thread picked up in a later page on cross-breeding. For now, hold on to the rule: both gametes must carry the same number of chromosomes for the zygote to work.
The red blood cell exception is worth remembering because it looks like a contradiction. Every body cell that arises from the zygote has the same chromosome number — except mature red blood cells, which lose their nucleus entirely and so contain no chromosomes at all. It is an exception to the rule, not a problem with it.
Worked examples
WE 1
Work out a haploid number
A species of frog has a diploid chromosome number of 26. State the number of chromosomes in its gametes and explain your answer. (2 marks)
Step 1: the calculation
26 ÷ 2 = 13Step 2: the reason
Gametes are haploid, containing one complete set of chromosomes, so they carry half the diploid number.
13 chromosomes per gametethe mark for “explain” is the word haploid plus the reason, not the number
WE 2
Spot an impossible value
A student states that an insect has a diploid chromosome number of 17. Explain why this cannot be correct. (2 marks)
Point 1: the rule
A diploid cell contains two complete sets of chromosomes, so the diploid number must be divisible by two.
Point 2: applying it
17 is odd, so it cannot be halved to give a whole haploid number, and the chromosomes could not form complete pairs.
A diploid number must always be evenlink it back to pairing — that is what makes an odd number impossible
WE 3
Interpret a set of chromosome numbers
A student notices that rice has 24 chromosomes while a chimpanzee has 48, and concludes that chimpanzees are twice as complex as rice. Evaluate this conclusion. (3 marks)
Point 1: what the data shows
The numbers are correct, and they do show that chromosome number varies between species.
Point 2: why the conclusion fails
Chromosome number is not linked to how advanced or complex a species is; it depends on changes during that species’ evolution.
Point 3: a counter-example
The horse threadworm has only 4 chromosomes, and other species with far more are not more complex, so no such relationship exists.
The numbers differ, but they say nothing about complexitya counter-example is the quickest way to break a claimed pattern
💡 Exam tips
Write 2n for diploid and n for haploid, and say which you mean.
Learn 46 for humans and 48 for chimpanzees. Nothing else.
Say gametes are haploid and are produced by meiosis.
Remember the diploid number is always even.
Never suggest chromosome number reflects complexity.
Mention red blood cells as the exception to “every body cell has the same number”.
⚠ Common mistakes
Confusing the number of pairs with the diploid number. Humans have 46 chromosomes in 23 pairs.
Saying gametes are diploid. They are haploid, or the number would double each generation.
Giving an odd diploid number. Impossible, since chromosomes come in pairs.
Thinking more chromosomes means more advanced. There is no such link.
Forgetting that a zygote needs matching gametes. Mismatched numbers cause problems.
Saying every human cell has 46 chromosomes. Gametes have 23, and red blood cells have none.
Up next: Karyograms (Skills) — how those chromosomes are photographed and sorted, and what they reveal about human and chimpanzee ancestry.
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