IB Biology HLClassifying Living DiversityPaper 1 & 2~10 min read
Chromosome Number: Cross-Breeding
A horse and a donkey can produce a mule. The mule is healthy, strong and completely unable to have offspring of its own. The reason is arithmetic: 32 chromosomes from one parent, 31 from the other, and one chromosome left with nothing to pair with.
📚 What you need to know
With rare exceptions, every individual in a species has the same chromosome number: human 46, chimpanzee 48, dog 78.
The diploid (2n) number is halved by meiosis to give haploid (n) gametes: human 23, chimpanzee 24, dog 39.
At fertilisation the nuclei of two (n) gametes fuse to give a (2n) zygote.
Both gametes must contain the same number of chromosomes for the zygote to be viable.
Because species have different chromosome numbers, their gametes do too, so different species usually cannot produce fertile offspring.
Three things can go wrong: the gametes cannot fuse; fusion gives a non-viable zygote; or a viable zygote forms but the diploid cells have an uneven number of chromosomes and cannot carry out meiosis.
The third case produces an infertile interspecific hybrid.
The mule is the standard example: horse 2n = 64, donkey 2n = 62, mule 2n = 63.
Numbers within and between species
Chromosome number is fixed within a species and different between species. That single fact does most of the work on this page.
Species
Body cells (2n)
Gametes (n)
Human
46
23
Chimpanzee
48
24
Dog
78
39
Horse
64
32
Donkey
62
31
Meiosis halves the diploid number to make gametes. Fertilisation adds two haploid sets back together. As long as both gametes carry the same number, the zygote ends up with a full set of matching pairs. If a zygote has too many or too few chromosomes, it may not survive.
What goes wrong between species
Even when two species are closely related enough to mate, they usually cannot produce fertile offspring. There are three separate failure points, and questions often want you to distinguish them.
Where it fails
What happens
At fusion
The gametes are unable to fuse at all
At the zygote
Fusion happens but does not lead to a viable zygote, because of the uneven number of chromosomes
At meiosis, one generation later
A viable zygote forms and grows into an animal, but its diploid cells contain an uneven number of chromosomes and cannot carry out meiosis. The result is an infertile interspecific hybrid
The mule itself is perfectly healthy. Every one of its body cells is made by mitosis, which copies chromosomes rather than pairing them — the problem only appears when meiosis is needed.
Following the mule through
Horses have 64 chromosomes, so their gametes carry 32.
Donkeys have 62 chromosomes, so their gametes carry 31.
Fusion gives a mule zygote with 63 chromosomes.
63 is an odd number, so the chromosomes cannot all pair up during meiosis, and the mule cannot produce gametes of its own.
The key is that meiosis needs homologous pairs. Every chromosome must find its matching partner and line up before the cell divides. With 63 chromosomes there is always one left over with nothing to pair with, so the process stalls. Mitosis has no such requirement, which is why the mule grows normally.
🧠
Odd number, no meiosis
Whenever a hybrid comes out with an odd diploid number, the answer to “why is it infertile?” is always the same: the chromosomes cannot form complete homologous pairs, so meiosis fails.
Tie it back to the species definition. This is exactly why the biological species concept says fertile offspring. A horse and a donkey can produce offspring, so the loose version of the test would make them one species. The mule’s infertility is what keeps them separate.
Worked examples
WE 1
Calculate a hybrid chromosome number
A species of deer has 70 chromosomes and a related species has 68. Calculate the chromosome number of a hybrid produced by crossing them, and predict whether it would be fertile. (3 marks)
Step 1: the gametes
70 ÷ 2 = 35 and 68 ÷ 2 = 34
Step 2: the zygote
35 + 34 = 69 chromosomes
Step 3: the prediction
69 is odd, so the chromosomes cannot form complete homologous pairs and meiosis cannot take place, making the hybrid infertile.
2n = 69, and infertilethe prediction needs the pairing reason, not just the word “odd”
WE 2
Explain why a mule is infertile
Explain why a mule cannot produce offspring, even though it is healthy. (3 marks)
Point 1: where it came from
A horse gamete carries 32 chromosomes and a donkey gamete 31, so a mule has 63 chromosomes.
Point 2: the problem
63 is an uneven number, so the chromosomes cannot all pair up into homologous pairs during meiosis.
Point 3: the consequence
Meiosis cannot be completed, so no gametes are produced, and the mule is an infertile interspecific hybrid. Growth is unaffected because body cells are produced by mitosis.
No pairing, no meiosis, no gametesthe phrase “infertile interspecific hybrid” is worth using by name
WE 3
Give the reasons a cross fails
Outline three reasons why two different species are usually unable to produce fertile offspring together. (3 marks)
Reason 1
The gametes may be unable to fuse at all.
Reason 2
Fusion may not produce a viable zygote, because of the uneven number of chromosomes.
Reason 3
A viable zygote may form, but its diploid cells contain an uneven number of chromosomes and cannot carry out meiosis, giving an infertile hybrid.
No fusion, no viable zygote, or no meiosis later onthese are three separate failure points — do not merge them into one answer
💡 Exam tips
Show the halving and adding: n from each parent, summed to give the hybrid’s 2n.
Explain infertility through homologous pairing failing in meiosis.
Use the term infertile interspecific hybrid.
Point out that mitosis is unaffected, which is why the hybrid grows normally.
Learn the mule numbers: 64, 62, 63.
Connect back to the word fertile in the biological species concept.
⚠ Common mistakes
Saying a mule is unhealthy. It is perfectly healthy and simply cannot reproduce.
Blaming mitosis. Mitosis works fine; meiosis is what fails.
Saying the mule has no chromosomes in pairs at all. Most pair up; the odd number leaves one without a partner.
Giving the mule 126 or 32 chromosomes. It is 32 + 31 = 63.
Treating “cannot mate” and “cannot produce fertile offspring” as the same thing. Horses and donkeys mate perfectly well.
Forgetting that this is why species remain separate.
Up next: Dichotomous Keys (Skills) — how to identify an organism in the field when you have no idea what it is.
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