The big idea: Chromosome number is a shared trait: all members of a species, and all of an organism's body cells, have the same number.
The points to remember
- Members of a species share a characteristic chromosome number: humans 46, dogs 78.
- Every body cell of an organism has that same number: root cells and leaf cells alike.
- Gametes hold half (one set); fertilization restores the full number.
- So the number is one more trait shared within a species.
Remember it as: Same species, same count; same body, same count.
So one count from any body cell tells you the count in all the others.
Real example: bluebells: The British bluebell, Hyacinthoides non-scripta, has 16 chromosomes in its root cells, so its leaf and petal cells have 16 too.
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The big idea: Closely related species with different chromosome numbers rarely give fertile offspring.
The points to remember
- A hybrid gets one set from each parent: horse 32 + donkey 31 = 63 in a mule.
- In meiosis, chromosomes must line up in matching pairs.
- With sets that do not match, they cannot all pair, so gametes fail.
- So the hybrid is sterile, and the parents stay separate species.
Real example: horse and donkey: Horses have 64 chromosomes and donkeys 62. Their mule offspring has 63 and is almost always sterile.
Matching numbers are needed, but they are not always enough:
Same number helps, it does not guarantee: Matching numbers make fertile hybrids possible, not certain: the gene order must match too.
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How it is tested: Paper 2 gives the chromosome number of one cell type and asks for another, or asks why a hybrid is sterile.
IB-style questionState[1 mark]
Cells from the tip of a garlic root have 16 chromosomes. State the number of chromosomes in a garlic leaf cell.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Do not halve it: Leaf and root cells are both body cells. Halve only for gametes.