The big idea: Meiosis makes every gamete genetically different in two ways: crossing over and the random orientation of bivalents.
The points to remember
- A bivalent is a pair of homologous chromosomes lying together in meiosis I.
- Crossing over (prophase I): non-sister chromatids swap sections, making new combinations of alleles on a chromosome.
- Random orientation (metaphase I): each bivalent faces either pole, independently of the others.
- So gametes get different mixes of chromosomes from the mother and the father.
- With n pairs there are 2 to the power n possible combinations, before crossing over.
Remember it as: Swap sections, then shuffle the pairs.
Real example: brothers and sisters. Brothers and sisters have the same two parents, yet (unless they are identical twins) they look different, because each came from a different egg and sperm, made with different shuffles in meiosis.
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Two sources, compared: Both happen in the first division of meiosis, and both are random.
| Crossing over | Random orientation | |
|---|---|---|
| When | Prophase I | Metaphase I |
| What happens | Non-sister chromatids swap sections | Each bivalent lines up facing either pole |
| What it mixes | Alleles along one chromosome | Whole chromosomes from each parent |
| Result | Recombinant chromatids | Different chromosome mixes in gametes |
And then fertilisation: Meiosis makes varied gametes; random fertilisation of any egg by any sperm from two different parents adds still more variation.
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How it is asked: A 1A item on what increases variation (random orientation of bivalents, not mitosis or asexual reproduction), or a long Paper 2 answer on how meiosis leads to variation.
IB-style questionExplain[4 marks]
Explain how meiosis produces gametes that are genetically different.
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See the answer broken into clear points, with guidance on how each point earns marks.