Meiosis and variation
Meiosis and variation
- A diploid cell (2n) has two sets of chromosomes; a haploid cell (n) has one.
- Gametes (sex cells) must be haploid — or the number would double each generation.
- They are made by meiosis, which also creates variation.
Practice
Why must gametes be haploid?
If gametes were diploid, fertilisation would double the chromosome number each generation; meiosis halves it.
Homologous pairs and meiosis
- Chromosomes come in homologous pairs: same length, same genes — but the alleles (versions) may differ.
- Meiosis halves the chromosome number, in two divisions (eight stages: prophase I → telophase II).
Practice
A homologous pair of chromosomes:
Homologous chromosomes match in length and gene positions, though they may carry different alleles.
Two sources of variation
- Crossing over: homologous chromosomes swap matching pieces, mixing alleles.
- Independent assortment: the pairs line up in a random order, so each gamete gets a random mix.
Practice
Crossing over creates variation by:
At a chiasma, homologous chromosomes exchange matching pieces, producing new allele combinations.
Practice
Independent assortment means that:
The random orientation of each pair at metaphase gives many different gamete combinations.
Random fertilisation
- At fertilisation, any gamete can fuse with any other.
- This random fusion makes every new individual genetically different.
Practice
Random fertilisation adds to variation because:
Which two gametes meet is random, so each new combination of alleles is unique.
You've got it
Key idea
- diploid (2n) = two sets; haploid (n) = one; gametes must be haploid
- homologous pairs: same genes, possibly different alleles; meiosis halves the number
- variation from crossing over + independent assortment
- plus random fertilisation → every individual is unique