Meiosis and variation
| English | Chinese | Pinyin |
|---|---|---|
| diploid | 二倍体 | èr bèi tǐ |
| haploid | 单倍体 | dān bèi tǐ |
| gamete | 配子 | pèi zi |
| meiosis | 减数分裂 | jiǎn shù fēn liè |
| homologous | 同源 | tóng yuán |
| allele | 等位基因 | děng wèi jī yīn |
| crossing over | 交叉互换 | jiāo chā hù huàn |
| independent assortment | 自由组合 | zì yóu zǔ hé |
| fertilisation | 受精 | shòu jīng |
Shuffling the genetic deck
- A diploid 二倍体 cell (2n) has two sets of chromosomes; a haploid 单倍体 cell (n) has one.
- Gametes 配子 (sex cells) must be haploid — or the number would double each generation.
- They are made by meiosis 减数分裂, which also creates variation.
Why must gametes be haploid?
If gametes were diploid, fertilisation would double the chromosome number each generation; meiosis halves it.
Meiosis halves the chromosome number, producing ______ gametes (one set of chromosomes each).
Two haploid gametes fuse at fertilisation to restore the diploid number.
Homologous 同源 pairs and meiosis
- Chromosomes come in homologous pairs: same length, same genes — but the alleles 等位基因 (versions) may differ.
- Meiosis halves the chromosome number, in two divisions (eight stages: prophase I → telophase II).
Meiosis runs two divisions, halving the chromosome number from a diploid cell to four haploid gametes
A human egg cell (ovum), one of the gametes produced by meiosis.
Meiosis
Step through it. Two divisions halve the chromosome number and shuffle the alleles, giving four unique gametes.
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 交叉互换: homologous chromosomes swap matching pieces, mixing alleles.
Crossing over swaps matching segments between homologous chromosomes, making new allele combinations
- Independent assortment 自由组合: the pairs line up in a random order, so each gamete gets a random mix.
Independent assortment: each homologous pair lines up at random, so many different gamete types form
A human karyotype: 23 pairs of chromosomes, ending in XY (male)
Crossing over creates variation by:
At a chiasma, homologous chromosomes exchange matching pieces, producing new allele combinations.
Independent assortment means each homologous pair lines up in a random order, so gametes get a random mix of chromosomes.
The random orientation of each pair at metaphase I gives an enormous number of possible gamete combinations.
Match each source of variation to what it does.
Meiosis (crossing over + independent assortment) and random fertilisation shuffle existing alleles; mutation creates new ones.
Random fertilisation 受精
- At fertilisation, any gamete can fuse with any other.
- This random fusion makes every new individual genetically different.
Random fertilisation adds to variation because:
Which two gametes meet is random, so each new combination of alleles is unique.
You've got it
- 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