減数分裂と変異

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.

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.

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)

Random fertilisation 受精

• At fertilisation, any gamete can fuse with any other.
• This random fusion makes every new individual genetically different.

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