Three shuffles: crossing over in prophase I, independent assortment in metaphase I, random fertilization after.
Meiosis doesn't invent new alleles. It reshuffles the ones that exist. Crossing over, independent assortment, and random fertilization deal out combinations neither parent had, which is why siblings differ.
Key terms
- Crossing over — in prophase I, non-sister chromatids of a homologous pair trade matching segments. It mixes alleles within a chromosome.
- Independent assortment — at metaphase I, each homolog pair orients randomly, regardless of the others. It mixes whole chromosomes.
- Random fertilization — any of one parent's gamete types can meet any of the other's.
- Recombinant — a chromatid or gamete with an allele combination that wasn't on either parental chromosome.
- Nondisjunction — homologs or sister chromatids failing to separate, so gametes end up with an extra or missing chromosome.
Where the variety comes from
- Prophase I: homologs pair and cross over. One exchange involves one chromatid from each homolog, making two recombinant chromatids and leaving two parental ones.
- Metaphase I: each pair lines up at random, with its maternal chromosome facing either pole. That's a coin flip per pair: n pairs give 2ⁿ chromosome combinations.
- Anaphase I: homologs separate, so each cell gets a mix of maternal and paternal chromosomes.
- Anaphase II: sister chromatids separate. After crossing over, sisters no longer fully match, so all four products can differ.
- Fertilization: any egg can meet any sperm, so the numbers multiply. Humans: 2²³ ≈ 8.4 million combinations per parent → about 70 trillion possible zygotes, before crossing over.
Why it matters
Natural selection needs variety to act on. In sexual species, meiosis and fertilization produce fresh allele combinations every generation. Crossing over also sets up gene mapping: the farther apart two genes sit on a chromosome, the more often an exchange separates them. Expect to count combinations, spot recombinants in gamete data, or work out which division an error happened in.
Exam traps
| Trap | Actually |
|---|
| Meiosis creates new alleles. | Only mutation makes new alleles. Meiosis and fertilization make new combinations of alleles that already exist. |
| Crossing over swaps segments between sister chromatids. | It swaps between non-sister chromatids of homologs. Sisters are identical copies, so trading between them would change nothing. |
| Each recombinant gamete needs its own crossover. | One exchange is reciprocal. It makes two recombinant chromatids at once and leaves the other two parental. |
| A nondisjunction in meiosis II makes all four gametes abnormal. | Only two: one gains an extra copy, the other lacks it. A meiosis I error, before the cells split, affects all four. |
Worked example
A man inherited 23 chromosomes from his mother and 23 from his father. Ignoring crossing over, what fraction of his sperm carry only chromosomes he got from his mother? A classmate says 1/2. Test that claim.
- Each homolog pair orients on its own at metaphase I. For any one pair, a given sperm gets the maternal member with probability 1/2.
- The 23 pairs are independent, so multiply: 1/2 × 1/2 × … for all 23 pairs = (1/2)²³.
- 2²³ is about 8.4 million, so the answer is about 1 sperm in 8.4 million.
About 1 in 8.4 million of his sperm, not 1 in 2.
The classmate pictures the maternal chromosomes traveling as one bundle. They don't: each pair is its own coin flip, and that independence is where the 2ⁿ variety comes from. Add crossing over, and a single chromosome is often a patchwork of both his parents' DNA, so an all-maternal sperm is rarer still.