Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Correct separation (homologues in meiosis I, sister chromatids in meiosis II) gives each gamete one haploid set that mixes maternal and paternal chromosomes.
- Crossing over (recombination): in prophase I, non-sister chromatids of a homologous pair swap matching segments at chiasmata. Result: recombinant chromatids with new combinations of existing alleles, and sister chromatids that are no longer identical.
- Independent assortment: each homologous pair lines up at metaphase I independently of the others, so whole chromosomes are shuffled into gametes.
- Random fertilization: any gamete from one parent can fuse with any gamete from the other.
- Nondisjunction: homologues (meiosis I) or sister chromatids (meiosis II) fail to separate, giving gametes that are not haploid. Fertilization then gives 2n + 1 (trisomy) or 2n − 1 (monosomy) zygotes.
Key relationships
| Quantity | Rule | Human value (n = 23) |
|---|---|---|
| Gamete combinations (no crossing over) | 2ⁿ | 2²³ ≈ 8.4 million |
| Zygote combinations from one couple | 2ⁿ × 2ⁿ | 2⁴⁶ ≈ 7.0 × 10¹³ |
| Probability a gamete has all maternal chromosomes | (1/2)ⁿ | 1 in 2²³ |
| Nondisjunction in | Gametes from one cell |
|---|---|
| Meiosis I | n + 1, n + 1, n − 1, n − 1 |
| Meiosis II (one cell) | n, n, n + 1, n − 1 |
Assumptions behind the numbers
- 2ⁿ ignores crossing over, so the real number of genetically different gametes is far larger.
- Homologues are assumed to differ in at least some alleles; otherwise shuffling them would change nothing.
- Details of different organisms’ sexual life cycles are outside the course; the rules apply to the chromosomes.
Mistakes to avoid
- Saying crossing over happens between sister chromatids.
- Saying crossing over creates new alleles.
- Using 2n or n² instead of 2ⁿ.
- Placing independent assortment in metaphase II.
- Giving the same gamete outcome for nondisjunction in meiosis I and II.
- Forgetting that fertilization adds variation as well as restoring 2n.
Quick self-check
- A species has 2n = 12. How many chromosome combinations can its gametes have, ignoring crossing over? (n = 6, so 2⁶ = 64)
- In the same species, sister chromatids fail to separate in one cell in meiosis II. Chromosome numbers of the four gametes? (6, 6, 7, 5)
- Why can all four products of one meiosis differ genetically? (Crossing over makes sister chromatids non-identical, so the two cells of meiosis II receive different chromatids.)
Next: practice questions.