Meiosis & Sources of Genetic Variation
- Explain how meiosis halves the chromosome number to produce four haploid gametes
- Describe how crossing over and independent assortment generate genetic variation
- Distinguish meiosis I from meiosis II and contrast meiosis with mitosis
Why sexual life needs a halving division
Body cells are diploid (2n): they carry two copies of every chromosome, one inherited from each parent, forming homologous pairs — chromosomes of the same length carrying the same genes (though possibly different alleles). If a sperm and egg simply fused, the chromosome number would double every generation. Meiosis prevents this by cutting the chromosome number in half, producing haploid (n) gametes with a single copy of each chromosome. Fertilization then restores the diploid number, so 2n stays constant across generations.
Two divisions, one round of replication
DNA is copied once, but the cell then divides twice, yielding four haploid cells. Meiosis I is the reduction division: homologous chromosomes pair up and are pulled to opposite poles, so each daughter cell ends up with only one chromosome from each pair — the cell goes from 2n to n here. Meiosis II looks just like mitosis: the sister chromatids of each chromosome finally separate. The end result is four genetically distinct haploid cells from one diploid parent cell.
Three engines of variation
Meiosis does more than count chromosomes — it shuffles them. Crossing over happens in prophase I: homologous chromosomes physically exchange matching segments at a chiasma, creating recombinant chromosomes that mix maternal and paternal alleles on a single chromosome. Independent assortment happens in metaphase I: each homologous pair lines up and orients randomly, independently of every other pair, so maternal and paternal chromosomes are dealt into gametes in every combination. Finally, random fertilization unites two already-unique gametes. Together these make each offspring genetically one of a kind.
An organism has a diploid number of 2n = 8. Ignoring crossing over, how many genetically different gametes can it produce by independent assortment, and how many chromosomes are in each cell right after meiosis I?
- 1.Find the haploid number: n = 8 ÷ 2 = 4, so there are 4 homologous pairs.
- 2.Each pair can orient two ways at metaphase I, and the pairs assort independently, so the number of gamete types is 2ⁿ = 2⁴ = 16.
- 3.Meiosis I is the reduction division: it separates homologous pairs, so each of the two cells receives one chromosome from each pair — that is 4 chromosomes (each still made of two sister chromatids until meiosis II).
What is separated from its partner during meiosis I, and what is separated during meiosis II?
Anchor the whole process on one fact: meiosis I reduces, meiosis II divides. If you can state what separates in each division (homologs, then chromatids), you can reconstruct the chromosome counts at every stage.
Crossing over is an important source of genetic variation. When and how does it occur?
A cell completes mitosis and, separately, an identical cell completes meiosis. How do the products differ?
On free-response questions about "how sexual reproduction increases variation," name all three mechanisms explicitly: crossing over (prophase I), independent assortment (metaphase I), and random fertilization. Listing the mechanism and its stage is what earns the point.
Answer the 3 checkpoints as you read.
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