5.2 Meiosis and Genetic Diversity

01Three sources of variation

Crossing over: homologs exchange DNA, creating recombinant chromosomes.

Independent assortment: homolog pairs line up randomly in metaphase I. Humans can make 2²³ ≈ 8.4 million combinations from this alone.

Random fertilization: any sperm can fuse with any egg: 8.4 million × 8.4 million ≈ 70 trillion combinations, before crossing over.

02Why it matters

Variation is the raw material for natural selection. Sexually reproducing populations adapt faster to changing environments and pathogens than clones.

03Notebook box

Number of combinations from independent assortment = 2ⁿ (n = haploid number).

Worked example: An organism has 2n = 6. How many chromosome combinations in its gametes (ignoring crossing over)? → n = 3, so 2³ = 8.

Key takeaways

✦Crossing over, independent assortment, random fertilization.
✦Combinations = 2ⁿ.
✦Variation fuels evolution.

Watch outUse the HAPLOID number in 2ⁿ, not the diploid number.

Quick check

Did it stick?

1.Random orientation of homolog pairs in metaphase I is called…

2.2n = 10. Gamete combinations from independent assortment?

3.Which does NOT increase genetic variation?