Lesson
Meiosis makes variation on purpose
One diploid cell becomes haploid gametes that are not carbon copies.
Learning goals
- Explain how meiosis I halves chromosome number.
- Name two meiotic events that create new combinations.
Explanation
Meiosis has two divisions. In the first, homologous partners separate, so the products are haploid. In the second, sister chromatids separate. Fertilization restores the diploid number. The math is the point: without the reduction, chromosome number would double every generation.
Crossing over swaps pieces between homologs. Independent assortment lines up maternal and paternal chromosomes in different mixes. Mutation adds new alleles over longer time. Together they make siblings different even when they share parents.
Key terms
- Homologous chromosomes. Partner chromosomes that carry the same genes, possibly with different alleles.
- Crossing over. Exchange of corresponding segments between homologous chromosomes during meiosis.
Common mistakes
- Saying meiosis “creates DNA” rather than reshuffling and reducing it.
- Drawing crossing over as a random chop between non-partners.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Why is independent assortment a source of variation only when the parents are heterozygous at more than one gene?
Answer. If both homologs carry identical alleles, shuffling them does not change the allele mix in the gamete.
Assortment rearranges existing differences; it does not invent them.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
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AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
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OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
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Open source