AP Biology · Lesson 2 of 6 · Unit 5

Meiosis and genetic diversity: crossing over, assortment and nondisjunction

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Meiosis does more than halve the chromosome number; it reshuffles maternal and paternal alleles to generate the genetic variation that natural selection acts upon. On the AP Biology exam (Topic 5.2), you will analyze models of chromosome segregation, predict the exact outcomes of nondisjunction in meiosis I versus meiosis II, and formulate testable questions about how recombination and assortment produce diverse offspring.

By the end you’ll be able to

Explain how correct separation of homologous chromosomes in meiosis I and sister chromatids in meiosis II gives each gamete a haploid set of maternal and paternal chromosomes.
Predict the chromosome numbers of gametes and offspring when nondisjunction occurs in meiosis I or meiosis II.
Explain how crossing over between non-sister chromatids in prophase I increases genetic diversity among gametes.

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Core ideas (continued)
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What the exam asks

Section I (Multiple Choice): Stimulus sets often show diagrams of chromosomes in prophase I or metaphase I, or karyotypes of gametes, asking you to identify how recombinant chromosomes formed or which division failed during nondisjunction.
Section II (Free Response): Frequently tested in Q4 (Conceptual Analysis) and Q5 (Analyze Model or Visual Representation). You may be asked to explain how a specific meiotic event increases genetic diversity, predict the chromosome number in gametes or zygotes after a chemical disrupts spindle fibers, or pose a testable question based on a model of chromosome behavior.

Core ideas

Correct separation and the three sources of genetic diversity

In a diploid ( $2n$ ) germ cell, chromosomes exist in homologous pairs—one inherited from the maternal parent and one from the paternal parent. Accurate chromosome segregation requires two distinct divisions:

Meiosis I (Reductional Division): Homologous chromosomes pair up in prophase I (forming tetrads) and separate during anaphase I. Each resulting cell is haploid ( $1n$ ) because it contains only one chromosome from each homologous pair, though each chromosome still consists of two sister chromatids.
Meiosis II (Equational Division): Sister chromatids separate during anaphase II, yielding four genetically distinct haploid ( $1n$ ) gametes.

Sexual reproduction generates genetic variation through three primary mechanisms:

Mechanism	When it occurs	How it increases genetic diversity
Crossing over	Prophase I	Non-sister chromatids of homologous chromosomes exchange corresponding DNA segments at chiasmata, creating recombinant chromatids with new combinations of maternal and paternal alleles on the same chromosome.
Independent (random) assortment	Metaphase I / Anaphase I	Each homologous pair aligns randomly on the metaphase plate independently of other pairs. Each gamete receives a random mix of maternal and paternal chromosomes ( $2^n$ possible combinations, where $n$ is the haploid number).
Random fertilization	Conception	Any genetically unique sperm ( possibilities, plus crossover variations) can fuse with any genetically unique egg, multiplying the potential diploid combinations ().