Meiosis & Genetic Variation

Meiosis I — reductional (separates homologs)

• Prophase I: chromosomes condense; homologs pair (synapsis) forming tetrads (bivalents); crossing over occurs at chiasmata. Major source of genetic variation.
• Metaphase I: tetrads line up at the metaphase plate; independent assortment of homologous pairs — each pair orients independently → 2ⁿ combinations of maternal/paternal chromosomes (2²³ ≈ 8.4 million in humans, before crossing over).
• Anaphase I: homologous chromosomes separate; sister chromatids stay together.
• Telophase I / Cytokinesis: two haploid cells, each chromosome still as sister chromatid pair.

Meiosis II — equational (separates sister chromatids)

• Resembles mitosis. No DNA replication occurs between Meiosis I and II.
• Sister chromatids separate at anaphase II.
• Result: 4 haploid cells, each genetically distinct.

Mitosis vs. Meiosis (high-yield comparison)

Feature	Mitosis	Meiosis
Divisions	1	2
Daughter cells	2	4
Ploidy	2n → 2n	2n → n
Synapsis / crossing over	No	Yes (Prophase I)
Genetic identity	Identical to parent	Genetically unique
Purpose	Growth, repair, asexual reproduction	Gamete formation

Sources of genetic variation

1. Crossing over (Prophase I) — exchanges segments between homologs at chiasmata.
2. Independent assortment (Metaphase I) — random orientation of tetrads.
3. Random fertilization — any sperm × any egg.
4. Mutation — the ultimate source of new alleles. The other three only shuffle existing variation.

Linked vs. unlinked genes

• Unlinked (different chromosomes): independent assortment — predicted dihybrid 9:3:3:1.
• Linked (same chromosome): tend to inherit together; recombination only occurs via crossing over. Recombination frequency (RF) ≤ 50%; 1% RF ≈ 1 map unit (centimorgan). Closer genes → fewer recombinants.

Nondisjunction

• Failure of chromosomes (Meiosis I) or sister chromatids (Meiosis II) to separate.
• Results in aneuploidy: one gamete has n+1 chromosomes, another has n−1.
• Examples in humans: Trisomy 21 (Down syndrome), XXY (Klinefelter), XO (Turner), XXX, XYY.

Example questions

MCQ During which phase do tetrads form and crossing over occur? (A) Prophase I (B) Metaphase I (C) Prophase II (D) Anaphase II

Answer: A. Synapsis pairs homologs into tetrads (bivalents) during Prophase I; crossing over occurs at the chiasmata that form between non-sister chromatids of homologous chromosomes.

FRQ Explain how meiosis generates genetic variation, and describe how this variation contributes to evolution by natural selection.

Answer: Meiosis generates variation through (1) crossing over in Prophase I, which recombines alleles between homologs; (2) independent assortment in Metaphase I, in which each homologous pair orients randomly, producing 2ⁿ combinations of maternal/paternal chromosomes; and (3) random fertilization, which combines two genetically unique gametes. This raw variation supplies the heritable phenotypic differences on which natural selection acts: individuals whose variants confer higher fitness reproduce more, gradually changing allele frequencies in the population over generations.

MCQ Two genes are 18 map units apart. From a test cross, what percent recombinant offspring are expected? (A) 9% (B) 18% (C) 36% (D) 50%

Answer: B. A map unit (centimorgan) is defined as 1% recombination, so genes 18 map units apart yield 18% recombinant offspring in a test cross.

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