Meiosis & Genetic Variation

Sexual reproduction and the origin of diversity

Meiosis consists of two sequential divisions (Meiosis I and II) that reduce chromosome number from 2n to n. In Meiosis I (reductive division), homologous chromosomes pair up as bivalents during Prophase I. Crossing over (recombination) at chiasmata shuffles alleles between homologs, creating new gene combinations. During Metaphase I, bivalents align randomly (independent assortment). After Meiosis I, two haploid cells result. Meiosis II (equational division) resembles mitosis — sister chromatids separate, producing 4 haploid gametes. Non-disjunction (failure of chromosomes to separate) causes aneuploidy: trisomy 21 (Down syndrome), monosomy X (Turner syndrome), XXY (Klinefelter syndrome).