Overview

The essence of sex is alternation between a haploid phase, when cell nuclei possess one set of chromosomes, and a diploid phase, when they possess two. Each set of chromosomes is homologous and usually nonidentical, having come from unrelated haploid sex cells. The transition from diploid to haploid requires meiosis, a process in which precisely one copy of each chromosome is apportioned to each haploid cell.

The events of meiosis determine quantitative, predictable patterns of genetic transmission from parent to offspring in sexual species. Two hallmarks of meiosis are the 1:1 segregation of gene copies and recombination of genes and chromosomes. In recombination, chromosomes and chromosome segments shuffle to make a virtually limitless number of new genetic combinations.

The first part of the chapter describes meiosis as a formal dance of chromosomes. The second part of the chapter explains the genetic consequences of meiosis – the segregation of homologous chromosomes and recombination. The third part of the chapter describes exceptional patterns of meiosis.

Recap of Ploidy and DNA Content

To recap what was explained in Chapter 19, for any eukaryon with sexual reproduction the haploid number of nuclear chromosomes is N and the diploid number is 2N; in a diploid cell the two sets of chromosomes are nonidentical and homologous. In asexually reproducing cells, chromosome number is constant through the life cycle. In sexual organisms, gametes (sperm and eggs, or their equivalents) are haploid, while zygotes (cells formed by the union of gametes) are diploid.