The New Yeast Is a Mouse

Sexual reproduction depends on a specialized type of cell division called meiosis to generate the sperm and egg cells (gametes) that fuse to form an embryo. Meiosis carries out two important functions: recombination, which generates the diversity on which evolution acts, and reduction of the chromosome number from the full complement (diploid) to half (haploid). Every somatic cell in the human body contains 23 pairs of chromosomes: one set from the mother and one set from the father. When these cells divide, every daughter cell gets one copy of each pair of chromosomes. However, if the gametes contained both sets of chromosomes when they combined during fertilization, the embryo would have twice the normal amount of genetic information. Meiosis (Figure 1) avoids this problem by ensuring that each gamete gets only one copy of each chromosome pair. When the correct partitioning of chromosomes fails (non-disjunction), parental infertility or offspring with an abnormal number of chromosomes result.