ABSTRACT
Sexual reproduction differs, in that it involves fusion of cells (gametes), one derived from each parent, to form a zygote. The genetic processes involved in the production of gametes allow for some genetic changes in offspring. Sexual reproduction is limited to species that are diploid or have a period of their life cycle in the diploid state. The production of gametes is referred to as gametogenesis. This may be a complex process involving sexual differentiation and the production of highly differentiated male and female gametes, or in lower eukaryotes identical cells may fuse (isogamy). Whatever the biology of the process, one fact is obvious: gametogenesis must involve a halving of the chromosome number otherwise each succeeding generation would have double the chromosome number of its parents. Halving of chromosome numbers is achieved in a specialized form of cell division, meiosis, that is only observed in gametogenesis. During the process of reducing the number of chromosomes by half the combinations of alleles are rearranged to give recombinant gametes. Two distinct processes are involved. These are independent assortment of chromosomes and crossing-over. These are described below.
