The process of sexual reproduction involves two parents, each contributing one gamete. Gametes are produced. by a process called meiosis, which starts by the duplication of the chromosomes, followed by two rounds of cell divisions and halving of the chromosome number.Gametes have half the chromosome number of other adult. cells of an organism.A key feature of meiosis is the exchange of chromosome pieces which occurs in the first division of this process, called recombination or crossing over.ametes have half the chromosome number of other adult. cells of an organism. A key feature of meiosis is the exchange of chromosome pieces which occurs in the first division of this process, called recombination or crossing over.