In mitosis, every daughter cell is exactly like the parent cell. Meiosis and sexual reproduction, however, result in a reassortment of the genetic material. This reassortment, called genetic recombination,
originates from three events during the reproductive cycle:
- Crossing over. During prophase I, nonsister chromatids of homologous chromosomes exchange pieces of genetic material. As a result, each homologue no longer entirely represents a single parent.
- Independent assortment of homologues. During metaphase I, tetrads of homologous chromosomes separate into chromosomes that go to opposite poles. Which chromosome goes to which pole depends upon the orientation of a tetrad at the metaphase plate. This orientation and subsequent separation is random for each tetrad. For some chromosome pairs, the chromosome that is mostly maternal may go to one pole, but for another pair, the maternal chromosome may go to the other pole.
- Random joining of gametes. Which sperm fertilizes which egg is to a large degree a random event. In many cases, however, this event may be affected by the genetic composition of a gamete. For example, some sperm may be faster swimmers and have a better chance of fertilizing the egg.