Genetic recombination and segregation
These are processes that occur during the formation of sex cells. In these processes, a pair of similar, i.e. homologous, chromosomes in the nucleus forms doublets and mutually recombine. In recombination, the DNA molecule is broken at the same place in both homologous chromosomes. If the original parts of the same chromosome subsequently rejoin, no recombination occurs; however, if a strand of one chromosome joins together with a strand from the second chromosome, the pair of recombined chromosomes will differ in the combination of their alleles from the two original chromosomes. While, prior to recombination, it was possible to state that one chromosome was derived from the father and one from the mother, the recombined chromosomes contain part of the alleles from the father and part from the mother. Segregation occurs during the separation of homologous chromosomes to the opposite ends of dividing cells. In this process, one of the chromosomes of each pair moves quite randomly to the opposite end of the cell. Even if recombination did not occur before this, the segregation of the chromosomes of paternal and maternal origin would give the newly formed cells their own combination of paternal and maternal alleles, different from the combination of alleles of either of its parents. Following separation of the pair of chromosomes in the first meiotic division, the two sister chromatids of each chromosome separate in the second meiotic division. Thus, four sex cells, haploid cells, can be formed from one diploid cell.