The attraction between homologous chromosomes occurs in the pachytene phase. Homologous chromosomes come close to each other and pair along their length. The process of pairing of chromosomes is called synapsis and each pair of chromosomes is bivalent. In bivalents, each chromosome divides longitudinally without the centromere to form two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids in bivalents come close together and form an X-shaped zygoma. At the chiasma segment, certain parts of the two non-sister chromatids are broken by the action of endonuclease enzymes. A broken chromatid is then joined to another non-sister chromatid with the help of ligase enzyme. In this way, parts are exchanged between two non-sister chromatids and is called crossing over. Crossing over results in new arrangements of genes on chromosomes. The position and arrangement of the genes changes. Genetic variation occurs in organisms. Hereditary changes occur in organisms. New characteristics emerge in the created organisms. As a result, genetic or heredity diversity is created.