According to the White House model, crossing over is done in four steps. These are-
1. Synapsis: The process by which homologous chromosomes join together is called synapsis. At this stage attraction is observed between homologous chromosomes. Homologous chromosomes come close to each other and pair along their length. The process of pairing of homologous chromosomes is called synapsis. Each pair of chromosomes is called bivalent.
2. Duplication of Chromosome: Each chromosome of bivalent divides longitudinally without 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.
3. Interchange of body parts: Two non-sister chromatids of bivalents come close to each other and form X-shaped zygoma. Chromatids are broken by the action of endonuclease enzyme at the chiasma segment. A broken chromatid joins another non-sister chromatid with the help of ligase enzyme. In this way parts are exchanged between two non-sister chromatids. It is called crossing over. Dr. Siddique Publications
4. Terminalization: Repulsion begins between the bivalent chromosomes after crossing over is completed. Chromosomes move away from each other. The chiasma gradually move towards the edge. This is called terminalization.