Tetrads, four-part structures made up of two homologous chromosomes, form during prophase I of meiosis. During this phase, chromosomes exchange genetic material with other chromosomes in the tetrad. The process is referred to as crossing over and is how genetic variation occurs.
Meiosis occurs in all sexually reproductive animals, plants and fungi. In meiosis, DNA replication is followed by two rounds of cell division that produce four daughter cells, each with half the number of chromosomes in the original parent cell. These cells mature into reproductive cells, which when fertilized, form a zygote containing a mixture of paternal and maternal chromosomes.