Hint: In prophase, I of meiosis the two homologous chromosomes align one next to the other. A chromosome consists of two sister chromatids thus two pairs of homologous chromosomes mean four homologous chromatids.
Complete answer: Tetrad formation occurs during the zygotene stage of meiotic prophase. It is a meiotic-specific process. Homologous chromosomes are DNA fragments within a diploid organism of the same size one from each parental source. In meiosis, a single cell divides to produce four cells containing half the original amount of genetic information. This type of cell division occurs in sex cells – sperm in males, eggs in females.
The tight pairing of the homologous chromosomes is called synapsis. Synapsis allows matching up of homologous pairs prior to their segregation. It allows chromosomal crossover and thus genetic material can be exchanged between the homologous chromosomes. The result of synapsis is a tetrad. Homologous pair of chromosomes that are close to each other and form a synaptonemal complex is called a tetrad. During synapsis, the homologous pairs of sister chromatids line up together and connect. Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes.
Note: The exchange of genetic material is called recombination. This results in a variety of genes which are passed to the offspring. Because of this process, there is genotypic and phenotypic diversity in the population.