Hint: In crossing over, genetic material is exchanged between the non-sister chromatids of two homologous chromosomes which results in recombination. Crossing over occurs during the pachytene stage of prophase I. Crossing over has greater significance as it brings genetic variability.
Complete answer- During sexual reproduction, meiosis occurs. The prophase I of meiosis consists of mainly five stages. During prophase I, a process called synapsis occurs in which homologous chromosomes arrange themselves to form tetrads or bivalent. The tetrads can be easily seen during the pachytene stage, which is the third stage of prophase I. During pachytene stage, recombinant nodules are seen where crossing over takes place. It is the site where genetic material is exchanged between two homologous chromosomes and results in recombinant chromosomes. Recombinase enzymes help in this process. At the end of pachytene stage, the recombinant chromosomes are linked. After the end of crossing over, the two daughter cells are formed by the separation of homologous chromosomes. During meiosis II, the sister chromatids separate from these two daughter cells which results in four gametes. Out of four, two gametes contain the same alleles as that of parents while the other two contain new allelic combinations.
Thus crossing over results in the recombination of linked alleles. It is the last phase of the recombination process which results in variations as the recombinants contain the genes that are not present in either of the parents.
Note: Homologous chromosomes are the chromosomes which contain two pieces of DNA. One piece is parental while the other is maternal. The homologous chromosomes are not similar but they contain the same genes with slight variation. When the DNA segment is broken and recombined, recombinants are formed. This is called recombination. Recombinants contain the new combination of alleles which has genetic variability.