A pair of homologous chromosomes are genetically different because in a set of homologous chromosomes, one of the chromosome belongs to the male parent and the other come from the female parent. Therefore, one of a pair will contain paternal genes and the other will contain maternal genes.
However, a pair of sister chromatids are genetically identical before crossing over as the chromatids are formed from the replication of DNA during the ‘S’ phase of interphase. DNA replication ensures that the DNA content is doubled with identical genes being copied from the original DNA. Therefore, there is no genetic variation because there is no exchange of genetic material between sister chromatids. If crossing over occurs, then it would he possible for some genes to he exchanged between the chromatids of. homologous chromosomes that have chiasmata, thus leading to genetic variation.