Genetic sister chromatides

A) Sister chromatids are genetically identical and therefore an exchange would not show any change in phenotype

When genetically identical chromatids cross over, sister chromatid exchange occurs. Sister chromatids exchange does not result in the recombination of alleles because the chromatids are genetically identical, however, homologous recombination can result in genetic recombination. Since the alleles are not recombined, it doesn't result in different observable phenotype traits. Basically, crossing over, or exchange, between sister chromatids does not lead to new gene combinations. It's important to not confuse sister chromatid exchange with recombination. One is observed in mitosis, and the latter one is observed in meiosis.