...or any configuration where the alleles at the same location on the two sister chromatids are NOT identical, then you are not on the right track. Try the following. Draw a homolog with ONE chromatid (i.e., as the chromosome would be in the G1 phase of the cell cycle, before replication). Mark two genes on the chromosome (any two genes -- e and f, a and b, whatever). Then draw the same chromosome as it would be after DNA replication, with the two resulting sister chromatids. Do you expect the two products of DNA replication to be identical to each other (barring new mutations)? If so, will the two sisters be identical with respect to the alleles of the genes they carry?

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