Final answer:
Sister chromatids are genetically identical and connected by a centromere. They contain the same alleles as they are formed through DNA replication during the S phase of interphase.
Step-by-step explanation:
During the S phase of interphase, DNA replication results in the formation of two sister chromatids that are genetically identical and connected by a region known as the centromere.
These sister chromatids, held together by cohesin proteins, contain the same alleles because they are exact duplicates made during the DNA replication process.
Therefore, when fully compacted, each chromosome consists of a pair of these identical sister chromatids. This formation is ready for the later stages of cell division, where each sister chromatid will eventually be allocated to one of the two daughter cells.