Here's why:

* Sister chromatids are two identical copies of a single chromosome that are joined together at the centromere. They are produced during DNA replication, where the original chromosome is duplicated.

* Centromere is a constricted region on a chromosome that serves as the attachment point for spindle fibers during cell division.

* Genetically identical: Sister chromatids are identical because they are formed from the same original chromosome. They carry the same genes and the same genetic information.

Therefore, sister chromatids are genetically identical and attached to each other at the centromere.