Answer:
Anaphase of meiosis II
Explanation:
I think you are asking during which phase of cell division sister chromatids are mostly likely to fail to separate properly, a phenomenon that can lead to genetic disease.
This is sometimes called non-disjunction, and it is most likely to occur in anaphase during meiosis II. During this stage, sister chromatids of the two daughter cells produced by meiosis 1 are separated and brought to opposite parts of the cell. If something goes wrong at this stage, the sister chromatids can fail to separate properly, meaning the daughter cells do not have the correct number of chromosomes (see attachment).
An example of such a genetic disorder is Down syndrome
