I have been told that interphase cells are better for gene mapping purposes than metaphase cells but don't know why.
In metaphase, chromosomes are at their highest level of condensation. FISH relies on being able to dissociate the two strands of DNA to anneal a sequence-specific probe. So, my guess is that it is technically easier to achieve a good FISH staining in interphase, when chromosomes are much less densely condensed. There might be other reasons I am not aware of (I am not a geneticist).