Since when are dimples or cleft chins considered a defect? 
Cheek dimples occur when the muscles underneath the surface of the skin are shorter than the skin on top, so when you smile, the muscle pulls the skin at its attachment point, thus folding the top skin as it is pulled back. Chin dimples are actually a cleft chin in which the bones do not fuse completely (to form the face, the bony plates grow from the outsides to meet in the middle to form the mouth and lower part of the nose) and the outer skin lies on top of the cleft bone, revealing the structure underneath.
If you inherit the genes for dimples, that is, genes for short facial muscles or cleft chin, there may be other factors during your development that could lengthen the muscles, or allow full closure of the cleft chin. We know that several environmental factors can influence the growth of the face, as well as several different gene products that are only around during the facial development (the face develops during the first and second months of the life of the embryo, before many women know they are pregnant). In fact, sometimes dimples can be a marker for an underlying facial defect other than short muscles or cleft chin.
