 In addition to the one-many relation between molecular and higher level kinds, Hull (1972, 1974, 1976) also pointed to the existence of many-one relations as a challenge for reductionism. The same higher level phenomenon (e.g., a Mendelian trait such as coat color) can be produced by several different molecular mechanisms or a higher level phenomenon is realized by different kinds of molecular configurations  (multiple realization).  This is a potential problem for both theory reduction and explanatory reduction (though to differing degrees). In defense of his model of theory reduction, Schaffner (1976) replies that it is sufficient to specify one such molecular situation. The higher level situation, which is determined by the molecular constellation, then can be derived logically from this molecular specification of the higher level situation. However, one has to be clear about the nature of the explanandum in a reductive explanation (i.e., what is the consequent of a reductive deduction). Is it a type of higher level phenomenon (described by higher level predicates) or merely a token phenomenon (an instance of a higher level phenomenon)? Theory reduction is the idea that a whole theory (such as classical genetics) can be reduced, or at least that its laws can be reduced. For example, the Mendelian principle of segregation is a law describing a phenomenon occurring in all sexually reproducing organisms, not just a token phenomenon, so this general principle—covering various lower level realizations—would have to be explained/deduced molecularly.