Multiple Characters and Correlated Response1

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Abstract
Selection of a new cultivar usually requires the manipulation of quantitative characters, many of which are correlated. These correlations may be due either to genetic linkages or to physiological and developmental relations. The latter two categories are considered here. The need for an understanding of these relationships is obvious because they impose constraints on the form of a plant and are frequently barriers to improved yield. The approach is based on Sinnott's law which states, “The size of an organ is proportional to the size of the meristem from which it develops.” In an attempt to relate this law to correlation between traits and to predict the tightness of the association, a dichotomy was proposed between: 1) the allometry associated with organs arising from the same meristem and 2) the allometry associated with organs arising from different meristems. It was predicted that generally the correlations in the first category would be expected to be harder to manipulate, but some of the latter category could also be recalcitrant. Examples were given for both categories including one in barley, Hordeum vulgare L., of an increase in yield due to manipulation of correlated traits not arising from the same meristem. Two corollaries to Sinnott's law were proposed. 1) Plasticity is inversely proportional to ontogenetic proximity. Traits arising from the same meristem will tend to be harder to manipulate than those separated in space and time of origin. 2) Numbers and size of organs tend to have an inverse relationship. Examples are given in support of both corollaries.