A WIDE collection of varieties of almost any crop plant often exhibits a wide range in variability for each quantitative character. Yield and other quantitative characters usually are the end result of the interaction of a large number of genes under particular environmental conditions. What is inherited is the manner of reaction and not the character itself. A recent review of quantitative inheritance studies has been made by Smith (1944). All that will be attempted in relation to yield inheritance will be a condensed summary of various types of gene action and interaction that may be considered under the heading of yield genes. Yield Genes. Resistances to plant diseases are of great importance in relation to yield. The various types of inheritance of disease reaction may be used as one illustration of the gelletic complexity of this factor in yield. For a disease such as stem rust of wheat caused by Putccinia graminis tritici there are many physiological races of the pathogen that are differentiated by their reaction on a series of host varieties and species of wheat. Some varieties of wheat are resistant under field conditions from heading to maturity in the spring wheat area of North America to all or nearly all races of stem rust, which has been called mature plant resistance. This resistance in certain crosses where Hope or H-44, or their derivatives, are used as one parent is dominant in F1 over susceptibility and the reaction in the segregating generations is dependent upon one or two factor pairs. This