The Segregation of Genes Affecting Yield of Grain in Maize 1

p RESENT-DAY maize breeding in the United States involves isolating self-fertilized lines, testing them, primarily for yield, in hybrid combination, and then utilizing the superior ones in F1 hybrids of one kind or another. The value of the different inbred lines in the breeding program depends upon their general contributions to their hybrid progeny and particularly to the value of specific individual hybrids involving them. Experience has demonstrated these two characteristics to be correlated and that inbred lines imparting a high average yield to their hybrid progeny may be expected also to produce high yielding individual hybrids. Accordingly, it has become practice to test new inbred lines in top crosses, i.e., crosses with an open-pollinated, heterozygous variety, using the top-cross data as a measure of their ability to impart high average yield to their hybrid progeny, or, more conveniently, their yield prepotency. Yield prepotency depends, of course, upon the number of dominant alleles favorable to yield carried by the different inbred lines and upon their relative importance. The rate at which fixation for yield prepotency occurs in an inbreeding program likewise is dependent upon these two factors. In an experiment reported by the writer in ~935 (2),3 it was found that inbred lines of corn showed their individuality as parents of top crosses very early in the inbreeding process and remained relatively stable thereafter. This rather surprising situation was explained on the basis that yield was controlled by a large number of dominant genes, many of which have approximately equal effects. Essentially equal numbers of dominant alleles will be preserved by chance through the successive generations of selfing even though accompanied by segregation for particular dominant alleles. These data are all that are available in maize (or in any other organism so far as is known to the writer) on segregation for yield prepotency. They do not permit a precise determination of the amount of segregation for various reasons. From the standpoint of practical corn breeding, it is important to know the earliest possible generation in which newly developed lines may be tested for yield performance. Early elimination of unpromising lines permits a concentration of effort on more promising material and should result in greater progress in the breeding operatiops. It was decided, therefore, to obtain data on which a more precise estimate of the segregation for yield prepotency might be based. Two assortments of the genes influencing yield will affect the segregation for yield prepotency. When a maize plant heterozygous for many pairs of yield genes is self pollinated, the complement of genes re-