Dominance of Genes Controlling Quantitative Characters in Maize 1

NDIVIDUAL effects of genes controlling quantitative charI acter:; cannot ordinarily be distinguished from one another. Consequently, it is not possible to determine the mode of action of single genes. By studying their combined effect in segregating populations, however, one can gain some insight into their behavior and can make inferences about ;:he average level of dominance involved in the expression of a particular character. The investigation reported herein was designed to provide estimates of the averagc: dominance for genes determining quantitatire characters in maize. Knowledge of the general nature of the actions and interactions of genes controlling the development of quantitatire characters and bringing about heterosis is very important in evahating various breeding procedures. Hull (1 3) has pointed out that current corn breeding procedures are inefficient :if over-dominance is of major importance in the action of genes conditioning yield and has proposed the method of “recurrent selection for specific combining ability” a:$ a Kreeding procedure where over-dominant genes are involved. Comstock et a1. ( 4 ) have suggested “recurrent reciprocal selection” as a method of corn breeding which will be effective with any of the types of gene action believed to be operative. Thc dominant favorable linked-gene hypothesis as discussed by Jones (15) has been the most widely accepted explanation of heterosis; however, the possibility of the heterclzygous genotype excelling either homozygote (now gener.dly referred to as “over-dominance”) has long been recognized (7). East (6) proposed a sort of complementary. gene action between alleles to account for the superiority of the heterozygote, and Henderson (12) discussed a case of inheritance of rust resistance in flax reported in the literature which he believed represented this type of gene action. Hull (14) applied a constant parent regression .technique (later outlined in detail by Grifing (1 1) ) to data on the performance of single crosses of corn and concluded that over-dominance of genes determining yield was indicated. From theoretical considerations, Crow (5), Fisher ( 9 ) , and Brieger (1) concluded that the dominance hypothesis alone is inadequate to explain all of the facts of heterosis, and Brieger offers an evolutionary explanation for the importance of over-dominance. The general pattern for a genetic interpretation of the variance of quantitative characters may be found in papers