PLASMAGENES AND CHROMOGENES IN HETEROSIS 227 



deletions, but do prevent other genes from liaving their usual exi)ression. 

 Most of these inhibitors control color characters and are usually not involved 

 in heterosis. If they were, there would be more negative heterosis than actual- 

 ly is found. 



GENES WITHOUT DOMINANCE 



Unlike the visible Mendelizing genes with their clear-cut dominance and 

 unfavorable action of one or the other allele, there are many genes that dif- 

 ferentiate size or number of parts, time of flowering and maturing. These are 

 the genes usually involved in normal variation. They are the ones the plant 

 and animal breeder are mainly concerned with and could expect to have a 

 major effect on heterosis. Since neither member of an allelic pair can be con- 

 sidered abnormal or deficient, both are designated with a capital letter with 

 some prefix to differentiate them, as for example A and A\ 



Genes of this type usually have simple additive effects such as the F endo- 

 sperm color gene in maize, in which each allele adds a definite increment in 

 total carotene content. Such additive genes without dominance are used to 

 interpret the inheritance of quantitative characters which have been shown 

 to segregate and recombine in a Mendelian manner. 



No clear distinction can be made between the A a and .4^1' types of genes 

 and this has led to much confusion. The first class shows complete or nearly 

 complete dominance. The second shows no dominance or very little domi- 

 nance, but one type integrates into the other. The principal question at issue 

 is whether either type shows over-dominance, or in other words, an interac- 

 tion between alleles such that Aa > A A or aa or AA^ > A A or .4 '.4'. Before 

 considering the evidence for or against over-dominance, two remaining types 

 of genes should be considered. 



CHROMOSOMAL REARRANGEMENTS 



By chromosomal rearrangements such as inversions and translocations, 

 genes without alteration are placed in different spatial relations with other 

 genes. In their altered position they have different effects. Dobzhansky and 

 his associates have studied many geographical races of Drosophila that differ 

 by chromosomal rearrangements. Crosses between these chromosomal types 

 from the same region exhibit heterosis, whereas the same chromosomal type 

 from different regions do not show such a high degree of heterosis. This 

 seems not to be a position effect, but is the result of an accumulation of gene 

 differences that are protected from random distribution by the prevention of 

 crossing over in hybrids of different chromosomal types. 



COMPOUND GENES AND GENES WITH MULTIPLE EFFECTS 

 In many organisms, loci are known which have different effects on differ- 

 ent parts of the organism. In maize the .1 , P, and R genes have been studied 

 in considerable detail by Stadler and his co-workers. These loci each have a 



