PLASMAGENES AND CHROMOGENES IN HETEROSIS 225 



logical efficiency, such as reproductive rate and survival ability. This may 

 even be shown with a reduction in productiveness as measured by economic 

 characters. Some confusion has arisen by not distinguishing clearly between 

 these two different manifestations of heterosis. 



In addition to these two general types of heterotic effects, there may also 

 be a reduction in both growth and survival ability; in other words, hybrid 

 weakness or a reversed or negative heterosis. This effect is much less com- 

 mon and is seldom found in cultivated plants and domesticated animals. 



TYPES OF GENE ACTION 



An understanding of the mode of action of heterosis has now resolved into 

 a study of the nature of gene action. The genes usually used to illustrate 

 Mendelism are the loss variations that have a major effect such as the inabil- 

 ity to produce some essential substance. This results in a block in the normal 

 chemical processes, finally resulting in an individual of greatly altered ap- 

 pearance, size, or ability to survive. The effect ranges in intensity from a com- 

 pletely lethal condition at some stage of development, up to individuals that 

 differ only slightly in appearance from normal with no appreciable reduction 

 in growth or survival ability. Such genes are illustrated by the long lists of 

 Mendelizing characters now tabulated for maize, Drosophila, mice, and many 

 other animals, plants, and lower organisms. 



DOMINANT AND RECESSIVE GENES 



In these cases, the normal allele is usually designated by a capital letter, 

 with the mutant, deficient allele denoted by the corresponding lower case 

 letter. In comparison with the normal allele, the recessive mutants are de- 

 ficient in some respect. In their inability to produce certain specific sub- 

 stances, as shown in the haploid Neurospora by Beadle and his co-workers, 

 they are referred to as .4 -less, 5-less, C-less, etc. In diploid organisms A is 

 usually completely dominant over a; that is, one A allele functions as well 

 or nearly as well as two. 



There is no question that the accumulation in a hybrid of the normal 

 alleles of this type results in heterosis. In the simplest example of a cross of 

 ^-less by B-less (aaBB XAAbb) the hybrid offspring are all AaBb, and 

 essentially normal for whatever effect A and B have. But since the mutant 

 recessive alleles of this type are so drastic in their effect, most of these 

 deficiencies are removed by natural selection in all species whether self- 

 fertilized or cross-fertilized. Therefore they have little part in the heterosis 

 that is shown by these organisms when crossed. Furthermore, genes of this 

 type are eliminated when naturally cross-fertilized species, such as maize, 

 are artificially self-pollinated. Yet such inbred strains show the largest 

 amounts of heterosis. 



There is evidence, as will be shown later, that there are many genes of this 



