226 DONALD F. JONES 



type having small effects that are not eliminated by natural or artificial 

 selection either in the wild or under domestication, and that these deficiencies 

 or degenerative mutants do have a large part in bringing about reduced 

 growth. Before presenting this evidence, there are other types of gene action 

 that should be considered. 



CHROMOSOMAL DELETIONS 

 In addition to the recessive mutant alleles that are deficient as compared 

 to their normal alleles, there are also chromosomal deletions which result in 

 the complete elimination of the normal locus. Large deletions are usually 

 lethal and are quickly eliminated. Small deletions that cannot be detected 

 cytologically are haplo-viable, and may persist indefinitely if they are closely 

 linked with essential loci. Changes of this type have been demonstrated by 

 McClintock (1931) and by Stadler (1933). They cannot be readily distin- 

 guished from recessive mutants of the .4 -less type. In fact there may be no 

 difference. In practically all cases they show varying amounts of germ cell 

 abortion, and do not mutate back to normal. Deletions of this type are 

 designated Ao. 



DOMINANT UNFAVORABLE GENES 



In many cases of deletion the heterozygote, or the hemizygote, is visibly 

 and unfavorably altered from normal, in which event the genes involved are 

 listed as dominant, and if partially viable they can bring about negative 

 heterosis or hybrid weakness. It is not known whether all dominant unfavor- 

 able genes are deletions of this type, but as far as their effect on heterosis is 

 concerned it makes little difference whether or not they are. An illustration 

 of this type of gene action may be seen in a cross of Ragged and Knotted 

 maize plants. Both of these genes result in a marked reduction in growth in 

 the heterozygous condition. They are not completely lethal in the homozy- 

 gous dominant condition, but seldom produce seed or pollen. When both 

 dominant genes are present together in the heterozygous condition, there is 

 a marked reduction in size, rate of growth, and reproductive ability as com- 

 pared with either parental type. 



Tunicate, teopod, and corn grass are also dominant genes that reduce 

 grain yields in both the homozygous and heterozygous condition. They are 

 probably reversions to a primitive condition which in suitable genetic com- 

 binations maybe favorable to survival in the wild. Dunn and Caspari (1945) 

 describe many structural abnormalities in mice that seem to be due to dele- 

 tions having a dominant effect in the hemizygote. Some of these counteract 

 each other and tend to restore a more normal condition, while others accumu- 

 late unfavorable effects. A similar situation has been reported in Drosophila 



by Stern (1948). . 



In addition to recessive deletions with a dominant effect m the hetero- 

 zygote, there are also dominant inhibitors that have no indication of being 



