192 MUTATION AND PLANT BREEDING 



kafir were reported by Karper (11) at a ratio of 1 mutation to 604 

 zygotes. Such a mutation rate produces almost 100 tall plants per 

 acre when plant population per acre is above 50,000. Tall plants 

 make fields unsightly and farmers are suspicious of seed production 

 practices when such large numbers of off-type appear in their fields. 



All varieties unstable for height are recessive dw s , except for 

 Early Hegari. Some varieties recessive for dw s , such as Shantung 

 Dwarf Kaoliang and Acme and Japanese Dwarf Broomcorns, are 

 stable. Hegari and Early Hegari have the same height constitution, 

 DiVi dw 2 Dw-s div 4 , but Hegari is stable for height and Early Hegari 

 is unstable. The unstable gene in Early Hegari has not been identi- 

 fied. The cause of the instability of height genes in sorghum is not 

 known. 



Sorghum hybrids of the future will probably be heterozygous 

 3-dwarf or homozygous 4-dwarf. The seed-parents will be 4-dwarfs. 

 The pollinators will exist in two versions: the 4-dwarf and the 

 stable 3-dwarf, dwi dw 2 Div s dw*. If the 4-dwarf version of the 

 hybrid is too short in stature in some areas, the heterozygous 3- 

 dwarf hybrid would be used. In the original height inheritance 

 paper (28), the possibility of using stable 3-dwarfs was pointed out. 

 Stephens (38) then suggested the conversion of unstable 3-dwarfs 

 to 4-dwarfs by a backcrossing process followed by a recovery of 

 stable 3-dwarfs following mutation to dominant Dw s . In producing 

 hybrids using 4-dwarf seed-parents and stable 3-dwarf pollinators, 

 mutations to 3-dwarf would still occur in the 4-dwarf female. The 

 mutated gamete would be dw r dw 2 Div- A dw 4 and the plant pro- 

 duced from it would be homozygous 3-dwarf. The rest of the hybrid 

 population would be heterozygous 3-dwarf and only a few centi- 

 meters shorter than the plants that carry the mutated gene. In the 

 field, the few homozygous 3-dwarfs would be inconspicuous. 



These dwarf genotypes in sorghum are brachytic. Brachysm, by 

 definition, is dwarfness characterized by shortening of the inter- 

 nodes only. Some unpublished data bearing on this point and col- 

 lected in 1947 are presented in Table 5. Plants of two Sooner milo 

 populations segregating for height were studied. The data from 

 plants heterozygous for height were discarded, leaving only the 

 data from homozygous height genotypes to be summarized. 



The mean difference in height between 1 -dwarf plants in pop- 



