218 MUTATION AND PLANT BREEDING 



is very simple, because the two mutants differ (practically) only by 

 two genes. This is in contrast to the great expenditure necessary 

 in combination breeding on the basis of variety crosses. Occasion- 

 ally, two or even more genes may be changed in one mutant at the 

 same time, but this does not complicate seriously the breeding pro- 

 cedure. Usually, however, crossing of two mutants results in normal 

 bifactoriel segregation (78, 93). 



In extensive crosses among different erectoides mutants of 

 barley, Hagberg (50) studied the additive effect of the combined 

 loci as expressed by the internode length in the ear. He succeeded 

 in the addition of up to four homozygous mutations in one plant. 

 By this, the average internode length was reduced from 33 mm of 

 the mother line (no mutation) to 24 mm in those containing a single 

 mutation, to 16 mm for those containing two mutations, to 13 mm 

 where three genes were involved, and to 12 mm in the line con- 

 taining all four mutated genes. This result indicates that the addi- 

 tion of each further gene had a smaller effect than the previous 

 one. Hoffmann (discussion of 50) reported that in another polyfac- 

 torial character, namely, earliness, the combination of two mutations 

 sometimes results in a multiplication effect. Thus, crossing a 3-day 

 earlier with a 6-day earlier mutant resulted in a double recessive 

 being about 18 days earlier. Crosses between various intermedium 

 mutants of barley (Hoffmann, loc. tit.) led sometimes to double reces- 

 sives which were dwarfs. These examples indicate that certainly not 

 all mutations can be combined without reducing the vitality. On the 

 other hand, crossing of small mutations inter se has led in peanuts to 

 heterosis (41, 42) and the use of micro-mutations should also be 

 explored in combination breeding. 



Also crosses of mutants with foreign varieties offer the chance to 

 obtain rapid and simple results. Their advantage is particularly 

 to be expected when a "rare" character, which is known only in 

 primitive or non-adapted forms, is to be transferred. If that char- 

 acter is available from a mutant collection of an adapted variety, it 

 may be much easier to use the mutant instead of the non-adapted 

 variety for a cross-breeding program. Characters from mutant collec- 

 tions which may be useful are for instance: resistance against fungi, 

 variability in relation to different soil and climatic conditions, 

 strawstiffness, high protein content of seeds (95, 95), earliness, and 



