mac key: induced mutation in crop improvement 349 



erably be combined with heterozygosity and/or outcrossing. Instead 

 of carefully purified parental stocks and rigorous isolation in order to 

 meet high demands on scientific exactness (12), the purely practical 

 mutation breeder should stimulate outcrossing and recombination 

 above what decreased fertility can give. It would thus be interesting 

 to try artificial induction of mutations in connection with composite 

 crosses or multi-cross bulks where male sterility was added to augment 

 genie exchange. 



In evolution, the mutation processes are not only restricted to 

 interior changes of the genes. Their deletion, duplication, and posi- 

 tion, alone or in blocks, interfere with evolutionary fitness. The loss 

 of genie material is definitely the most common type of chromosome 

 mutation, and the rare evolutionary advantage of such a process is 

 largely responsible for the discredit of this whole group of gross 

 mutations. A loss may have a positive effect as a sequence of duplica- 

 tion (70, 72, 94). Duplications are, however, definitely more interest- 

 ing, since they may imply a cumulative effect and also a buffering 

 which allows vital genes to mutate in directions that might have been 

 impossible otherwise. In genetically well-studied objects like maize 

 and barley, systematic production of duplications starts to become a 

 reality (1, 31, 45, 46). 



In barley, the breeder is interested in duplicating a segment of 

 chromosome 6 carrying the gene "orange lemma". This gene is strong- 

 ly associated with high tx-amylase activity and a duplication may thus 

 offer a chance to improve malting properties. Another idea is to 

 duplicate parts of the 40 centimorgans long segment of chromosome 5, 

 where 13 of the 14 known loci for mildew resistance are incorporated 

 (22). Such a procedure may augment the degree of resistance of some 

 of the weaker genes, and it may offer a possibility to overcome difficul- 

 ties with very close linkage. It would also allow more than one allele 

 to be present simultaneously in the homozygous condition necessary 

 for a stable barley variety. The last-mentioned possibility to fix a 

 heterozygous condition would give a chance to utilize superdom- 

 inance also in autogamous plants. Such a relation is found by Wiebe 

 (cited by Hagberg, 46) to exist between the alleles V for 2-row and v 

 for 6-row barley. By a proper duplication in chromosome 2, a geno- 

 type of the constitution VV, vv would allow the advantageous inter- 

 action between V and v without the risk of segregation and 

 separation. 



