POLYPLOIDY 231 



duplication of parts of chromosomes, but the simplest and most 

 plausible explanation of the common occurrence of duplicate 

 factors in tetraploid and triplicate factors in hexaploids is that 

 they were present in the original diploid species, and have been 

 brought together in the cultivated cereals by allopolyploidy. 



The speltoid and fatuoid mutations of wheat and oats seem to 

 be clear cases showing the effect of polyploidy in producing 

 mutations. Their appearance in cultivated varieties of wheat and 

 oats is correlated with chromosome aberration. Different genetic 

 types of these mutants, all similar in appearance, may apparently 

 arise, as Huskins has shown, through the interchange of a pair of 

 chromosomes, or through gain or loss of a chromosome, and the 

 heterozygous mutant form can therefore be obtained with 41, 42 

 and 43 chromosomes. The " homozygous " mutant types segre- 

 gated from these have 40, 42 and 44 chromosomes respectively, 

 and by making the appropriate crossings or through further 

 chromosome aberration they can be obtained with 41 and 43 

 chromosomes also. These results are explicable if the characters 

 of cultivated wheat and oats involved be determined bv the inter- 

 action of rather distinct pairs of factors, situated on similar (but 

 not identical or truly homologous) chromosomes which have been 

 brought together by allopolyploidy. The mutant form will then 

 probably arise whenever the balanced interaction is disturbed. 



Age apparently plays a part in t!he production of mutant forms, 

 esjDecially of speltoids and fatuoids. Some of the oldest European 

 varieties of oats very rarely produce fatuoids, but these are of 

 frequent occurrence in forms of recent origin. Hybrids between 

 hexaploid species or even varieties of cereals sometimes have 

 irregular chromosome behaviour, and the crossing of varieties 

 may introduce aberrations in later generations. This is to be 

 expected if they be allopolyploids. 



One of the most important features of work on speltoids and 

 fatuoids in cereals in connection with polyploidy bears on the effect 

 of the latter in hiding harmful mutations and also on the problem 

 of the origin of dwarfs. Fatuoids or speltoids with 41 instead 

 of 42 chromosomes are almost as vigorous as normal plants. The 

 production of grains, however, in such plants is much below normal, 



