322 AN INTRODUCTION TO MODERN GENETICS 



of variation made available for breeding is provided by the potato. The 

 European varieties are derived from a single species. Solarium tuberosum^ 

 but in South America there are at least thirty cultivated species, form- 

 ing a polyploid series with 2n = 24, 36, 48, 60 and 70. Among these 

 are frost-resisting forms, which can perhaps be cultivated in the far 

 north of Europe, and also types which form tubers under conditions of 

 short periods of daylight, which may perhaps be useful and adaptable 

 to tropical conditions. The economic and social possibiHties of any 

 large extension of the potato belt are bound to be enormous. 



Artificial polyploids (p. 256) made from species hybrids have already 

 been introduced into horticulture, e.g. Primula kewensis. None have 

 yet been made which are satisfactory from an agricultural point of 

 view, but great hopes are held out for the future of the doubled wheat- 

 rye hybrid described by several Russian authors ;^ it may prove a very 

 valuable cereal on Hght dry soils. The method may be expected to find 

 considerably greater appHcation in future. 



Perhaps the most important achievements to date of the method of 

 wide crossing are those obtained by wheat breeders.^ English wheat, at 

 the beginning of the century, was extremely proHfic but lacked the 

 hardness of grain required to give good baking flour. This hardness 

 was present in the lower-yielding Canadian wheats. Red Fife (p. 316), 

 and its derivative Marquis. Biffen crossed Red Fife with an English 

 variety. Rough Chaff, and showed that hardness depended on a dominant 

 gene; and by selection in the F2 and subsequent generations he isolated 

 a segregate, Burgoyne's Fife, which combined hardness with some of 

 the high-yielding qualities of the English wheats. Subsequently he 

 developed Yeoman i and later Yeoman 11 from a Red Fife-Browick 

 cross, and in these two varieties the hardness is transferred to the 

 EngHsh wheats with Uttle, if any, loss of yield. 



Biffen also investigated the inheritance of resistance to the important 

 wheat disease known as Rust, and found that it was determined by a 

 single recessive factor in crosses between varieties of ordinary vulgare 

 42 chromosome wheats (cf. p. 258). The picture soon became more 

 compUcated, however, and it was discovered that there are very many 

 different physiologic forms of the rust fungus, and that resistance to one 

 form did not necessarily imply resistance to another. An attempt was 

 therefore made to get into vulgare wheats the factors determining the 

 general resistance to all forms of rust which can be found in the 14 

 chromosome Emmer and 28 chromosome Durum wheats. The Fi of a 

 ^ Meister 1930. - Rev. Hunter and Leake 1933. 



