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rniversiti/ of California PvbUcations in Jr/rindtnral Sciences [Vol. 2 



only three pairs of chromosomes. A cross involving these two cliar- 

 acters gave the following results (-f indicates the presence and — the 

 absence of the character named) : 



TABLE 11 



DiHYBRiD Segregation of Smooth X Revolute in a Culture which Gave a 

 15: 1 Ratio for Each Character Separately 



The calculated numbers agree fairly well with those obtained 

 except in the third class w'here the observed numbers are more than 

 twice as large as the calculated number. This class may have been 

 increased at the expense of the first class by placing in it some plants 

 which genetically belonged in the latter. The observed number in 

 the first class is considerably less than the calculated number for that 

 class. These figures indicate that the genes are arranged in the three 

 pairs of chromosomes as follows: R, s, — (R'S') (r's') — r, S, where 

 primed genes are the complements of the unprimed genes. Were the 

 linkages as follows (R's) and (r'S), the P. population should consist 

 of three classes in the proportion of 14:1:1, assuming that little or 

 no crossing over occurs. A high percentage of crossing over in the 

 latter type of linkage would give approximately the results obtained. 

 It appears, therefore, that either the dominants are linked, as stated 

 above, or that there is a high percentage of crossing over between the 

 linked genes. This inference can be tested experimentally, for races 

 have been obtained which gave 3 to 1 ratios for both of the characters. 



Effects of Inhreeding 



The flowers of Crepis are perfect and, although self-fertilization 

 can take place, the arrangement of the stigmas in respect to the 

 stamens is such as to jiermit cross-pollination before self-pollination 

 can be naturally effected. The stamens are united into a tube sur- 

 rounding the style, and the pollen is shed on the inside of this tube. 



