Genetic Systems II | 185 



is involved. In Crepis (false dandelion, Compositae) the chromo- 

 somes may be recognized individually. Species considered to have 

 retained primitive characteristics in other features of the phenotype 

 have X = 7, 6, or 5. The most specialized species have x = 4 and 

 X = 3. The correctness of the above-described model of Darlington 

 has been shown by Tobgy, who demonstrated that Crepis fuliginosa 

 (n = 3) was derived from C. neglecta or its ancestor (n = 4). One 

 arm of the C chromosome of C. neglecta apparently is inert, and 

 the other arm was translocated to the A chromosome. This is shown 

 by pairing behavior in the Fi hybrid between the two species. 



Fig. 9.3 I Diagram showing how basic chromosome number can be in- 

 creased or decreased by reciprocal translocation of unequal chromosome 

 segments. Nonhomologous chromosomes are white and hatched; black- 

 indicates supposedly inert segments. {From Stebbins, 1950, Variation 

 and Evolution in Plants, Columbia University Press, and after Darling- 

 ton, 1937, Recent Advances in Cytology, Blakiston.) 



Origina 

 n + 2 pairs 



offspring from similar gametes 



n+3 pairs 



