104 CELL HEREDITY 



of recessive lethals induced by mutagenic treatment. Muller s demon- 

 stration of the mutagenic effects of X-ray in 1927 required a reliable 

 screening procedure to detect and count rare mutations; the ClB method 

 as shown in Figure 4.6 is just that. 



Paracentric inversions are commonly found in natural populations of 

 plants and of the dipteran insects; they have been looked for in few 

 other forms. Because the presence of heterozygous inversions results 

 in the suppression of recombination within the inverted segment, it 

 may be supposed that consequences of evolutionary interest might result. 

 A long and fascinating investigation of inversion patterns in many species 

 of Drosophila in relation to their ecological habitats, bv Dobzhansky 

 and co-workers, has provided a wealth of information about the evolu- 

 tionary role of inversions in this genus. There is no doubt that the 

 locking together of certain gene combinations within inverted chromo- 

 somal segments has great selective advantages. There seems even to be 

 an optimum inversion length, long enough for the accumulation of new 

 mutations, but short enough so that few viable double crossovers will 

 occur. 



VARIATIONS IN CHROMOSOME NUMBER 



Polyploidy of Organisms 



Integral number increases in the chromosome complement are of wide- 

 spread occurrence in natural populations, particularly of plants, in 

 which polyploidy represents a popular evolutionary device. Polyploidy 

 has played an important role in the improvement of agricultural plants, 

 both by the selection of more favorable strains, subsequently found to 

 be polyploid, as in the evolution of domestic wheat, and in planned 

 breeding programs. 



For example, a rare hybridization between distantly related strains can 

 sometimes be stabilized by a subsequent doubling of the entire comple- 

 ment, as shown in Figure 4.7. The homologous chromosomes from the 

 rather different original parents no longer pair in meiosis; and pairing 

 is limited to the closely related homologues of each parental complement. 

 Thus, regularity of meiosis and fertility of the new hybrid, called an 

 amphidiploid, are assured. 



In general, polyploid plants are far more common than polyploid ani- 

 mals. Perhaps the mechanism of sex determination in animals, which is 

 based upon the segregation of sex chromosomes and balance in number 

 with the autosomes, cannot readily be applied to accidental polyploidy. 



