RECOMBINATION IN SEXUAL ORGANISMS 



81 



over or because of structural variation along the chromosome, or both. 

 The same is true of comparisons with usual somatic chromosomes where 

 the position of X-ray-induced breaks cannot be ascertained so precisely. 

 A real difficulty with genetic mapping comes from the fact that the 

 nature of the map depends upon the methods of analysis. In the first 

 place, only mutations with detectable effects can be mapped. Secondly, 

 only mutants which occur with a detectable frequency can be located. 

 In haploid organisms, mutations with a lethal effect must be repre- 

 sented as gaps in the genetic map. Which of these classes a given 

 mutant will occupy depends upon conditions. Take for example, the 

 case of the r mutants of the bacteriophage T4, multiplying on a film of 

 the bacterium, Escherichia coll strain B, on the surface of agar; the r 

 phages form large plaques with a sharp margin; wild-type r"*" phages 



Centromere 



Metaphase 

 chromosome 



Tip of right 

 arm of salivary 

 chromosome 



FIGURE 3.7. A comparison of genefic with cytological maps of fhe second chromo- 

 some of Drosophila melanogaster (from Dobzhansky, in Duggar et al., 1936, Biolog- 

 ical Effects of Radiation, Vol. 2, p. 1167, New York, McGraw-Hill; and Bridges, 1937, 

 Cytologia, Fujii Jub., 2:745). 



