OF MUTANT CHARACTERS. 



227 



The calculation of the locus of streak on the basis of all the available 

 data places it at 13.6 units to the left of darhs and 15.4 units to the 

 right of star, star being the zero-point for the chromosome. 



A summary of the linkage data directly involving streak is given in 

 table 74. 



Table 74. — Summary of cross-over data involving streak. 



VALUATION OF STREAK. 



There is only one drawback — but that one very serious — to the use- 

 fulness of streak, namely, the difficulty of separating all the streaks 

 from the wild-type. In most of the experiments conducted by l^ridges 

 this difficulty was not great enough to impair the accuracy of the result. 

 However, a streak dachs back-cross was attempted and abandoned, 



and in the quadruple back-cross ( J^' ) the separation is not com- 



\ Sf. a / 



plete in all cultures. In these cultures the first separation performed 



was that of the streak from the not-streak, without regard to the other 



