GENETICS PURPLE EYE COLOR DROSOPHILA 



291 



. TABLE 17 



A summary of the four types of black -purple vestigial black cross, with inviability 



balanced 



COINCIDENCE 



Another and very important advantage of these more complex 

 crosses is that the process of double crossing over can be ex- 

 amined. Thus there were 23 double crossovers, or 0.46 per cent 

 of the total flies. If the proportion of double crossovers were 

 determined by chance alone the percentage should have been 

 6,4 per cent of 10.8 per cent or 0.69 per cent of the total. The 

 observed per cent of coincident crossovers (0.46) is only 61 per 

 cent of the theoretical per cent (0.69). This percentage, 61, is 

 called the 'coincidence' for black purple vestigial. This index 

 can be more conveniently calculated directly from the black- 

 cross numbers as follows: 



No. doubles X Total flies x 100 

 Total firsts x Total seconds 



2 3 X 5001 X 100 ^ ^ 

 322 X 539 



(Weinstein, '18) 



