OF MUTANT CHARACTERS. 



143 



Table 12. 



appear. If x is the percentage of cross-over gametes, and lOOx of 

 the non-cross-over gametes, then the ratio of star to not-star is 200x : x. 



Obviously, if snub is truncate, then by mating such a star-truncate 

 heterozygote by a similar star-snub heterozygote, one should get this 

 same 200x : x ratio of star to not-star. That is, the occurrence of a 

 ratio of star to not-stars in which the not-stars are less numerous than 

 in the ordinary 2 : 1 ratio would mean that 

 snub is truncate, and by calculation from the 

 observed ratio we can find out how far the locus 

 of truncate is from star. The experiment as 

 carried out gave (table 12, Wallace) four cul- 

 tures in which the ratio was significantly 

 different from 2:1. The total of stars was 

 950 and of not-stars 149, or a ratio of 6.3 : 1. 

 , From this we may conclude that snub is 

 truncate or an allelomorph so similar in its 



obvious characteristics that to demonstrate a difference would require 

 a refined biometrical study or elaborate 'interaction' tests. 



In table 12 will be found the ratio of star to not-star given by crosses 

 of females heterozygous for star and truncateto males heterozygous 

 for star and snub. 



The solution of the proportion 200x : x : 950 : 149 gives a value of 

 27.1 for x; that is, there is 27.1 per cent of crossing-over between star 

 and truncate. Nearly 1 per cent should be allowed for double cross- 

 ing-over within the distance from star to truncate, or the map distance 

 should be about 28.0 on the basis of this data. This position also 

 agrees with what is known of the location of truncate. 



INTENSIFICATION OF TRUNCATE BY CUT. 



That cut intensifies the original truncate in the same manner as it 

 does snub is sho\\Ti by the results of a test of this point (table 13). 

 When a cut female was crossed to a truncate male the ratio of not- 

 truncate to truncate among the males was 1:0.79; that is, quite a 



Table 13. — Intensification of truncate by cut — Pi, cut 9 X truncate cT. 



close approach to the expected 1:1; but among the sisters, which 

 were not cut, this ratio was 1 : 0.40, which means that about 43 per 

 cent of the females genetically truncate failed to show the character. 

 The intensification by cut is more extensive than appears at first 



