224 



THE SECOND-CHROMOSOME GROUP 



the well-known case of the yellow mouse, and was the first of many 

 to be found in Drosophila, where a homozygous dominant is lethal. 



Outerosses of streak by w^ld gave in Fi streaks as approximately 

 half of the flies. The records of these early out-crosses were lost (note- 

 book S II), but similar out-crosses made later illustrate the fact as well 

 (table 68). 



CHROMOSOME CARRYING STREAK. 



The next task was to determine in which chromosome the gene for 

 streak is located. This was done by back-cross tests of the male for 

 black (II chromosome) and pink (III chromosome). 



Table 69. — Pi, streak 9 X pink cf; Fi streak cf X pink 9 of stock. 



Streak males heterozygous for pink were produced from the mating 

 of streak female by pink male. Two of these Fi males were back-crossed 

 to pink females and produced a total of 667 offspring, 329 of which 

 were recombinations (table 69) . The presence of the streak-pink and 

 the wild-type flies as 49.3 per cent of the whole proved that streak was 

 not in the third chromosome, since independent assortment was demon- 

 strated. 



Table 70. — Pi, streak 9 X black cf ; Fi streak cf X black 9 of stock. 



In the back-cross test of the streak male heterozygous for black no 

 recombination occurred. Every one of the 19 not-black flies was 

 distinctly streak, and likewise none of the 21 black flies showed a trace 

 of streak (table 70). This result was due to the fact that the locus of 

 streak is in the second chromosome and the lack of crossing-over in 

 the male. 



LOCUS OF STREAK. 



Immediately following the appearance of the first flies in the pre- 

 ceding back-cross tests of the male, a test of the locus of streak was 

 made by means of the mutant morula, which had itself just been 

 mapped at the right end of the second chromosome. 



Back-cross tests of Fi streak females, from the cross of streak females 

 by morula males, gave a total of 876 flies, of which 405 or 46.2 per cent 

 were cross-overs (table 71). 



