OF MUTANT CHARACTERS. 



279 



its activity was in the chromosome which had contained dachs; and 

 further, that its locus, as judged from the linkage with star, was 

 certainly very close to that normally occupied by dachs. 



The case for dachs-deficiency was further strengthened when, as the 

 result of tests, a third correspondence to forked-deficiency was estab- 

 lished. A female having forked-deficiency in one X and forked in the 

 other is in effect haploid for the forked gene, and accordingly such a 

 female shows the forked character just as in the male, which is nor- 

 mally haploid for forked (and for all other sex-linked genes). 



TABLE 127. PI, star flies from dachs-lethal stock out-crossed to dachs. 



If a fly were carrying " dachs-deficiency' ' in one second chromosome 

 and star in the other and were out-crossed to dachs, then half the off- 

 spring should be dachs, since these flies should carry the dachs gene 

 in one second chromosome and in the other no normal gene to oppose 

 its action. When the test was made half of the offspring were dachs 



and half were not (table 127). In appearance these dachs flies f j 



were indistinguishable from the dachs flies of regular stock. The 

 dachs flies were distributed according to the usual linkage relations 

 of star and dachs. One pair failed to give dachs offspring (2458), 

 corresponding to the crossing-over that occurs normally between star 

 and dachs whereby a certain proportion of the star descendents are not 

 heterozygous for the lethal. 



BALANCED LETHALS. 



It was obvious that the stock had to be carried on as a recessive 

 autosomal lethal that is, by mating together two flies each heterozy- 

 gous for the lethal. The most advantageous method of doing this 



(Of \ 

 r ), 

 d t / 



since in this case advantage could be taken of the fact that most of the 

 flies which would be homozygous for not-lethal would at the same time 

 be homozygous for star and hence be eliminated. Most of the stars 

 in each generation would continue to be heterozygous for the lethal. 



