BIOLOGY. 285 



percentage of the mutant flies emerge, and those live only a few days. In 

 order to carry on this stock it had to be balanced, A male was crossed to 

 a curly female. Curly is a dominant which carries a dominant cross-over sup- 

 pressor in chromosome II. When the Fi curlys are inbred ( ^ .^-^^ by __j— 1 



the offspring in all subsequent generations are flies heterozygous for curly 

 and for no-wing (most of the homozygous curly die), and a few flies homozy- 

 gous for no-wing. This balanced stock of no-wing f -^^HLZ_ i c^n always 



be used in out-crosses as though it were a stock pure for no-wing, because 

 all the Fi offspring that are not curly are known to have received the gene 



for no-wing. For example, if ( ^"^i^ j is crossed to lobe-^ females (lobe-^ 



is a dominant in chromosome II) all the lobe-^ not-curly offspring are heterozy- 

 gous for lobe-^ and for no-wing. The amount of crossing-over between these 



two mutants is found by crossing such a female to a male that is I _£HIZ_ \ 



'' ° Vno-wingy 



The curly offspring are discarded, and the remaining flies fall into four 

 classes as in a typical back-cross result. Thus: -y by — = (1), curly 



(50 per cent); (2), lobe (non-cross-over); (3), wild type (cross-over); (4), no- 

 wings (non-cross-over); (5), lobe no-wings (cross-over). The actual results 

 were (I), 1581; (2), 725; (3), 130; (4), 362; (5), 67. The percentage of cross- 



197 

 ing-over is thus :j-2oj^X 100= 15.3. 



A second case has arisen, in which, according to genetic evidence, the right 

 end of chromosome II has become transferred to chromosome III. In still 

 another case a piece of chromosome II, containing the recessive gene speck, 

 has been transferred to the left end of the X-chromosome. 



There is another kind of genetic result, which may be interpreted as due 

 to deficiency (a loss or inactivation of a section of a chromosome), of which the 

 mutant notch is a typical case. Two new notches that are deficiencies, 

 extensive enough to cover the locus of white, have appeared, and others not 

 so extensive have been met with. A sex-linked character, minute-k, similar 

 in behavior to notch has been found and similarly interpreted. The locus of 

 minute-k is very close to that of miniature. 



Further work is being done on the stock "double yellow" in which two 

 X-chromosomes are present that are attached to each other and that conse- 

 quently fail to separate at the reduction division. Occasionally these two X's 

 break apart, and the separation does not always occur at the original point 

 of union. This leads to the production of a number of unusual combinations 

 of X's and fragments of X, giving complicated genetic results that are now 

 being studied. 



The unique behavior of the stock called bar (eye) has aroused a good deal 

 of interest. As first observed by May, and more thoroughly studied by 

 Zeleny, this dominant mutant reverts to normal eye with a frequency much 

 greater than that of any other mutation in Drosophila. By arranging the 

 experiment in such a way, that whenever crossing-over occurred near the 

 bar locus it could be detected, it has been demonstrated that whenever a 

 reversion occurs there is a cross-over in that region. Since crossing over 

 occurs only in the female it was to be expected that bar-reversion would take 



