GENES MODIFYING NOTCH. 387 



SUMMARY. 



(1) Mass selection on a dominant character called Notch was carried 

 out through 24 generations of Drosophila melanoga&er, with the result 

 that a change occurred in the direction of selection. Notch wing 

 is caused by a dominant gene in the sex-chromosome. In addition 

 to its dominance, the gene produces a recessive lethal effect, killing 

 every male that carries the gene. Notch females are heterozygous for 

 the Notch gene, i. e., one X chromosome carries the gene for Notch, the 

 other X chromosome its normal allelomorph. The latter saves the 

 female from the lethal effect of the Notch gene. Since no Notch males 

 exist, it is not possible to state whether the Notch gene would also be 

 lethal in double dose in the female, but that such is almost certainly the 

 case is shown by the absence of such females that might arise through 

 equational nondisj unction, i. e., by two Notch-bearing chromosomes 

 remaining in an egg that was then fertilized by a Y sperm. Such a 

 female, if she could be produced, would have no sons, and all of her 

 daughters would be Notch (instead of half of them as usual). No such 

 female appeared. The case of two females with high sex-ratios de- 

 scribed in this paper are shown to be due to a lethal factor that had 

 appeared in the "normal" X chromosome of the father of the female in 

 question, etc. 



(2) By a suitable method described in the text it is shown that the 

 changes brought about by selection were due to the presence in the 

 stock of a recessive modifying factor in the second chromosome. Notch 

 females homozygous for this factor give the "selected group." Those 

 heterozygous for it or lacking it altogether give the atavistic or original 

 group. . 



(3) Since in every one of the 24 generations of this experiment the 

 gene for Notch is in a heterozygous condition an extraordinarily 

 favorable chance exists for contamination of the Notch genes, if such a 

 thing is possible. Were it possible the results of the selection might be 

 supposed to be due to an influence of the normal gene on the Notch 

 gene. Mass selection was practiced in the same direction that such a 

 supposition would lead to. That the result was not reached in this 

 way is shown not only, as stated above, through the demonstration of 

 the specific modifier involved, but also by out-crossing; for if at any 

 time the selected Notch females (even those not showing any Notch at 

 all) are bred to flies of almost any wild stock, the atavistic Notch is re- 

 covered in the first generation. Here, owing to the dominance of the 

 character, one can obviate completely the difficulty that Castle met 

 with when studying the influence of selection on a recessive character. 

 Castle was obliged to out-cross his rats and then inbreed the FI. The 

 chance, unless guarded against scrupulously, of introducing new 

 genes into the result is ever present under such conditions and does 



