GENES MODIFYING NOTCH. [^J 



SUMMARY. 



(1) Mass selection on a dominant character called Notch \v:i.s ,■ irntni 

 out through 24 generations of Drosophila melanoga^icr , with the rc-sult 

 that a change occurred in the du-ection of selection. Notch win^ 

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

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

 every male that carries the gene. Notch females are heterozyKou.s for 

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

 other X chromosome its normal allelomorjih. The latter sa\-e«' the 

 female from the letlial effect of the Notch gene. Since no Notch niiilen 

 exist, it is not possible to state whether the Notch gene would also l>e 

 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 throiigli 

 equational nondisjunction, i. e., by two Notch-bearing chromosomeh 

 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 show^i to be due to a letlial fact<>r that had 

 appeared in the ''normal" X chromosome of the father of the fcnuile 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 extraordiiuirily 

 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 wat> not re^iched in this 

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

 the specific modifier involved, but also by out-cros.sing; 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 n^ 

 covered in the first generation. Here, ow4ng to the dominance of the 

 character, one can obviate completely the difficulty that Castle met 

 with w^hen studying the influence of selection on a recessive cluiractor. 

 Castle was obliged to out-cross his rats and then inbrecd the Fi. The 

 chance, unless guarded against scrui^ulouslj', of introducing new 

 genes into the result is ever present under such conditions an- 1 >\'»'< 



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