66 PHYSIOLOGICAL GENETICS 



The threshold conception had already been used by Gold- 

 schmidt in his firsl analysis of the phenocopic effect (19206), as 

 well as before, in his first analysis of intersexuality (the epistatie 

 minimum, Goldschmidt, 1912, 1920c). Wright also (1916) had 

 realized its importance in explaining the action of the pigmenta- 

 tion genes in guinea pigs (see page 88), which he conceived in a 

 way similar to that presented in Goldschmidt's simultaneous 

 work. It has since been used considerably by many authors who 

 discuss questions of rate (Crew, Haldane, Huxley, Plunkett, 

 Wright, Mohr, et al.; see also its interesting application to certain 

 problems of sex determination by Dobzhansky and Goldschmidt, 

 1934c). W r e shall meet it over again in connection with different 

 problems. Here only one of its aspects will be mentioned, which, 

 though highly important, has not yet been analyzed sufficiently. 

 In Mohr's table (page 57), we found that the lowest members 

 of the v ^-series, which produce only a small nick in the wing, do 

 it not in all individuals but only in a small percentage. In 

 higher gene combinations, this percentage increases finally up to 

 100 per cent, parallel with a quantitative increase of scalloping. 

 Here is a very important problem from the standpoint of gene 

 action. 



Timofeeff-Ressowsky (1926) has called this peculiarity of the 

 action of some genes or alleles, which in Drosophila melanogaster 

 is typical for some lower alleles in a series and which is in other 

 species of Drosophila the most frequent form of gene expression. 

 the penetrance; this term might sometimes be useful for a short 

 description of such cases. An explanation of the phenomenon in 

 terms of threshold is suggested, when this phenomenon appears 

 as a member of a simple quantitative series of effects at one end 

 of the series, as in the vg-ca.se. Here a further feature is also 

 added, viz., a parallel increase of the percentage phenotypic 

 effect (penetrance) with the increase of the degree of the phe- 

 nomenon, in this instance the amount of scalloping. (Timofeeff 

 calls this the expressivity, a term that might lead to rather 

 dangerous misunderstandings.) Such a series of facts of an 

 orderly linear nature requires an explanation of the same type. 

 This is easily conceived if the threshold concept is applied. To 

 show that t he same general type of interpretation may work with 

 different conceptions in detail, we may apply two such concep- 

 tions to the vestigial case as follows: 



