70 PHYSIOLOGICAL GENETICS 



of the individuals t<> reach the threshold. In those cases, the 

 position of the threshold was determined by the quantity of a 

 determining substance or the time of its onset. It may be safely 

 assumed that in most eases of penetrance this is the actual 

 situation. But it must be kept in mind that in each case the 

 action of modifying genes will first have to be excluded, before 

 any conclusion upon gene action may be drawn from the material. 

 We may conveniently group the material into the following 

 types: (1) genes that under normal conditions do not manifest 

 their action but have a visible effect under other definite condi- 

 tions; (2) genes that under standard conditions (and excluding 

 modifiers) produce a visible effect in a certain percentage of the 

 individuals containing the gene. It is understood that in such 

 cases the normal individuals must be proved to have the same 

 genetic constitution as the others. In this case, genetic or 

 environmental conditions may be shown to control the percentage 

 of changed phenotypes. Examples for these types of behavior 

 are abundant in genetic literature. We shall report only those in 

 which experimentation has yielded information relevant to the 

 problems under discussion. 



There is the frequently quoted case of reduplication of legs in 

 Drosophila due to a sex-linked mutant gene. Hoge (1915) 

 found that this type appears only in a certain percentage of the 

 homozygous flies but that if reared at a low temperature the per- 

 centage increases from between 10 and 15 to between 30 and 68 

 per cent. The action of low temperature was found to be limited 

 to a sensitive period which is situated very early in development. 

 This is, of course, to be expected, as the action must take place 

 before the imaginal disks of the legs are finally determined. One 

 might conclude from the facts that the action of the mutant gene 

 takes place at a time very near to the time of final determination 

 of the Anlage, a time limit beyond which a fluctuation occurs 

 depending upon temperature (see end of this section, page 72). 

 Similar results have been reported by Morgan (19156) for the char- 

 acter abnormal abdomen where humidity is the decisive agent, by 

 Komai (1926) for crippled legs in Drosophila, by Astauroff (1930) 

 for the mutant tetraptera. (Here the penetrance increases from 

 1 to 35 per cent with a rise in temperature from 17 to 27°C.) 



Of a somewhat different type is the work of Wright (1934a) on 

 extra toes in guinea pigs. This character is not the result of a 



