TYPES OF MENDELIAN HEREDITY 41 



abnormal as though the female had herself been abnor- 

 mal. The reciprocal cross, viz., abnormal females by 

 normal males, gives abnormal sons and daughters, if the 

 food is suitable, but normal if the food is dry, etc. In 

 both cases the F2 gives the expectation for a sex-linked 

 dominant factor if the medium is suited to bring out 

 the abnormal character, and the result is entirely ob- 

 scured if the food is dry. Here, at will, we can demon- 

 strate a regular Mendelian ratio by control of the 

 environment, and conversely, we can conceal com- 

 pletely what is taking place by substituting another 

 environment. That the same genetic process is going 

 on in both cases can be demonstrated by suitable 

 tests. 



A case similar in principle occurs in a mutant stock 

 of Drosophila that produces supernumerary legs. 

 This stock was observed in winter to produce a con- 

 siderable percentage of flies with supernumerary legs, 

 but few or none in summer, especially in warm 

 weather. Miss Hoge, who has studied this stock, 

 finds that when the flies are kept in an ice chest at a 

 temperature about 10° C. a high percentage of flies 

 with supernumerary legs occurs. Sometimes several 

 legs or parts of a leg are doubled, or the doubling 

 may occur twice in the same leg. The general rule 

 that Bateson pointed out for duphcated legs in other 

 insects appears to hold here, viz., the adjacent parts 

 are mirror images of each other. 



In the cold the dupHcate leg gives a regular 

 Mendehan result; but at normal temperature the 

 duphcation is a rare event and its mode of inheritance 



