l6 SEX-LINKED INHERITANCE IN DROSOPHILA. 



such a female should all (excepting a rare one due to crossing-over) 

 give 2 : i ratios, because each daughter must get one or the other X 

 chromosome of her mother, that is, one or the other lethal. Although 

 the mother was fertilized by a normal male, every daughter is hetero- 

 zygous for one or the other of the lethal factors. The daughters of the 

 two-lethal females differ from the daughters of the one-lethal female in 

 that the former mother, as just stated, gives all lethal-bearing daughters; 

 the latter transmits her lethal to only half of her daughters. 



INFLUENCE OF THE ENVIRONMENT ON THE REALIZATION OF TWO 



SEX-LINKED CHARACTERS. 



The need of a special environment in order that certain mutant 

 characters may express themselves has been shown for abnormal 

 abdomen (Morgan, 1912^, 191 5&) and for reduplication of the legs 

 (Hoge, 1915). In a third type, club, described here (page 69), the 

 failure of the unfolding of the wing which occurs in about 20 per cent 

 of the flies is also without much doubt an environmental effect, but as 

 yet the particular influence that causes the change is unknown. 



A very extensive series of observations has been made on the char- 

 acter called abnormal abdomen. In pure cultures kept moist with 

 abundance of fresh food all the flies that hatch for the first few days 

 have the black bands of the abdomen obliterated or made faint and 

 irregular. As the bottles get dry and the food becomes scarce the flies 

 become more and more normal, until at last they are indistinguishable 

 from the normal flies. Nevertheless these normal-looking flies will give 

 rise in a suitable environment to the same kind of flies as the very 

 abnormal flies first hatched. By breeding from the last flies of each 

 culture, and in dry cultures, flies can be bred from normal ancestors for 

 several generations, and then by making the conditions favorable for 

 the appearance of the abnormal condition, the flies will be as abnormal 

 as though their ancestors had always been abnormal. Here, then, is a 

 character that is susceptible to the variations in the environment, yet 

 whatever the realized condition of the soma may be, that condition 

 has no effect whatever on the nature of the germ-plasm. A more 

 striking disproof of the theory of the inheritance of acquired characters 

 would be hard to find. 



A demonstration is given in this instance of the interaction between 

 a given genotypic constitution and a special environment. The char- 

 acter abnormal is a sex-linked dominant. Therefore, if an abnormal 

 male is mated to a wild female the daughters are heterozygous for 

 abnormal, while the sons, getting their X chromosome from their 

 mother, are entirely normal. In a wet environment all the daughters 

 are abnormal and the sons normal. As the culture dries out the 

 daughters' color becomes normal in appearance. But while the sons 



