THE 



AMERICAN NATURALIST 



Vol. XLV February, 1911 No. 530 



THE APPLICATION OF THE CONCEPTION OF 

 PURE LINES TO SEX-LIMITED INHERI- 

 TANCE AND TO SEXUAL DIMORPHISM 1 



PROFESSOR T. H. MORGAN 

 Columbia University 



In the same sense in which our ideas concerning 

 variation and heredity have been entirely revolutionized 

 since 1891, so has a similar change taken place in regard 

 to our theories of sex determination. Sex is now treated 

 by the same methods that are used for Mendelian char- 

 acters in general. From this point of view I propose to 

 consider to-day three questions, intimately associated. 

 First, the treatment of sex as a Mendelian character; 

 second, the relation between sex and the inheritance of 

 secondary sexual characters; third, the bearing of the 

 recently discovered cases of "sex-limited-inheritance" 

 on the problem of the transmission of characters in 

 general. 



Most modern theorists are in agreement that the 

 heredity of sex can be best understood when one sex is 

 regarded as a pure line, or homozygous, and the other 

 sex is treated as a phaenotype, i. e., as heterozygous. 

 The experimental evidence has made it plain that in 

 some animals and plants it is the female that is hetero- 

 zygous, and in other animals and plants it is the male 

 that is heterozygous. Hence have arisen through the 

 necessities of the situation the two following classes of 

 formulae : 



'From a symposium on "The Study of Pure Lines of Genotypes," 

 before the American Society of Naturalists, December 29, 19 A. 



65 



