MENDELISM AND INTRODUCTION TO THE NEW HEREDITY 395 



zygote) leaves the individual derived from such a zygote lacking in 

 something essential for life. All such individuals in any breeding 

 experiment will fail to survive, and their absence will be noted when 

 the ratios of the various combinations are worked out. The failure 

 of a certain expected combination to appear in the F2 generation is 

 attributed to the presence of a lethal factor in the stock. It can readily 

 be proven that many of the surviving individuals possess the lethal 

 factor in a heterozygous condition, having one dose of the normal 

 allelomorph along with the lethal factor. These lethal factors can be 

 identified and located as readily as characters that actually appear. 

 The subsidiary hypothesis of lethal factors has had a far-reaching in- 

 fluence upon some of the most advanced phases of modern genetic 

 practice. 



THE RELATION OF SEX TO GENETICS 



Not many years after the rediscovery of Mendel's work, the chro- 

 mosome theory of sex determination, already outlined in chapters xxiv 

 and xxviii, grew out of our knowledge of MendeUan heredity. It came 

 to be recognized that sex is inherited in Mendelian fashion, as follows: 

 If we suppose that the male sex is a heterozygous dominant and the 

 female sex pure recessive, we can understand why males and females 

 are produced in equal numbers, for it is equivalent to the familiar 

 condition where an Fi hybrid is back-crossed with the pure recessive 

 parent: the F2 offspring are half hybrid dominants (Dr) and half pure 

 recessive (rr). The discovery that, as a rule, the male is hetero- 

 zygous (produces two kinds of gametes) and the female is homozygous 

 (produces but one kind of gamete) was confirmed by cytological study 

 and went a long way toward the establishment of the chromosome 

 theory of heredity. It is because of the discovery of the sex chromo- 

 somes that it became later possible to locate many other genes in the 

 same chromosomes as those responsible for sex; and the fact that the 

 heredity of these characters could be followed along with sex made it 

 possible to develop the hypotheses of linkage and crossing-over, as well 

 as several important hypotheses that now form essential links in the 

 series of generaUzations that make up modern genetics. 



