106 GENETICS 



Sometimes when dominance is not pronounced it is 

 possible to distinguish the heterozygote dominant from 

 the homozygote dominant. Correns has described an 

 excellent instance of this type. When plants of a 

 white-flowering race of the four-o'clock, MirabUis 

 jalapa, are crossed with those of a red-flowering race, 

 all the offspring in the first filial generation, unlike 

 either parent, exhibit rose-colored flowers. When, how- 

 ever, these rose-colored flowers are crossed with each 

 other, they produce red, rose, and white in the Men- 

 delian ratio of 1:2:1; that is, three colored to one 

 white. The red-flowering race thus proves to be 

 homozygous and the rose-flowering race heterozygous. 

 Here color dominates the absence of color, or white, 

 but the degree of the color depends upon whether the 

 dose of pigment is double, from both parents, or single, 

 from only one parent. 



9. THE "PRESENCE OR ABSENCE" HYPOTHESIS 



In place of Mendel's conception that every dominant 

 character is paired with a recessive alternative or 

 allelomorph^ there has been proposed the presence or 

 absence hypothesis which was first suggested by Correns 

 but later logically worked out by others, particularly by 

 Hurst, Bateson, and Shull. According to this inter- 

 pretation, a determiner for any character either is, 

 or is not, present. When it is present in two parents, 

 then the offspring receive a double, or duplex, "dose," 

 to use Hurst's word, of the determiner. When it is 

 present in only one parent, then the offspring have 



