GENETICS AND EUGENICS 



829 



recessives, the homozygous negro would have PPP'P' while the ho- 

 mozygous white person would have ppp'p' and the mulatto resulting 

 from the first cross would have PpP'p'. The cross between two mulat- 

 toes would be a dihybrid cross which could be plotted by a checker- 

 board essentially similar to that of Fig. 432. The accompanying chart 

 (Fig. 432) shows that there can be four, three, two, one, or no dom- 

 inant factors present, thus accounting for the series known under 

 the following technical terms: ''negro," "chocolate," ''mulatto," 

 "quadroon" and "pass for white." 



ppl 



Pp* 



pp« 



PP* 



Fig. 432. — The inheritance of color by children of mulatto parents can be shown by 



the checkerboard for a dihybrid cross. 



The other type of case in which multiple genes occur is exemplified 

 by a certain brown-seeded variety of oats in which there are two pairs 

 of genes concerned with the determination of color. Here the pres- 

 ence of the dominant gene of either pair produces the entire effect. 

 This case, too, can be plotted on the dihybrid checkerboard, but the 

 result is a 15 :1 ratio, for fifteen of the sixteen squares would show at 

 least one dominant gene. Crosses between a red and a white variety of 

 wheat in which there are three pairs of genes for color, produce a 63 :1 

 ratio. This is a trihybrid cross in which each dominant gene can pro- 

 duce the entire effect. 



Complementary Genes. — In a number of cases color is produced 

 by two allelomorphic pairs of genes, the dominants of which must 

 react on each other to produce the color effect. In the sweet pea, if 

 the dominant factor of the red allelomorph (Rr) is acted on by the 



