GENETICS AND EUGENICS 



243 



the person to pass for white. This is explained by the fact that there 

 are two allelomorphic pairs concerned with the inheritance of color, 

 the dominant of either of which will produce a certain amount of pig- 

 ment per square millimeter of skin surface. The negro of pure skin 

 inheritance is homozygous for both pairs. Using P and P' to repre- 

 sent the dominant factors of these two allelomorphs and p and p' the 

 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 similar to that of Fig. 78. The accompanying summary shows 

 that there caij be four, three, two, one, or no dominant factors present, 

 thus accounting for the series: ''negro," ''chocolate," "mulatto," 

 "quadroon" and "pass for white." 



ppt Ppt ppt ppt 



ppt 



Pp 



pp 



PP* 



Fig-. 78. — 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 plural 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 produce the entire effect. 



