184 



GENETICS AND EUGENICS 



qualitative change. By others it was assumed that the ob- 

 served character changes were not due to changes in single 

 genes but to the supplemental or modifying action of the 

 other genes. For example, the hooded pattern of rats (Figs. 

 124 and 125) clearly behaves as a simple unit-character alle- 

 lomorphic to Irish pattern or to self in crosses. But the 

 hooded pattern as seen either in pure-bred or in cross-bred 

 litters of young (Fig. 124) varies slightly, and such variations 

 have a genetic basis since by selecting either the whitest or 



Fig. 124. Inheritance of a recessive pattern of white spotting seen in " hooded " rats. The parents 

 (at the left) are a homozygous hooded mother and a heterozygous " Irish " father (black with white 

 belly). An entire litter of their young is shown at the right. Four are homozygous hooded like the 

 mother, five are heterozygotes like the father. Note fluctuation in both classes. Such fluctuations 

 are found to be in part heritable. 



the blackest individuals, one can either whiten or blacken 

 the average racial condition. (See Tables 24a and 25a.) 

 Races corresponding with the extremes of the series shown in 

 Fig. 125 were thus produced. The question now arose 

 whether the observed changes had occurred as a result of 

 change in the single unit-character or gene clearly concerned 

 in the case, or whether this was due to other agencies. To 

 test the matter the selected races, now modified genetically 

 in opposite directions, were crossed repeatedly with a non- 

 hooded (wild) race. The recessive hooded character dis- 

 appeared in Fi but was recovered again in F2 in the expected 

 25 per cent of this generation. Compare Fig. 56. These 

 extracted hooded individuals, following each cross, were less 

 divergent than their hooded grandparents from the ordinary 

 hooded pattern. After three successive crosses (six genera- 



