198 GENETICS AND EUGENICS 



class 6, though they range all the way from class 2 to class 9. 

 The sixty-three Fa offspring likewise have their mode in 

 class 6, and are slightly more variable than Fi, though only 

 one aberrant individual falls beyond the range of Fi. This is 

 a case in which apparently many independent factors of 

 approximately equal influence on weight are concerned and 

 which do not segregate in linked groups. The result is that 

 both Fi and F2 vary symmetrically about the same strictly 

 intermediate mode (class 6). 



A case in which fewer factors are involved or in which the 

 factors are either not all of equal influence or occur in linked 

 groups is the following. Punnett and Bailey crossed two 

 breeds of fowls differing w^idely in weight, the larger breed 

 being represented in a gold-penciled Hamburg cock, the 

 smaller in silver Sebright bantam hens. The relative size 

 of the breeds is shown in Table 29a. As male fowls are larger 

 than females, the weight of each sex is tabulated separately 

 in absolute weight units (grams). The weight of the Fi birds 

 was much nearer that of the larger than that of the smaller 

 parent breed, an indication that one or more of the factors 

 for large size show dominance. An alternative interpretation 

 would ascribe the large size of Fi to hybrid vigor. (See 

 Chapter XXVII.) Possibly each explanation is in part 

 correct. The F2 generation showed very great variability in 

 weight, covering the ranges of both parent breeds, so far as 

 those ranges had been ascertained for the material studied. 

 But the variation curve for F2 was not symmetrical about an 

 intermediate mode,as in the case of ducks studied by Phillips. 

 The mode was close to the Fi mode, but the variation was 

 very '*skew," ending abruptly above, but sloping gradually 

 downward to bantam size. When the more extreme F2 in 

 dividuals were mated, large with large and small with small, 

 broods were obtained which averaged larger than pure Ham- 

 burgs and smaller than pure Sebrights respectively. 



Punnett interprets the case as involving four independent 

 factors having among themselves unequal influence on the 

 total weight. He supposes that three of the four factors are 



