236 



The Journal of Heredity 



dicated below that females are prob- 

 ably somewhat lighter on the average 

 than the males. The standard devia- 

 tion of amount of black was 265 units 

 while the coefficient of variation was 

 61 per cent. It is evident then that the 

 spotting of these cows is extremely 

 variable. 



We then compared the two sexes in 

 amount of spotting, using the offspring 

 group in which the number of males 

 and females was equal. The mean of 

 all males was found to be 555.4 - 23.9 

 units, while the mean of all females 

 was 388.0 1 20.8. The difference in 

 mean grade was 67.39 - 30.86, or 

 slightly greater than twice its probable 

 error which can hardly be accepted as 

 statistically significant. We believe, 

 however, that in a larger sample the 

 males would be found to average 

 somewhat darker than the females, 

 and that this sexual difference ac- 

 counts for the fact that the random 

 sample of dams was somewhat lighter 

 than the mean.'' This probable sexual 

 difference does not affect the conclu- 

 sions which follow, since these are 

 based on averages of equal numbers of 

 male and female offspring. 



We next compared the mean grades 

 of the offspring of the light, medium 

 and dark bulls. The results of this 

 comparison are given in Table 1. The 

 most obvious fact shown by this table 

 is that the offspring of the three bulls 

 which differed considerably in dark- 

 ness, themselves differ in darkness in 

 the same way as their sires. The 

 progeny of King Model (Figure 15), a 

 very light bull, are much lighter on 

 the average than the offspring of King 

 Pieterje (Fig. 20), a very dark bull, 

 although the dams of the two lots 

 were of similar darkness. The light 

 bull produced light calves ; the medium 



bull produced calves of medium dark- 

 ness and the dark bull produced dark 

 calves. The amounts of influence of 

 the various sires on their offspring 

 cannot be measured accurately by this 

 method, for the group of cows with 

 which each bull was mated did not 

 prove to be of the same mean grades, 

 although the method of random choice 

 was designed to avoid such differences. 

 The most serious departure in this re- 

 spect is in the cows mated to the me- 

 dium bull. King of the Pontiacs, which 

 proved to be 200 units (20%) darker 

 than the cows mated to either the light 

 or dark bull. All groups of females, 

 however, fall within the medium class 

 and the mean grades of their calves 

 clearly show the influence of the sire 

 and point to the heritability of differ- 

 ences in darkness. 



Each male, however, differs in dark- 

 ness from the average of the females 

 with which he was mated. The calves 

 produced are in each case intermediate 

 in darkness between the sire and the 

 average of the dams. The average 

 grade of the calves is very near to the 

 average between the sire and dam as 

 shown bv the followingf tabulation : 



Spotting of Sire Light Medium Dark 



Av. Grade of Parents.. 220 

 Av. Grade of Offspring 201 



449 

 430 



623 

 638 



This might be taken as evidence of 

 a purely blending type of inheritance 

 in which dominance was absent, such 

 as that which characterizes some cases 

 of size inheritance. But mass statis- 

 tics alone are inadequate to establish 

 the mode of inheritance, since indi- 

 vidual differences are extinguished by 

 averaging and we know now that in- 



''An interesting parallelism is thus established between the spotting of Holstein cattle 

 and the piebald spotting of guinea pigs. Wright (Proceedings Matio)iaI Academy of 

 Sciences, 6:321, 1920) found that the males of all the piebald stocks of guinea pigs which 

 he had observed had about seven per cent more color than the females. In the pre- 

 liminary figures given above the Holstein males are darker than the females by about the 

 same amount. In both animals sex, as Wright has suggested, acts as a modifier for the 

 colored and white pattern, which is inherited independently of sex. A comparison of spot- 

 ting in the males and females of two inbred stocks of piebald mice reveals no significant 

 sexual difference (unpublished data). 



