PRINCIPLES OF LIVESTOCK BPvEEDHSTG. 35 



to the common belief that proved prepotency in one respect, such as 

 color, indicates prepotency in others. Experiments, however, easily 

 prove the falsity of this claim. 



A white-faced red Hereford cow with normal horns produces polled, 

 white-faced, black calves when bred to a polled black Aberdeen- 

 Angus bull. An Aberdeen-Angus cow produces the same kind of 

 calf when bred to a Hereford bull. Evidently prepotency lies neither 

 in the sex, the breed, nor the individual, but in the characteristics, 

 polled head, black color (where there is any color besides white), and 

 white face. 



Somewhat similarly, a cross made in either way between an 

 Aberdeen Angus and a white Shorthorn produces polled blue-roan 

 calves. Polled head and black color are prepotent, as before, but 

 prepotency is lacking as regards the third pair of opposed character- 

 istics, the solid color of the Aberdeen Angus and the nearly solid 

 white of the Shorthorn. 



It is not the whole story, however, to say that certain character- 

 istics are always prepotent. If in the case above, the polled, blue- 

 roan Shorthorn-Angus crossbreds are bred back to a white Shorthorn, 

 only half the calves will be polled, the rest having good horns, and 

 only half will be black in the colored parts of the coat (that is, they 

 will be blue-roan or white, with black ears), the rest being red roans 

 and whites with red ears. Thus, the characters which were 

 fully prepotent in the purebred cease to be so in the crossbred. The 

 difficulty is that the crossbred produces more than one kind of re- 

 productive cell. In the present case, half of the reproductive cells 

 transmit the polled condition and half transmit horns ; half transmit 

 black and half transmit red; half transmit the tendency to develop 

 color in the entire coat, as in the Aberdeen Angus, and half transmit 

 the highly reduced condition of color, as in the white Shorthorn. 

 Moreover, the representatives of the three sets of opposed characters 

 are shuffled up and sorted out into the reproductive cells independ- 

 ently of one another. Some of the reproductive cells transmit the 

 combination polled, solid color and black, others polled, solid color 

 and red, and so on through the eight possible combinations. 



In this illustration we have used characteristics which have already 

 been discussed as examples of simple Mendelian heredity. Most 

 characteristics probably depend on a much larger number of heredi- 

 tary units, but, nevertheless, the nature of prepotency is believed to 

 be essentially the same. So far as there is prepotency, it is a property 

 of characteristics (or really of the hereditary units back of the char- 

 acteristics), not of individuals, breeds, or sexes, and whatever the 

 characteristic, there can be no prepotency unless the individual 

 produces only one kind of reproductive cell so far as it is concerned. 



