30 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



MENDELIAN HEREDITY IN LIVESTOCK. 

 POLLED CATTLE. 



We know much less about the details of heredity in the larger 

 animals than in a number of small ones, such as the guinea pig, 

 rabbit, rat, mouse, and especially Prof. Morgan's fruit fly. Never- 

 theless unit factors have been demonstrated in a considerable number 

 of cases. Polled and horned cattle, for example, differ by a single 

 unit in their heredity. The factor which determines the polled con- 

 dition is nearly fully dominant over its alternative in horned cattle. 

 In a cross between the polled Aberdeen Angus and the Shorthorn, 

 most of the calves are wholly polled, and the rest, as a rule, merely 

 have loose scurs in the skin. These scurs are more frequent in males 

 than females. The same factor has appeared within the Shorthorn, 

 Hereford, and other breeds and has permitted the formation of polled 

 subbreeds. 



Polled bulls produce 100 per cent polled or nearly polled calves if 

 they are homozygous polled like the Aberdeen Angus breed (PP). 

 Otherwise (Pp) they produce 50 per cent polled and 50 per cent 

 horned in crosses with horned cattle (pp). The polled animals can 

 be crossed generation after generation with horned stock without 

 reducing the per cent of polled calves below 50 to a significant extent. 

 Their horned descendants, on the other hand, have no more tendency 

 to transmit the polled condition than ordinary horned cattle. The 

 polled character can easily be fixed if the mode of inheritance is 

 borne in mind in making all matings. The most important point is 

 to use exclusively bulls which have been proved to be homozygous 

 polled (PP). Such bulls can be produced only when both parents 

 are polled. All polled calves produced by a polled bull from a polled 

 cow are not, however, homozygous. Unless it is known that both 

 parents were homozygous, the most promising bull calves among 

 those with scurs small or absent should be picked out and tested 

 with a number of horned cows. It should be easy to find one which 

 transmits only the polled condition. 



COLORS OF CATTLE. 



The black Aberdeen-Angus, Galloway, and Holstein-Friesian cattle 

 differ in their hereditary make-up from the red breeds (Shorthorn, 

 Hereford, Ayrshire, Devon, etc.) by a unit factor. Black is dominant 

 over red, and thus may transmit it. The red may be handed on for 

 generations out of sight, to appear when two blacks are mated both 

 of which transmit it. Even to-day a red calf occasionally is born in 

 a respectable black herd of Aberdeen-Angus cattle. In such case it is 

 well to remember that the sire, as well as the dam, is transmitting it 

 in half his reproductive cells and had best be replaced. 



