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 (Pf) 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. 
