252 IOWA DEPART]MENT OF AGRICULTURE. 



one parent is dominant, it will manifest itself in the first cross, while 

 the corresponding character of the other becomes recessive. For example: 

 When a horned animal is crossed with a polled individual of the same or 

 of a different breed the offspring will be uniformly polled, because the 

 polled character is stronger than the horned character — in other words, 

 it is dominant. It is customary to represent the dominant character by 

 a capital and the recessive character by a small letter. From this it will 

 be seen, if we let "p" represent the polled and "h" the horned character, 

 that the offspring will be represented by ph. The germ cells ph of 

 the hybrid are not able to maintain themselves as such; they do not 

 seem to be stable, but segregate into individual germ cells or into their 

 component parts — in this case, polled and horned. This segregation 

 occurs in the egg and sperm cells alike, and in the pollen grains in the 

 case of plants. So instead of having the character pair ph represented 

 in the mature germ cell of the hybrid, we have the two separate charac- 

 ters, p and h (polled and horned), in separate germ cells; hence when 

 the hybrids are interbred, we have two kinds of germ cells, of both the 

 male and the female to deal with and to form possible combinations with 

 each other. The egg cells contain either the p or the h characters and 

 the sperm cells either of the same two characters. Suppose twenty egg 

 cells containing both these characters are fertilized with twenty sperm 

 cells containing the same characters, it is evident that we would have 

 ten egg cells with p characters and ten with h characters; the same would 

 be true of the sperm cells — there would be ten with p and ten with h 

 characters. A sperm cell with a p character fused with an egg cell with 

 the same character would produce offspring whose germ cells would con- 

 tain only p characters, which offspring would, therefore, be a pure poll. 

 It would also be possible for a sperm cell with an h character to unite 

 with an egg cell of the same character, and the resulting offspring would 

 produce germ cells with pure h characters and hence would be horned. 

 But opportunity would also be afforded for the combinations of ph and 

 hp to form, both of which would be alike and identical with the original 

 hybrid. 



From the above it will be seen that the polled hybrid is not capable 

 of reproducing polled offspring in every instance, but a second generation 

 cross between two like hybrids will produce three classes of animals, pure 

 polls, hybrids like the parents, and pure horned individuals. Further- 

 more, the pure polls will constitute 25 per cent, the hybrids 50 per cent 

 and the pure horned individuals 25 per cent of the total offspring. The 

 pure polls, so far as the polled character is concerned, will remain pure 

 forever afterwards. When bred to other pure polls the offspring will 

 always be polled, or if they are bred to horned animals the offspring will 

 be polled hybrids, the same as in the first instance. The horned individ- 

 uals, on the other hand, of which 25 per cent were produced, will transmit 

 the horned character to their offspring and never polled characters. The 

 50 per cent of hybrids will be further broken up if crossing is continued, 

 as in the first instance and in the same proportion, namely, into one- 

 fourth pure polls, one-half hybrids and one-fourth pure horned individuals. 

 In this way it will be seen that from 100 crosses of polled hybrids the 

 breeder may expect 25 pure polls, 50 hybrids and 25 with horns. It is 



