148 DOMESTICATED ANIMALS AND PLANTS 



shown in Fig. 25. Here we have a hybrid offspring from an 

 albino sire and a black dam. The offspring are all black, so 

 black is dominant over white. Their offspring are, however, of 

 two kinds, both black and white, but in the proportion of 3 to 1 . 

 Of this group of four, therefore, only one, the white, can with 

 certainty be counted upon to breed true. Some of the blacks 

 will also breed true, but only the breeding test will determine 

 which they are. 



This whole matter is up in full force in all attempts at im- 

 provement by crossing, whether among plants or animals, which 

 is the reason why animal breeders especially avoid this form of 

 breeding, though it is a favorite method in the improvement of 

 plants, which can be produced in large numbers. 1 



When the parents differ in two unit characters, the case is 

 more complicated, but the principle remains the same, namely, 

 that all possible combinations will occur and a perfectly definite 

 number of each may be expected. Again, Professor Castle's work 

 with guinea pigs illustrates the point especially well. 



In Fig. 26 are shown a dark-colored smooth-haired and an 

 albino rough-haired parent. Their offspring were all dark and 

 rough as shown in the middle figure, but some of their progeny 

 were smooth and white as shown in the lower figure, while others 

 were like each of the original parents, and still others like the 

 first hybrid ; that is, all possible combinations had been made. 

 In this case the Mendelian expectation is 3 : 3 : 9 : 1 . 



Pure races may spring from crossing. The facts just pre- 

 sented show XhdX for characters that blend, the hybrid will breed 

 pure as a single new race, but that for characters that do not 

 blend, the individuals may or may not be pure and may or may 

 not breed true. 



All the facts go to show that whether the offspring of hybrid 

 parents consist of three groups as when only one character is 

 involved, or whether they consist of many groups as when two or 



1 The student of breeding should understand, however, that crossing is 

 equally effective with animals and plants, except that the very large numbers 

 involved makes it too expensive for most individual animal breeders. 



