398 ANIMAL BREEDING 



some worse than the parental combinations. For this 

 reason continuous selection of the best offspring of each 

 generation to become the parents of the next is always 

 necessary to secure any improvement or even to avoid 

 degeneration of the stock. It appears to be true that 

 animals are especially rich in supplementary genes (page 

 386) which modify the quantitative extent of a 

 character. Since these are numerous there is excellent 

 opportunity for continued selection to isolate genotypes 

 with more and more of these genes in both members of 

 the chromosome-pairs and so increase the quantitative 

 expression of the character. Of course selection is usually 

 equally effective in the opposite direction. 



Inheritance OF Coat-color in Guinea Pigs. — These 

 small animals are much used for experimental work in 

 biology on account of the satisfactory way in which they 

 endure captivity. Two characters of their hair will serve 

 as illustrations (Fig. 116) of the fact that many traits 

 in animals behave in strict accordance with Mendel's 

 laws of segregation and independent assortment. A 

 smooth-haired black guinea pig crossed to a rough-haired 

 white one gives hybrid offspring with rough black hair. 

 When these hybrids are interbred they yield a second 

 hybrid generation of 9 Black Rough; 3 Black smooth; 

 3 white Rough; 1 white smooth. Hair color in many 

 other animals behaves in essentially the same way, 

 although there are often more pairs of genes involved 

 and so the results are less simple. 



Inheritance of Hooded Pattern in Rats. — In an 

 extensive series of experiments with rats Professor Castle 

 showed that a pattern of the coat color can readily be 

 modified by selection. If a pure colored race is crossed with 

 one having a colored patch or hood over the head and 

 shoulders the hybrid offspring are all fully colored (Fig. 

 117). When the hybrids are interbred the second hybrid 

 generation shows 3 colored to 1 hooded. The hooded 

 ones are, of course, recessive and pure for the " hooded " 



