178 GENERAL ZOOLOGY 



eration follow definite laws. Of course, in a single litter the 

 numbers may be different, but if large numbers of individ- 

 uals are considered the second generation of young (called 

 F 2 for convenience) from the black and white grandparents 

 will be in the ratio of three blacks to one white. If white 

 individuals of this second generation are bred together they 

 produce nothing but white young even though the grand- 

 parents were coal black. If the black guinea pigs of the 

 second, or F g} generation are bred together we find that 

 though they all look the same there are two different kinds 



X Cf^j) Pen cuts 0dP X <X ^3 

 F 



F 9 



r 



x 



j_ 



F I ' 1 I 1 1 1 I ' 1 



Fig. 98. A diagram showing how hair color is inherited in guinea pigs 



of the black individuals. One out of three of the black 

 animals of the F g generation can produce nothing but black 

 offspring, and two thirds of the black individuals of the F 2 

 generation, when bred together, may produce either white 

 or black young. Those individuals of the F 2 generation 

 which are capable of passing only their own color to their 

 offspring are said to be homozygous. In contrast, those 

 which may have offspring either like themselves or showing 

 the recessive character are called heterozygous. 



Since each individual has its origin as a pair of germ cells 

 which unite to form a fertilized egg, we must go back to the 

 germ cells to seek an explanation of how heredity operates. 

 It is very generally agreed that the chromatin within the 

 nuclei of the germ cells is the material concerned in matters 



