288 GENETICS 



cause in time there occur genetic variations, changes in the 

 genetic system, such as are dealt with in Chapters XIII and 

 XIV, Most such genetic variations are harmful, so that 

 under the severe conditions of nature the individuals in 

 which they occur die or fail to propagate. Thus in a state 

 of nature the uniform and homozygotic conditions may be 

 perpetuated by this selective elimination of the individuals 

 in which genetic change has occurred. In man and in do- 

 mesticated animals this does not happen, because the weak 

 or defective individuals resulting from genetic variations 

 are given special care, so that they survive and propagate. 

 Thus such organisms come to be extremely varied as to the 

 genes that they carry. 



J. Usually there exist within species, even those living 

 under natural conditions, several or many different stocks, 

 having different combinations of genes and of character- 

 istics; these are known variously as stocks, races, varieties, 

 subspecies, and the like. When two stocks having different 

 gene combinations mate there may appear in their descend- 

 ants may different combinations of genes and of character- 

 istics. 



Thus, suppose two stocks differ in four pairs of genes, 

 although both are homozygous. Representing by capital 

 letters the genes that are dominant and by lower case letters 

 those that are recessive, the two stocks may be, for example, 

 AAbbccDD and aaBBCCdd. The germ cells from these two 

 stocks will be AbcD and aBCd respectively. When these 

 unite there are produced individuals (F i ) , all of which have 

 the constitution AaBbCcDd; they are heterozygotic for all 

 the four pairs of genes. 



Suppose now that these individuals of Fi interbreed. Each 

 produces numerous different types of germ cells, having dif- 

 ferent gene combinations. From the four heterozygotic 

 pairs there are i6 types of germ cells producible; these are: 

 ABCD, ABCd, ABcD, AbCD, aBCD, ABcd, AbCd, AbcD, 



