GENERAL RELATIONS IN INHERITANCE 313 



having the new combination of characteristics that it is de- 

 sired to preserve, this stock is permanent (so long as it 

 does not again interbreed with other stocks). 



The mechanism of the operation of inbreeding may be 

 illustrated from simple cases. Suppose that a stock is heter- 

 ozygotic for a single pair of genes, Aa, and that it is desired 

 to obtain a permanent stock with the characteristic repre- 

 sented by the dominant gene A. If we breed together two 

 members of this stock, we obtain the three combinations 

 AA, Aa, and aa. The two former have the character A 

 which we wish to preserve. If we breed together such indi- 

 viduals as have this character, we obtain in different cases 

 the pairs AA X AA, AA X Aa and Aa X Aa. The first of 

 these, AA X AA, will yield offspring AA, with no admixture 

 of other types. Continuing to breed together AA X AA, 

 we obtain a stock in which all the individuals have the char- 

 acter A. 



Or similarly, if we wish to obtain a stock all of which 

 have the recessive character represented by a, we have but 

 to breed together the recessive individuals aaXaa; the 

 descendants are all aa. 



If the original parents are heterozygotic for two pairs, 

 so that they may be represented as AaBb, when they inter- 

 breed we obtain the nine different gene combinations given 

 on page 1 17. Some of these show the characters represented 

 by the two dominant genes AB, some show one dominant 

 and one recessive, Ab or aB, some show two recesslves, ab. 

 Suppose that we wish to obtain a permanent stock with 

 characters Ab. We Interbreed all the individuals showing 

 those characters. Some of them will not breed true; such for 

 example are Aabb X AAbb; these will yield some recesslves, 

 abab. But if in some of our matlngs we breed together the 

 homozygotes AAbb X AAbb, these give only offspring 

 AAbb. These when bred further remain constant. 



When the number of gene pairs In which the parents 



