182 INBREEDING AND OUTBREEDING 



alone, and if it usually segregates germ cells of the types 

 AbC and aBc, making the vigor thus obtained a practical 

 corollary of heterozygosity, there is still the chance, no 

 matter how closely linked these factors may be, of breaks 

 occurring which will bring about the production of a gam- 

 ete ABC. This gamete, if it meets another of the same 

 type, will result in a homozygous individual, and if dom- 

 inance is but partial, this individual, through the very 

 fact of its homozygous condition, will be even more vig- 

 orous than those of the first hybrid generation. 



Practically the difficulties in the way of obtaining such 

 pure combinations may be very great or even insurmount- 

 able, but the hypothesis holds out the hope of thus ob- 

 taining types of great economic value. The rearrange- 

 ment of factors in all possible recombinations is not pre- 

 vented by linkage as long as there are breaks in the 

 linkage. But since these breaks occur with varied fre- 

 quency between different factor loci, and in some cases are 

 very rare, the problem is exceedingly complicated; and 

 when many linked factors are involved the chance of ob- 

 taining an individual which is completely homozygous 

 in all factors in the first segregating generation is so ex- 

 tremely small that for all practical purposes it may be 

 disregarded. In later generations the chance may become 

 somewhat greater because of the formation of new linked 

 groups. If these are combinations which are favorable to 

 development, natural selection will increase individuals 

 possessing them at the expense of those having less favor- 

 able combinations. In time, there is the possibility, how- 

 ever remote, of all the more favorable factors being 

 brought together in a homozygous combination which, 

 therefore, will not be reduced by inbreeding. 



