174 INBREEDING AND OUTBREEDING 



abnormal in order to have a reasonable explanation of the 

 increased development of hybrids in the first generation 

 over the average of the parents or subsequent generations. 

 In the first hybrid generation the maximum number of 

 different factors can be accumulated in any one individ- 

 ual ; and because of factor linkage it is extemely difficult 

 to recombine in one organism in later generations any 

 greater number of homozygous characters than were 

 present in the parents, provided the factors are distrib- 

 uted at random in all of the chromosome pairs. This 

 view of the situation makes more understandable 

 why the effects of heterozygosis result in an increase 

 in development, and why they remain throughout 

 the life of the sporophyte, even though innumerable 

 asexual generations. 



The abstract view of the dominance hypothesis may 

 be somewhat clearer if a concrete diagrammatic illustra- 

 tion is made. A case will be assumed, in which two homo- 

 zygous individuals, having three chromosome pairs, both 

 attain the same development as represented by any meas- 

 urable character. This development will be considered to 

 amount to 6 units, 2 of which are contributed by each 

 chromosome pair. One of these individuals, which we will 

 call "X," attains its development through the operation 

 of factors distributed in the three pairs of chromosomes, 

 each differing from the others in its contribution. Any 

 number of factors can be chosen, but, for the sake of sim- 

 plicity, only three in each chromosome will be employed. 

 These are numbered 1, 3, 5; 7, 9, 11; and 13, 15, 17 in the 

 accompanying diagram (Fig. 36). The second individual, 

 "Y," develops to an equal extent in the character meas- 



