GENERAL RELATIONS IN INHERITANCE 295 



inferior in one or several ways, as illustrated in figure 61. 



In such ways are produced most of the hereditarily de- 

 fective individuals. They may arise by unfortunate recom- 

 binations in normal or even superior families. They may 

 arise also, of course, from defective parents, as set forth in 

 paragraph D, above. But the number of normal or inter- 

 mediate families are so much greater than the number that 

 are distinctly inferior or superior that the greater propor- 

 tion of inferior, as well as the greater proportion of superior 

 individuals, in such an organism as man, arise from the 

 usual mediocre families. 



Two points mentioned in the foregoing paragraphs are 

 worthy of separate emphasis. Superior offspring are more 

 likely to be produced through the mating of unrelated stocks, 

 while under usual conditions, defective offspring are most 

 likely to be produced through the mating of closely related 

 individuals. These two facts constitute the genetic basis for 

 the undesirability of inbreeding. When extensive inbreed- 

 ing occurs, many defective offspring are produced. 



Yet through long continued inbreeding, combined with 

 selection of the best individuals for further propagation, 

 permanently superior stocks may be produced. This matter 

 is dealt with later (pages 312-314). 



H. Production of new gene combinations through bring- 

 ing together genes from diverse ancestors: The genes borne 

 by the offspring of any two parents are of course a combina- 

 tion from the genes of the parents, a combination that con- 

 tains half the genes from each parent. If traced farther 

 back they are a combination of the genes borne by the four 

 grandparents, by the eight grandparents, and so on. 



These relations are indicated in the two diagrams, figures 

 62 and G^)-, which give two of the possible different combina- 

 tions resulting in the fourth generation (IV) from the com- 

 mingling of the genes of the eight ancestors of generation I, 

 the four of generation II and the two of generation III. 



