GENERAL RELATIONS IN INHERITANCE 29 1 



bearing one of the defective genes (so that he Is Aa). Then 

 half of the offspring (aa) are Hke the defective parent, the 

 other half (Aa) like the normal parent. 



D. Both parents defective, each having two defective 

 recessive genes In a certain pair (so that both are aa). Off- 

 spring (aa) are like the two parents, defective. 



E. Both parents defective, but owing to two recessive 

 genes In different pairs (figure 47, page 221). Offspring un- 

 like both parents: normal, not defective. 



In such cases defective parents produce normal offspring, 

 because each parent carries a normal gene to correspond 

 with the recessive gene of the other parent. The two parents 

 may In such a case be defective for the same characteristic. 

 For we know that every characteristic is affected by many 

 genes, and can be made defective by modification of many 

 different pairs of genes. Thus, one of the parents may be 

 defective In the pair AA, the other in the pair BB; the 

 former parent Is to be represented as aaBB, the latter as 

 AAbb. The offspring are therefore AaBb, normal and 

 dominant for both the pairs. 



F. The two parents supplement one another In several 

 or many pairs of genes. — Both parents may be recessive, 

 and therefore defective or weak in respect to a number of 

 different pairs of genes, but these may be different pairs In 

 the two parents (figure 60, P and M). In such cases the 

 offspring have a normal dominant gene for each of the pairs 

 (figure 60, F) and will therefore show none of the numer- 

 ous defects observed in the two parents. The offspring are 

 much more vigorous than the parents, and superior to them 

 in many ways. 



This situation commonly occurs when two stocks, not too 

 diverse, are crossed. The two stocks have lived apart for 

 many generations. In each there has In the course of time 

 collected through genetic variation of some sort a con- 

 siderable number of recessive genes, causing weakness and 



