300 GENETICS 



tain degree of correlation or resemblance between indi- 

 viduals of the same descent. Brown-eyed parents produce a 

 greater proportion of brown-eyed offspring than do blue- 

 eyed parents, and vice versa. Vigorous and capable parents 

 produce a larger proportion of vigorous and capable off- 

 spring than do parents that are weak and inefficient. Weak 

 and inefficient parents produce a larger proportion of weak 

 and inefficient offspring than do vigorous and capable par- 

 ents. Yet it is all a statistical matter, a question of relative 

 proportions when large numbers of individuals are taken 

 into consideration. Usually there is great variety among the 

 offspring of any particular type of parents, though with a 

 tendency toward the parental characteristics. 



It is to be observed that the offspring of different pairs 

 of parents show very different degrees of similarity. The 

 offspring of parents that are alike in their genes and nearly 

 or quite homozygous will be nearly identical in their char- 

 acteristics. Thus, offspring of two parents that could both 

 be represented as AAbbCCDD would all have this same 

 constitution and would be, except for sex differences, as 

 similar as identical twins. In such a family identical twins 

 could not be distinguished from fraternal twins in which 

 the two members were of the same sex, nor indeed, except 

 as to age, from children of the same sex born at different 

 births. 



On the other hand, offspring of parents that are heter- 

 ozygous and unlike in their genes will be extremely varied. 

 Parents whose genotype could be represented as AaBbCcDd, 

 etc., will produce many types of offspring that may be ex- 

 tremely diverse. In such a family, identical or one-egg twins 

 would differ greatly from fraternal twins. 



Thus it is clear that the same degree of conventional re- 

 lationship, such as that of parent and offspring, or of broth- 

 ers and sisters, has very different genetic significance in dif- 

 ferent cases. If the genes of the two parents are nearly the 



