290 GENETICS 



dominant represents the normal condition, while the various 

 recessive conditions are different grades of imperfection, 

 so that the individuals that manifest the recessive condition 

 are at a disadvantage. In other cases, much less frequent, 

 the dominant condition represents a defect. 



In view of the usual association of weakness or defective- 

 ness with recessiveness, it is clear that there is an advantage 

 to the organism in the doubleness of its genes. A gene de- 

 rived from one of the parents may be defective; if this were 

 the only gene of that kind present, the individual carrying it 

 would be defective. Some of its required functions would 

 be ill-performed. But the corresponding gene from the other 

 parent may be normal. Then as a rule this normal gene is 

 dominant; it performs the required function properly so 

 that the individual is normal. In consequence, defective indi- 

 viduals are much less numerous than would be the case if 

 their genes came from a single parent only. The doubleness 

 of the genes acts as an insurance, preventing the harmful 

 results of many defective genes. 



Examine now the relation of the characteristics of parents 

 to those of offspring, particularly as to relative fitness and 

 defectiveness, in some of the more significant results of the 

 recombinations that occur. We deal with simpler cases first. 



A. One parent defective, owing to the presence of two 

 defective genes (aa) in one of his gene pairs. The other 

 parent normal, having the two normal dominant genes 

 (AA) in that pair. 



All the offspring (Aa) are normal, having the char- 

 acteristic of one parent to the exclusion of that of the other. 



B. Both the parents are normal, but each has a defective 

 gene in the same pair (so that they are Aa). Then some of 

 the offspring (aa) resemble neither parent, but are defec- 

 tive. Other offspring (AA and Aa) are normal, like the 

 two parents. 



C. One parent defective (aa), the other normal, but 



