32 



PHYSICAL BASIS OF HEREDITY 



The case of Mirabilis, or of the Andalusian fowl, might 

 be represented (Fig. 9) in the following scheme: 



A 



\. 



Fig. 9. — Relation of Andalusian to splashed white and to black as shown by classes 



of F2 birds. 



Here all three types are fnlly separable, in which case 

 either homozygote might be considered the dominant. 



Finally, to return to the case of the tall and short 

 peas, the following scheme (Fig. 10) represents the F2 



Fig. 10. — Relation of tall to short peas as shown by F2 classes. 



group: Here the tall and the heterozygous group are 

 alike, and inseparable by ordinary inspection, even at 

 the extreme end of their variation curves, and short is 

 ^'completely'' recessive. 



In cases in which the environment enters more 

 obviously into the result (as in ' ' abnormal abdomen, ' ' Fig. 

 5), the following scheme (Fig. 11) represents the relation : 



Dry 



Wet 



Fig. 11. — Relation of normal to abnormal abdomen as shown by classes of Fi flies. " Dry" 

 signifies conditions that make for normal; wet for abnormal. 



In this case both the heterozygous and the parental 

 *' abnormal'' type may show '^ normal" abdomen like the 



