130 THE FIRST PRINCIPLES OF HEREDITY 



four visible types of individuals in the following propor- 

 tions : F 2 = 9 AB + 3 Ab + 3 aB + 1 ab. 



It will be seen on closer examination that out of the 

 nine AB, only one is a pure dominant, having the characters 

 ABAB, and therefore breeding true. All the others, though 

 they look like the dominant, contain a recessive character 

 in addition, either a or b or both together, and are therefore 

 impure dominants. Of the three Ab, one AbAb is pure, 

 while the other two have a as a recessive. Of the three aB, 

 one is a pure aBdB ; the other two contain & as a recessive. 



round 



a green 

 b angular 



ab 



? t 



ABat 



Ab 



A B a b AblAb AbUb a 5 



D I 



F, 



labab 



P F 



aB aB ab ab 



FIG. 53. DIHYBRIDISM. (After Lotsy.) 



Lastly, abab is a pure recessive. It follows from this that, 

 on further inbreeding of the above four types (each 

 separately), we get in the F 3 generation one true-breeding 

 line from each viz., ABAB, aBaB, AbAb, and abab 

 while all the others, being impure strains, will, on further 

 breeding, show the phenomenon of segregation. 



One more most important result can be seen to flow 

 from this fact. We started with two pure types viz. 

 AB, yellow and round, and ab, green and angular. Now 



