212 GENETICS 



AaBb, and since A and B are dominant, all the individuals 

 of Fi have red eyes. 



Now mate together individuals of Fi ; they yield the com- 

 binations shown in table 2, page 1 17. And as there shown, if 

 we classify these as to the characteristics that are manifested 

 (that is, as to what dominant genes they contain), the pro- 

 portions are the following: 



9AB + 3Ab + 3aB + lab 



All those that contain both the dominant genes A and B 

 have red eyes. Those that contain the dominant A but not 

 the dominant B have pink eyes. Those that contain the 

 dominant B but not the dominant A have purple eyes. Thus 

 the individuals with different eye colors occur in F2 in the 

 proportions: nine red-eyed (AB) to 3 pink-eyed (Ab) to 3 

 purple-eyed (aB) to i pink-purple-eyed (ab). Or we can 

 classify them as 9 red-eyed to 7 not-red. 



Thus in this case the character red gives in F2 still dif- 

 ferent proportions from those that were found in the two 

 previous cases. It now gives "two-factor" autosomal in- 

 heritance. It illustrates anew the fact that the type and ratios 

 of inheritance given by any character depend upon what it 

 is mated with. 



/f. Parents differing in two gene-pairs in the same auto- 

 some. — When the gene pairs in which the parents differ are 

 located in the same chromosome, the ratios are altered by 

 the fact that genes in the same chromosome usually go to- 

 gether to the descendants; that is, they show linkage. An 

 ideal case will illustrate the results. 



Let one parent carry the two recessive gene pairs aa and 

 bb, located in the same autosome. The corresponding domi- 

 nant genes, in the other parent, may be called as usual AA 

 and BB. The two parents will then show the constitution 

 illustrated in figure 45, P. Their Fi offspring will then be a 



