Two Pairs of Genes in Two Pairs of Autosomes 129 



binations can be worked out in the same way. It is customary 

 to express the results by a ratio of whole numbers rather than of 

 fractions, and the ratio of 9 : 3 : 3 : 1 is well known as the F2 

 dihybrid ratio. 



Similar instances of dihybrid ratios could be cited by the 

 thousands in the many plants and animals that have been 



Monohybrid Dihybrid Dihybrid 



genotypic ratios genotypic ratios phenotypic ratio 



1 CrCr 1 PI PI CrCr 



1 PlPl ^ 2 Crcr 2 PlPl Crcr 



1 PlPl crcr 



K} 



2 Plpl CrCr 

 2 Plpl ^ 2 Crcr 4 Plpl Crcr --^^ 



2 Plpl crcr 



1 plpl CrCr 



1 plpl ^ 2 Crcr 2 plpl Crcr 



9 purple, normal 



3 purple, crinkly 



3 green , normal 



crcr _1_ plpl crcr >■ 1 green . crinkly 



16 16 



Phenotypic ratios only 



^^„^ 3 normal — 9 purple, normal 

 3 purple cCT 



^'^ 1 crinkly — 3 purple, crinkly 



^ 3 normal — 3 green, normal 

 1 green =C[T 



^^^ 1 crinkly — 1 green, crinkly 



Fig. 42. The determination of the F2 from the cross PlPl CrCr X plp^ 

 crcr by the method of combined ratios. The PI and pi genes segregate into 

 a genotypic ratio of 1 PlPl : 2 Plpl : 1 plpl and the Cr and cr genes into a 

 ratio of 1 CrCr : 2 Crcr : 1 crcr. When the two independent ratios are 

 combined, the Fo genotypic ratio is obtained. Similarly, by combining the 

 phenotypic ratios of 3 purple : 1 green and 3 normal : 1 crinkly, the dihy- 

 brid phenotypic ratio results. 



investigated genetically. An example from Drosophila may be 

 used to show that the same phenomenon is observed in the 

 Animal Kingdom. Curved wing (c) is recessive to normal wing 

 (C) and is located in the second chromosome; ebony body (e) 

 is recessive to wild-type or gray (E) and is in the third chromo- 

 some. If a curved, ebony fly {cc ee) is crossed with one that is 

 homozygous for normal wing and gray body {CC EE), the Fi is 

 phenotypically normal-winged, has a gray body, and is geno- 

 typically Cc Ee. The Fo segregates into 9 normal-wing, gray : 3 



