Trihybrids 



137 



3a: 



la. 



,W 



lb 



.3B 



lb 



-3C- 



~lc- 



'3C- 



-Ic 



-3C 



^Ic- 



-3C- 



-^27 ABC 



— 9ABc 



-^9A6C 



9aBC 



3 Abe 



-*3aBc 



^3abC 



-"1 abc 



plicated. If two homozygotes are crossed, such as AA BB CC 

 X aci bb CC, the Fi is Aa Bb Cc. It is heterozygous for all three 

 pairs of genes and is known as a trihybrid. Since each pair of 

 homologous chromosomes acts independently of every other pair 

 at meiosis, there are eight different kinds of gametes formed by 

 the Fi and eight different kinds of 

 phenotypes in both the F2 and the 

 backcross of the Fi to the triple 

 recessive. 



In the F2 of the cross AA BB CC 

 X aabb CC, genes A and a give a 

 phenotypic ratio of 3A : la and 

 genes B and b a ratio of 35 : 16. 

 Of the A plants, three-quarters will 

 be B and one-quarter b, and the a 

 plants will fall into the same ratio 

 with respect to the B and b pheno- 

 types. Thus, if these two pairs of 

 genes are considered alone, the F2 

 is ^AB : 3^6 : 3ai? : lab. But C 

 and c must also be considered. A 

 CC plant or animal crossed with 

 one that is cc will produce an F2 

 phenotypic ratio of 3C : Ic. Since 

 the segregation of these genes is 

 independent of both the other pairs, 

 three-quarters of the AB plants 

 will also be C and one-quarter will 



be c; similarly, three-quarters of each of the other pheno- 

 types {Ab, aB, and ab) will be C and one-quarter of each 

 of them will be c. The F2 phenotypic ratio can be obtained very 

 easily by the method of combined monohybrid ratios, as in Fig. 

 46. It is customary to write this ratio 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. 

 This F2 phenotypic ratio is obtained whether the cross of the 

 Pi generation is AA BB CC X eta bb cc or AA BB cc X aa bb CC 

 or AA bb CC X aaBB cc or aaBB CC X AA bb cc and irrespec- 

 tive of which genotype is the male and which is the female in 

 each of these four crosses. 

 The checkerboard method of determining the F2 is just as 



Total = 64 



Fig. 46. The F2 trihybrid ra- 

 tio by the method of com- 

 bined ratios. The three inde- 

 pendent ratios are : 3A : la ; 

 SB : lb; 3C : Ic. The F2 ra- 

 tio is found if the original 

 crosses are AA BB CC X «« bb 

 cc, or AA BB cc X aa bb CC, or 

 AAbbccXaaBBCC, or AA 

 bb CC X aa BB cc, and irre- 

 spective in each cross of which 

 is the male and which the fe- 

 male. 



