INDEPENDENT MENDELIAN INHERITANCE 93 



former are intermediate in coloration between the full-colored RR 

 individuals and the flesh-colored rr individuals, and form a distinct class 

 in themselves. But disregarding these differences the phenotypes in F 2 

 are in the following ratio: 



27 Plants with zygomorphic, red on ivory flowers. 

 9 Plants with zygomorphic, red on yellow flowers. 

 9 Plants with zygomorphic, flesh-colored on ivory flowers. 

 9 Plants with peloric, red on ivory flowers. 

 3 Plants with zygomorphic, flesh-colored on yellow flowers. 

 3 Plants with peloric, red on yellow flowers. 

 3 Plants with peloric, flesh-colored on ivory flowers. 

 1 Plant with peloric, flesh-colored on yellow flowers. 



This 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 ratio is typical for trihybrids, if dominance 

 occurs in the three pairs of factors involved. Like the dihybrid ratio 

 it is derivable from the monohybrid 3 : 1 ratio by subdividing the mem- 

 bers of each term in the 3 : 1 ratio and then by again subdividing each 

 term of the dihybrid ratio thus obtained in the ratio 3:1. To illustrate 

 with our example, if the contrasted characters zygomorphic and peloric 

 are considered segregation is in the ratio 3 zygomorphic: 1 peloric. 

 When the contrasted characters red against flesh-colored, which also 

 segregate in the simple ratio, are introduced into the analysis the ratio 

 becomes 3 zygomorphic (3 red : 1 flesh-colored) : 1 peloric (3 red : 1 

 flesh-colored) = 9 zygomorphic red : 3 zygomorphic flesh-colored : 3 

 peloric red : 1 peloric flesh-colored. When finally the contrasted 

 characters ivory against yellow are introduced this becomes 9 zygomorphic 

 red (3 ivory : 1 yellow) : 3 zygomorphic flesh-colored (3 ivory : 1 yellow) : 

 3 peloric red (3 ivory : 1 yellow) : 1 peloric flesh-colored (3 ivory : 1 yellow) . 

 This gives the final distribution tabulated above. 



It is important to note that in this phenotypic ratio one member only 

 of each phenotype is homozygous for all its factors and will breed true 

 thereafter. From a Mendelian standpoint an individual is either homo- 

 zygous or heterozygous for a given factor, if it is homozygous it is pure 

 bred with respect to that factor and will breed true thereafter, irrespec- 

 tive of its derivation. It is therefore possible in Ft to obtain a pure race 

 with respect to any combination of parental factors provided only that 

 a large enough F 2 generation is grown and tested. The increasing diffi- 

 culty of fulfilling these conditions as the number of factors involved 

 increases is obvious, so that from the standpoint of practicability it is 

 usually necessary to work with crosses involving a relatively small num- 

 ber of factor differences. 



Another fact which is apparent from this trihybrid case is the 

 greater ease with which homozygous individuals may be obtained from 

 the classes which are represented in the smallest numbers. In the above 



