io6 Heredity and Environment 



the developed organism are the resultants of all these reactions 

 and interactions. 



Some progress has been made, in identifying certain structures 

 of the germ cells with certain hereditary units, but quite irrespec- 

 tive of what these units may be and where they may be located it 

 is possible, by means of the Mendelian theory of segregation of 

 units in the germ cells and of chance combinations of these in fer- 

 tilization to predict the number of genotypes and phenotypes 

 which may be expected as the result of a given cross. 



2. Modifications of the Principle of Dominance. Incomplete 

 Dominance. A large number of animal and plant hybrids show 

 one contrasting character completely dominant over the other one 

 as Mendel observed in the case of his peas. But in a considerable 

 number of cases this dominance is incomplete or imperfect. When 

 white-flowered strains of "four o'clocks" are crossed with red- 

 flowered ones the F x plants bear neither white nor red flowers 

 but pink ones, and the F 2 plants are white-flowered, red-flowered 

 or pink-flowered. The whites and reds are always homozygous, 

 the pinks heterozygous; pure white and pure red are produced 

 only when their factors are duplex (WW}, (RR) ; when they are 

 simplex (WR) pink is produced. In this case red is not com- 

 pletely dominant over white, but the hybrid is more or less inter- 

 mediate between the two parents (Fig. 28). 



It has long been known that the race of fowls called Blue An- 

 dalusian does not breed true, but in each generation produces a 

 certain number of blacks and whites as well as blues. Bateson 

 found that the blues are really hybrids between blacks and whites 

 in which neither of the latter is completely dominant. Black and 

 white appear only when they are pure (homozygous), blue only 

 when both black and white are present ( heterozygous). 



Again a cross of red and white cattle produces roan offspring, 

 but the latter when interbred give rise to reds, roans and whites 

 in the proportion of 1:2:1, showing that the roans are heterozy- 

 gotes in which red is not completely dominant over white, while 



