Phenomena of Inheritance 105 



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 1 plants bear neither white nor red flowers 

 but pink ones, and the F 2 plants bear white, red and pink flowers. 

 The whites and reds are always homozygous, the pinks heterozy- 

 gous ; pure white and pure red are produced only when their fac- 

 tors are duplex (WW), (RR) ; when they are simplex (WR) 

 pink is produced. In this case red is not completely dominant ovef 

 white, but the hybrid is more or less intermediate 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 

 the reds and whites are homozygotes and consequently breed true. 



Lang found that when snails with uniformly colored shells 

 were crossed with snails having bands of color on the shells the 

 hybrids were faintly banded, thus being more or less interme- 

 diate between the two parents ; but when these hybrids were inter- 

 bred they produced banded, faintly banded and uniformly col- 

 ored snails in the ratio of 1:2:1, thus proving that Mendelian 

 segregation takes place in the F 2 generation, and that dominance 

 is incomplete in the heterozygotes. Many other similar cases of 

 incomplete dominance are known. 



