Phenomena of Inheritance 107 



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. 



Sometimes dominance is incomplete in early stages of develop- 

 ment but becomes complete in adult stages. Davenport found that 

 when white and black fowls are crossed the chicks, especially the 

 females, are speckled white and black, but in the adult fowl domi- 

 nance is complete and the plumage is white. Similar conditions of 

 delayed dominance are well known in the color of hair and eyes 

 of children, though dominance may become complete when they 

 have reached adult life. 



Reversible Dominance. In a few instances a character may be 

 dominant at one time and recessive at another. Thus Davenport 

 found that an extra toe in fowls is dominant under certain cir- 

 cumstances and recessive under others. Tennent found that char- 

 acters which are usually dominant in hybrid echinoderms may be 

 made recessive if the chemical or physical nature of the sea water 

 is changed. Such cases seem to show that dominance may depend 

 sometimes upon environmental conditions, sometimes upon a 

 particular combination of hereditary units. 



Dominance Not Fundamental. In all cases dominance means 

 merely the development in offspring of certain characters of one 

 parent, while contrasting characters of the other parent remain 

 undeveloped. The appearance of any developed character in an 

 organism depends upon many complicated reactions of germinal 

 units to one another and to the environment. Under certain con- 

 ditions of the germ or of the environment some characters may 



