CHARACTERS IN HEREDITY 147 



sidered to be alternative and interchangeable unit 

 characters. 



Similarly yellow color of the cotyledons in the 

 seed of peas was found to be a unit character 

 alternative with green color. In animals we find 

 similar simple unit characters to exist. Thus in 

 mammals black pigmentation is due to the pres- 

 ence of a unit character which may be replaced 

 by another changing the pigmentation to brown. 

 Among horned ruminants, such as cattle and 

 sheep, development of the horns depends upon 

 the presence of a unit character which may be 

 replaced by (or perhaps become associated with) 

 another, in the presence of which horns fail to 

 develop. 



EECENT EXTENSIONS OF THE THEORY OF 

 HEREDITARY UNITS 



In cases less simple than these, a unit character 

 may have more than a single manifestation, as 

 where a plant having flowers of a certain color 

 has also a similar but fainter coloration of the 

 stem, or a mammal with black hair-pigment has 

 also black skin-pigment, while one with brown 

 hair-pigment has also brown skin-pigment. In 

 still other cases, two or more independent unit 

 characters must be present together to produce 

 a single visible effect. This fact was unknown 

 to Mendel. Its discovery constitutes one of the 

 most recent and important advances made in our 



