MENDELIAN TERMS 99 



Mendelian characters exist in contrasted pairs which are 

 alternatives of each other, as black and white, rough and 

 smooth, long and short. A gamete may from its nature trans- 

 mit only one of a pair, either black or w^hite, but not both. 

 Its nature is simplex. A zygote is duplex in nature; it may 

 contain a character twice represented (when it is a homo- 

 zygote) , or contain both a character and its alternative (when 

 it is a heterozygote) . The same zygote may be a homozygote 

 as regards one character (say hair-color) and a heterozygote 

 as regards another (say hair-length). 



Unit-character or unit-factor or gene. Such characters of 

 animals and plants as follow Mendel's law^ in heredity, i. e., 

 are inherited as independent units, are often called unit- 

 characters. But it has been shown in numerous cases that 

 an independent factor, which follows Mendel's law in trans- 

 mission, may affect or condition the inheritance of a supposed 

 unit-character, without itself producing any other discover- 

 able effect. Thus the agouti (or yellow-ticked) character of 

 the fur of rodents is not developed unless along with the 

 other genetic factors which produce a black or a brown coat, 

 a particular " agouti " factor is present; yet we have no 

 other evidence of the existence of this factor, except the form 

 which the black or brown coat assumes when this factor is 

 inherited. But it can be shown unmistakably that the in- 

 heritance of this unseen factor is that of an independent 

 Mendelian character. 



Some have sought to avoid the difficulty presented by such 

 cases by making a distinction between unit-characters and 

 unit-factors, the former being the recognized morphological 

 or physiological parts or properties of the organism, the 

 latter their hypothetical determiners. But this distinction 

 is of doubtful utility, since the only objective evidence which 

 we possess that unit-characters exist is the occurrence of 

 classes among the F2 individuals and their numerical fre- 

 quencies. But this same evidence also forms our only indi- 

 cation that determiners exist. In fact the " unit-characters " 

 about which we talk are the hypothetical determiners. For 



