50 Nature of Dominance [CH. 



Dominance: the Heterozygote Character. 



The character of the heterozygote, the " hybrid character" 

 of Mendel, gives no indication as to the system by which 

 the parental characters are transmitted. The expressions, 

 "blended" inheritance, "particulate" inheritance and so on, 

 terms formerly devised by Galton for describing the zygotic 

 appearances, are now seen to be descriptive not so much of 

 the mode of transmission as of the consequences of certain 

 groupings of special allelomorphs ; and as it is obviously 

 preferable in all possible cases to use the ultimate descrip- 

 tions reduced to terms of gametic composition, such terms 

 are now seldom requisite. Dominance must be discussed 

 more fully when other facts have been set forth, and in this 

 preliminary notice of the more salient features of the 

 phenomena it will be enough to point out that dominance 

 is no inseparable attribute of Mendelian inheritance. The 

 essential phenomenon is segregation. 



The occurrence of dominance is often an assistance to 

 the investigator and may greatly simplify the analysis of 

 the various generations. Seldom however is dominance 

 uniformly complete, and in certain cases, as those of the 

 combs in poultry, where dominance is quite definite, it is 

 still possible for an observer thoroughly familiar with the 

 material to distinguish the homozygous dominants from the 

 heterozygous with fair certainty. Provided the recessives 

 as a class can be identified the application of Mendelian 

 analysis is almost equally easy whether the heterozygotes 

 show definite dominance or some intermediate condition. 



The statement made by de Vries that dominance is an 

 attribute of the phylogenetically older character has not been 

 borne out by more extended investigation. In the lists 

 given above many examples to the contrary occur. No one, 

 for instance, can doubt that the various types of dominant 

 comb (rose, pea, &c.) in fowls and the colour called " Brown- 

 breasted" have arisen since domestication. This colour- 

 example is illustrated by Fig. n, where the distinction 

 between the striped Black-red type and the almost uni- 

 colorous Brown-red, or " Brown-breasted " type is shown. 

 The striped type is practically that of the wild Gallus 

 bankiva, but the unicolorous type of down-colour is a com- 



