136 THE SCIENCE OF BIOLOGY 



found : Black X White = Blue; Blue X Blue =25% Black: 

 50% Blue : 25% White; Black X Black = Black; White X 

 White = White. Similar results appear in all subsequent 

 generations. 



It thus appears that by following the middle percentage 

 of Blues one might trace an exclusively blue line of descent 

 through many generations. If nothing were known of the 

 collaterals, it would seem that the black and white had 

 permanently blended into blue. In small numbers, only 

 blues might occur and what is essentially a non-blending 

 type of inheritance would not be recognized as such. This 

 might easily be the case in records of human families. The 

 essential feature in Mendelian heredity is this absence of 

 blending, as shown by the segregation of characters in the 

 F 2 and in subsequent generations. The black and white 

 are united in the hybrid, but the latter does not give rise 

 to a hybrid race that breeds true. Hybrids do appear, but 

 along with them are the two original stocks in the funda- 

 mental 1:2:1 ratio. 4 



The adult individual may thus be pure or hybrid. In 

 a case like the foregoing, the hybrid is recognized by its 

 appearance. In some cases of Mendelian heredity, a phe- 

 nomenon known as dominance occurs. One member of a pair 

 of heritable qualities dominates the other to such an extent 

 that the hybrid cannot be distinguished from a pure in- 

 dividual of the dominant type. The cross between gray and 

 white mice will illustrate the facts. In this instance the 

 gray is dominant, the white recessive. 5 The results are as 



4 The blue fowls described above have long been known as Blue Andalusians. 

 This supposed breed was found impossible to establish in any degree of purity, 

 because when Blue Andalusians were bred together an annoying percentage of 

 black and of white birds appeared. The mystery is now explained. The blue 

 is a Mendelian hybrid and, therefore, can never be established as a pure breed, 

 although 100% of blues can be secured by the simple expedient of mating the 

 black and white wasters which so annoyed the breeders. Blacks and whites 

 are the pure stock. 



6 There is no adequate explanation of why one member of a contrasting pair 

 of characters should thus dominate the other. It is simply observed to be so 



