EVOLUTION AND HEREDITY 



353 



believe that there is no such thing as the perfect blending of 

 hereditary characters. 



Mendel's Law. There are many qualities which do not 

 " blend" but are inherited as "unit characters" that is, 

 they are present or absent, or show one of two alternative 

 conditions without intermediates. Such cases follow Men- 

 del's Law, which supposes (1) unit characters which are 

 (2) usually dominant or recessive and which are (3) in- 

 herited according to the laws of chance. Dominant and re- 

 cessive characters may be present in a single individual 

 but in such instances the former usually masks the latter 











FIG. 117. Diagram showing the scheme of inheritance in guinea pigs when 

 black and albino forms are crossed. (From Guyer's Being Weil-Born, Copy- 

 right, 1916. By special permission of the Publishers, The Bobbs-Merrill Co.) 



so that only the dominant character is shown. In other 

 cases the unit characters may produce an apparent " blend" 

 in the first hybrid generation but later separate out and 

 occur in later generations as they were in the original 

 parents. Guinea pigs, for example, may possess a colored 

 coat or be albinos with white hair, and pink eyes. The pig- 

 mented condition is dominant to the albino. Therefore, 

 if a black guinea pig (Fig. 117) is crossed with an albino the 

 offspring in the first generation will all be black. These 

 individuals are hybrids which possess both black and albino 

 qualities, but show only the former because it is dominant. 



