280 EVOLUTION AND GENETICS 



modify characters but that during the process of reproduction 

 they may be separated or segregated anew. 



Dominance. The fact that of the two characters present in a 

 hybrid one may completely conceal the presence of the other 

 illustrates the principle of dominance. 



Behaviour of Allelomorphic Characters. It is now known that 

 many allelomorphic characters are not completely dominant and 

 recessive to each other, but that both may be expressed in a 

 hybrid. Three kinds of inheritance of allelomorphs are recognized, 

 viz., alternative, mosaic and blending. 



Alternative Inheritance. This is the type described above, 

 in which one character completely dominates the other and the 

 recessive makes its appearance only in homozygous recessive indi- 

 viduals. 



Mosaic Inheritance. Mosaic inheritance differs from alterna- 

 tive inheritance in that both of the allelomorphic characters may 

 be fully expressed, but in different parts of the body. Black and 

 white spotted offspring of self-colored parents may be of this 

 type, although they are sometimes explained in another way 

 (see Chapter XVII). 



Blending Inheritance. This type of inheritance differs from 

 both of the preceding types in that neither character is fully 

 expressed in any part of the body when both are represented. 

 Such inheritance produces a character in hybrids which is inter- 

 mediate between the parental characters. Many flowers inherit 

 color in this way. The offspring of a red and white cross, for 

 example, may have pink flowers. In the Fo generation derived 

 from the pink flowers, however, red, pink and white individuals 

 appear. 



Regardless of the manner of inheritance the integrity of the 

 unit characters is preserved and their segregation may occur anew 

 with every generation. 



Effects of Allelomorphic Relationship. We have seen that in 

 Mendelian monohybrids the ratio 3:1 appears in the F2 generation 

 when one character dominates the other, although breeding tests 

 indicate that this is based on a 1:2:1 genotypic ratio. When 

 characters are inherited in a blended or mosaic conditions, however, 

 heterozygous individuals belong to a different phenotype as well 

 as to a different genotype from the homozygous, and consequently 

 the 1:2:1 ratio is evident without further breeding. 



