126 NATURE OF PLAXT VARIATIONS 



variation in the offspring so that they conform to a more con- 

 stant and uniform type. On the other hand many consider that 

 fertilization is designed to bring about variation in the offspring. 

 So numerous, however, are the traits or unit characters of each 

 parent that it is often impossible at present for the holders of this 

 view to interpret the nature of the variations that sometimes 

 appear in the offspring. In some cases there appears to be a 

 complete mingling of the parental characters. When, for ex- 

 ample, the parents vary in color or some other character, the 

 offspring may show a blending of these two colors as when 

 two pigments are mixed. More commonly certain characters of 

 one parent appear to be stronger in controlling the development of 

 the offspring and suppress certain characters of the other parent. 

 As a result the offspring resemble one parent more than the 

 other. Why this occurs can not be explained. We can only say 

 that the hereditary substance of one parent is perhaps more 

 potent and has more influence in the development of the off- 

 spring. These stronger features that appear in the offspring are 

 called the dominant characters. It usually happens in the second 

 and succeeding generations that some of the offspring will show 

 certain characters that were suppressed by the dominant charac- 

 ters in the first generation. These weaker traits are called re- 

 cessive characters because they reappear in the later genera- 

 tions. The number of offspring with dominant characters often 

 stand in definite ratio to those with recessive characters. For 

 example taking a single unit character of two plants, as the 

 black and yellow color of their seeds, Shull has shown when two 

 such plants are crossed that only black seeds may be produced 

 in the first generation. This color is therefore called a dominant 

 character because it suppresses the yellow. If these seeds are 

 grown and the resulting flowers are crossed among themselves 

 it will be found that tin- seeds produced in the second genera- 

 tion vary in color. Approximately one-fourth are pure black, 

 two fourths a mixture of black and yellow, with the black domi- 

 nant, /'. c, masking the yellow, and one fourth pure yellow. The 

 pure yellow and tin- pure black seeds continue to breed respec- 

 tively yellow and black seeds in all subsequent fenerations and it 



