NATURE OF PLANTS 133 



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 are at first suppressed and re- 

 appear in the later generations. The number of offspring with 

 dominant characters often stands 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 

 are 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 the seeds produced in the 

 second generation vary in color. Approximately one-fourth are 

 pure black, two fourths a mixture of black and yellow, with the 

 black dominant, i. e., masking the yellow, and one fourth pure 

 yellow. The pure yellow and the pure black seeds continue to 

 breed respectively yellow and black seeds in all subsequent 

 generations and it is evident that they represent pure stock in 

 which the black and yellow unit characters have been entirely 

 separated. The seeds that were a mixture of black and yellow 

 produce when crossed among themselves in the third generation 

 the same ratio of dominants, recessives and mixtures as appeared 

 in the second generation, namely, one-fourth pure black, two- 

 fourths black and yellow with the black dominant, and one fourth 

 pure yellow. So we see that each generation after the first is 

 characterized by the appearance of a rather definite proportion 

 of offspring with pure dominant or pure recessive characters, 

 and with mixed characters. This ratio of variation is known as 

 Mendel's Law. 



