298 Cell as Basis oj Organic Activity 



THE FIRST MENDELIAN LAW 



Although numerous individuals before Mendel's time had studied 

 the effects of hybridization (cross-fertilization) upon plants, none of 

 them had actually confined his attention to single characters. The par- 

 ticular genius of Mendel lay in the fact that he had the intelligence to 

 observe but a single character and to ignore the others. For example, 

 he first noted and counted the color of the flowers without regard 

 to whether the plants were tall or short or to how they bore their 

 leaves. Later he was able to consider combinations of characters. Thus 

 this attack of the problem from individual items finally led to the over- 

 all concept of inheritance. 



The Study of a Single Character. — One of the first crosses which 

 Mendel made involved flower color. He crossed white and red flowered 

 plants. The plants resulting from the seeds of this cross all had red 

 flowers ; however, the progeny of these red-flowered peas produced both 

 red- and white-flowered plants in a definite ratio. In the first gen- 

 eration, these characters were brought together, and in the second they 

 were separated. It would appear that the factors determining the 

 characters were separated or segregated at each cross. 



It must be realized that Mendel knew nothing of the physical 

 basis of this heredity. It was not until many years later that the 

 explanation of his discovery was found within the cells themselves. 



From his observation, Mendel's first law, the law of segregation, 

 may be stated as follows : Factors jor cacJi character are segregated 

 or separated at each generation. 



A Monohybrid Cross. — Mendel formulated his first law from his 

 observations of the type of crosses described above. As these crosses 

 involved but a single pair of characters, they were known as mono- 

 hybrid crosses. 



For the parental generation, Mendel used a pure-breeding line of 

 red flowers and one of white flowers. These lines were established 

 by long, patient selection. 



From many crosses similar to this one, Mendel was able to draw 

 several different conclusions. First he noted that with this contrasting 

 pair of characters, one character expressed itself over the other in 

 the first generation. There was no intermediate color between the red 

 and white. He termed the red color dominant to the white. Inasmuch 

 as the white did not appear in the first cross, he termed it recessive. He 



