448 



ELEMENTARY BIOLOGY 



parents, that is, split up again into dominant-appearing and recessive- 

 appearing individuals in the proportion of three to one. The two 

 classes are produced in the ratio of one pure dominant to two mixed 

 or hybrid, although the two classes may have the same appearance. 



The simplest explanations for these 

 mathematical relations between the char- 

 acters is to be seen from the diagram 

 in Fig. 239. 



472. Law of Unit Characters. In the 

 meantime we must not overlook the fact 

 that every organism is made up of many 

 characters. After finding out that his 

 peas behaved according to the law of 

 dominance and according to the law of 

 segregation with respect to several of the 

 pairs of characters, Mendel went farther 

 and studied the results of his crossings 

 to find out the behavior of combinations 

 of characters. For example, suppose 

 that two plants used in an experiment 

 differed with respect to height as well as 

 with respect to seed color, what would 

 be the results ? Experiments showed 

 that the tall, green-seeded parent trans- 

 mitted the tallness as a dominant and 

 the greenness as a recessive. In other 

 words, if a tall green was crossed with a short yellow, the 

 next generation appeared to be all tall yellow, resembling 

 one parent altogether in one character, and the other parent 

 altogether in the other character (see Fig. 240). 



The conclusion from this experiment, and many others like 

 it, is that each pair of dominant-recessive characters behaves 

 according to the two laws described above, without regard to 

 what other characters may be present. This general fact has 

 been found to hold for many species of plants and animals 



FIG. 240. Inheritance of 

 two or more characters 



The offspring of two parents, 

 A and B, resembles both par- 

 ents, but it does not, as a rule, 

 stand midway between the 

 parents with respect to the 

 several characters. Instead, 

 the offspring will be like one 

 parent in some characters and 

 like the other parent in other 

 characters 



