PLANT BREEDING 



513 



B 





that his garden peas, on crossing, manifested dominance and 

 subsequent segregation for each pair of characters, Mendel 

 went farther and experimented on the results of crossing plants 

 with different combinations of characters. Two plants, for ex- 

 ample, differ not only as to the color of the seed but also as to 

 tallness. What happens when they are crossed ? Experiments 

 showed that when a tall green-seeded 

 parent is crossed with a short yellow- 

 seeded one, the next generation appeared 

 to be all tall yellow. The hybrids re- 

 sembled one parent altogether in one 

 character, and the other parent entirely 

 in the other character (see Fig. 216). 

 In the following generation the offspring 

 of such hybrids would appear in four 

 types: tall yellow, short yellow, tall 

 green, short green. 



Experiments of this kind have since 

 been repeated by the thousand. From 

 them we conclude that each pair of al- 

 ternative characters behaves according 

 to the first two laws (dominance and 

 segregation), regardless of the other 

 characters present. This general fact is 

 called the law of unit characters. This 

 law helps us to understand how there 

 can be such diversity among the indi- 

 viduals of any species of plants or 

 animals. The greater the number of 



characters that make up a species, the greater is the possible 

 number of combinations, and the smaller is the chance that any 

 given combination will occur again. 



These three laws of hevediiy— dominance, segregation, and 

 unit characters— dive known as IMendel's laws or principles, 

 because they were first discovered by Gregor INIendel. Unfortu- 

 nately :\Iendel did not publish his results, and his researches 





Fig. 216. Inheritance of 

 two or more characters 



The offspring of two parents, 

 A and B, resembles both 

 parents, but it does not, as a 

 rule, stand midway between 

 the parents with respect to 

 the several characters. In- 

 stead, the offspring will be 

 like one parent in some char- 

 acters and like the other 

 parent in other characters 



