51 



An explanation of the origin of new combinations is afforded by Mendel's 

 Law of Hybrids. In fact, this law is now the basis of practically all investi- , en e . s 

 gation in the realm of hybridism and should be understood by all breeders. 

 Before the law of Mendel became known cross-fertilization was looked upon 

 as a means of stimulating or creating variation, making the selection of superior 

 variants possible. The varieties or sorts used for this purpose was not a 

 matter of great concern as almost any two, it was thought, were capable of 

 producing, when crossed, variations which might form the basis of new and 

 better sorts. When Mendel's law became better understood, crossing came 

 to be regarded not as a means of inducing variation, but as a means of com- 

 bining already existing units, allowing certain characters of one parent to be 

 combined with those of another. 



One of the requisites for the application of this law is that the two 

 parents possess characters which are opposed to each other. As examples 

 may be cited the simple characters, Baldness and Beardedness in wheat, 

 Roundness and Wrinkledness in pease, Smoothness and Hairiness of wheat chaff, 

 etc. These two opposing characters in each case are termed a "character 

 pair." When one of the characters belonging to a certain pair is " stronger" 

 than the other it is said to be Dominant. In this case only this character will 

 appear in the first generation hybrids, the other remaining recessive or con- 

 cealed. In wheats, Baldness is dominant over Beardedness. The first genera- 

 tion from a crossing between a Bald and a Bearded sort will therefore be 

 Bald, but in the second generation there will again be found both bald and 

 bearded forms. Mendel showed that where simple characters, such as those 

 now under consideration, are involved, the individuals in the second genera- 

 tion fall unto two main groups, one group representing the character of the 

 recessive parent and the other similar to or approximating the dominant one. 

 Those resembling the former parent represent about 25% of the whole 

 number. These breed true in succeeding generations. Of the second group, 

 which represents 75% of the whole, will produce true dominants while the 

 remaining f will again divide or segregate in the next generation producing 

 the same constant and inconstant forms. This fact of segregation is one of 

 the essential discoveries in Mendel's law. 



The proportions which are obtained when two sorts possessing simple 

 alterating characters are crossed (monohybrid combination) may be repre- 

 sented as follows: 



