Experimental Hybridizing 71 



Each of the four kinds of egg-cells may be met by any one of 

 these same four kinds of male cells, giving in all nine combina- 

 tions; viz. AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBb, 

 aaBB, aabb. But since these are combinations of both dominant 

 and recessive characters, the offspring will appear to be of only 

 four kinds. Thus the first four terms will belong to the type 

 AB; the next two kinds will be Ab; the next two aB; and the 

 last ab. It will be found by making all the possible combina- 

 tions that the AB type occurs nine times, Ab three times, aB 

 three times, and ab only once. 



Mendel carried out an experiment of this sort in which two 

 contrasted characters were involved. The results fulfilled the 

 expectations of the hypothesis. He used two races of peas, in 

 one of which the form of the seed was round, A, in the other 

 angular, a. In the round seeds the albumen was yellow, B; 

 in the angular seeds the albumen was green, b, thus 



AB, seed parent ab, pollen parent 



A, form round a, form angular 



B, albumen yellow b, albumen green 



The outcome of the experiment conformed to the scheme given 

 above. It should be noted that in the first generation, A Bab, 

 the hybrid will contain all of the possibilities, although A domi- 

 nates a and B dominates b. In the germ-cells the characters 

 separate on the assumption of pure gametes, but in such a way 

 that A will always be associated with B or b, i.e. the round 

 form will always be yellow or green. Similarly, a will always 

 be associated with B or b. Hence, as stated above, the only 

 combinations possible will be 



AB, round and yellow, 

 A b, round and green, 

 aB, angular and yellow, 

 ab, angular and green. 



From the foregoing account it will be obvious that the prob- 

 lem will become more complex when three contrasted charac- 



