MENDELISM AND OTHER METHODS OF DESCENT I97 



SUMMARY OF THE FIVE ELEMENTS OF DESCENT. 



Every test of the behavior of different parental characteristics 

 in crosses or hybrids involves these five cases or elements, which 

 may be summarized briefly as follows : 



1. Parent Organisms. — The individual organisms selected 

 for the experiment in crossing. 



2. Pareiit Gametes. — The sex-cells produced by the parent 

 organisms which are brought together to test the results of con- 

 jugation. 



3. Conjugate Organisms. — The organisms which are built 

 up after the parent gametes have united, but before their con- 

 jugation is completed. 



4. Pcrjugate Gametes. — The gametes produced by the con- 

 jugate organisms, representing the first results of a completed 

 conjugation of the gamete parents. 



5. Pcrjugate Organisms. — The organisms formed by unions 

 of pcrjugate gametes, representing the first generation of organ- 

 isms resulting from a completed conjugation of the gamete 

 parents. 



Even the simplest experiment in Mendelism involves consid- 

 eration of at least these five distinct periods of existence. The 

 adult organisms of three generations are included, and the 

 gametes or sex-cells of two generations. One generation is 

 required for the conjugation of the gametes of the original 

 parents to be accomplished, and another generation for the 

 results of the combination to be brought to expression in adult 

 form. The third generation differs profoundly from the pre- 

 vious generation in its relation to the original parents of the 

 cross, and not merely in the proportional representation of the 

 parental characters, as shown in experiments in Mendelism. 



latter can be more easily demonstrated in cases where it is associated with the 

 former. The principle of gametic purity rests upon the assumption that gamete 

 formation is the reverse of fertilization. In fertilization gametes A and B unite 

 to form a zygote ^^; when this zygote in turn forms gametes, they will be 

 again A and B. From a knowledge of the somatic form alone of pure ^'s and 

 5's, one can make no trustworthy prediction as to the form of AB. . . . But, 

 no matter what the somatic form of AB is, we may with confidence predict that 

 its gametes will be essentially pure ^'s and B's,, and in equal proportions. This 

 is the Mendelian expectation in all cases of alternative inheritance. Whether it 

 applies to other cases also, and, if so, to what extent, is not yet known." 



