362 PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 



crossed with a pure bred or homozygous white guinea pig, all 

 of the oifspring in the first filial or f^ generation will be black. 

 While these f^ blacks look like their black parent, a simple breed- 

 ing experiment will show that they are really different, — that 

 they are really a mongrel or heterozygous type. This experiment 

 consists in crossing such an f^ black with a pure bred white; 

 one-half the offspring will be black, one-half white. Returning 

 to the main experiment, suppose the i^ heterozygous black ani- 

 mals to be bred together; the oft'spring in the next generation 

 (second filial or f, generation), will be in the proportion of three 

 blacks to one white. Testing of the blacks permits the further 

 analysis of this fo proportion into one homozygous black : 2 

 heterozygous blacks ■ 1 homozygous white. 



With this experiment in mind, let us examine briefly Men- 

 del's three main theses. 



1. Unit Characters. The total inheritance of an organism 

 may be analyzed into a number of characters each of which is 

 inherited in its entirety (unit character) and independently of 

 other unrelated characters. In our illustration, blackness and 

 whiteness are two contrasting unit characters, and totally in- 

 dependent of other characters such as coat quality or hair length. 

 Recent work on linkage, to be discussed later, shows that this 

 view of independence of characters must be materially modified. 



2. Dominance. When contrasting unit characters are in- 

 herited from the two parents, only one (the dominant character) 

 appears in the offspring, the other (recessive character) disap- 

 pearing from sight although remaining in a condition to affect 

 the next generation. In our illustration, blackness is seen to 

 be dominant, whiteness recessive. Later work has shown that 

 the principle of dominance is not of universal applicability ; nor 

 is it essential to Mendelism. 



3. Segregation and Purity of the Gametes, — the centre 

 and heart of Mendel's work. Even when produced by a heter- 

 ozygous parent, a germ cell or gamete (egg or spermatozoon) 

 can carry but one of a pair of contrasting unit characters. In 

 fertilization the gametes unite in pairs (egg and sperm) to form 

 the new individual. If like gametes unite, the resulting animal 



