MENDELIAN HEREDITY 



279 



M 



0) 



+-) 



0) 



B 



a 



15 

 S 



Terminology. The examples given above represent in all cases 

 hybridization for a single pair of characters and are called mono- 

 hybrids. Dihybrids and trihtjbrids for two and thnn^ pairs of 

 characters respectively will be treated later. Any singles character 

 is called a unit character and the two alternative characters of a 

 pair are called allelomor'phs. It is customary to refer to the 

 original parents as the parental or P generation, to the hybrid 



as the first filial generation 



-r, , ,1 £f ■ f. Male gametes 



or Fi, to the otfsprmg of S s 



the hybrid as the F2 genera- 

 tion or second fihal and so 

 on. In any example certain 

 differences among individ- 

 uals are found which are 

 expressed in Figure 159. 

 Here it will be seen that of 

 the three similar F2 indi- 

 viduals, in two the charac- 

 ter wrinkled is represented 

 even though it does not ap- 

 pear. Such a character is Fig. 159. 

 said to be recessive, while 

 the allelomorph which con- 

 ceals it is dominant. The 



three individuals look the same, and are therefore said to belong to 

 the same phenotype, while the characters actually represented 

 within them determine their genotypes. When pure, like the SS 

 individual, they are homozygous, when mixed hke the two Ss indi- 

 viduals, heterozygous. Obviously where dominance is perfect only 

 the test of breeding will disclose the genotype, except in the case 

 of a homozygous recessive. 



Fundamental Principles. The results of these experiments and 

 the many others conducted since demonstrate three cardinal 

 principles of inheritance, viz: 



Unit Characters. Organisms are composed of many characters 

 which may be combined in the same individual or distri])uted 

 among different strains without losing their distinctive properties. 

 These are commonly known as unit characters. 



Segregation. The reappearance of the original unit characters 

 in the progeny of a hybrid indicates that association does not 



Diagram to illustrate the F2 

 generation of Mendel's hybridized smooth 

 and wrinkled peas. Ratio of phenotypes 

 3:1; ratio of genotypes 1:2:1. 



