MENDELISM AND MUTATION 339 



being formed at fertilization and the members of each allelomorphic pair 

 of factors separating when the germ cells are formed. 



The Mendelian proportion of pure forms and hybrids is more easily 

 followed in cases of "incomplete dominance," the pure dominants here 

 being visibly distinguishable from the hybrids. Such a case is that of 

 Mirabilis jalapa, the four-o'clock (Fig. 130). If plants bearing pure red 

 flowers (var. rosea) are crossed with those bearing pure white flowers 

 (var. alba) the result is an FI generation of intermediate pink-flowered 

 plants. When these pink hybrids are bred among themselves the result- 

 ing F 2 generation comprises plants of three visibly different types: pure 

 dominants with red flowers, hybrids with pink flowers, and pure recessives 

 with white flowers, in the numerical ratio of 1:2:1. 



Terminology. We may here introduce certain terms prominent in the 

 literature of genetics. The genotype is the entire assemblage of factors which an 

 organism actually possesses in its constitution, irrespective of how many of 

 these may be expressed in externally visible characters. The phenotype is the 

 aggregate of externally visible characters, irrespective of any other, factors, 

 unexpressed in characters, which may be present in the organism. For illustra- 

 tion : in the case of the tall and dwarf peas there are in the second hybrid genera- 

 tion (F 2 ) three genotypes (with respect now only to the single character pair 

 discussed): TT, Tt, and tt, represented respectively by pure tall plants, tall 

 hybrids, and dwarfs ; but there are only two phenotypes : tall and dwarf, because 

 of the fact that the complete dominance of tallness over dwarfness renders the 

 hybrids externally indistinguishable from the pure tall individuals. Thus one 

 phenotype (tall plants) here includes individuals with two genotypic constitu- 

 tions, and the two can be distinguished only by a study of their progeny. In 

 Mirabilis, however, there are in the F 2 generation not only three genotypes 

 represented, but also three phenotypes, since the incomplete dominance renders 

 the hybrids externally unlike either of the pure forms. 



An individual is said to be homozygous for a given allelomorphic character 

 pair if it has received the same factor from the two parents a pea, for example, 

 with the constitution TT or tt. If it has both members of the pair, such as Tt, 

 it is said to be heterozygous. It may be homozygous for some allelomorphic pairs 

 and heterozygous for others, or it may conceivably be either homozygous or 

 heterozygous for all of its characters. Thus an organism with the genotypic 

 constitution AABbcc is homozygous for the characters represented by A A and 

 cc, and heterozygous for those represented by Bb. It is thus a pure dominant 

 with respect to A and a, a pure recessive with respect to C and c, and a hybrid 

 with respect to B and 6. The phenotypic appearance of the organism would be 

 determined by the dominant factors A and B and by the recessive c; a given 

 dominant factor dominates only its recessive allelomorph, and not the recessive 

 factors belonging to other pairs. It is a common practice to represent dominant 

 factors or characters by capital letters and their respective recessive allelomorphs 

 by the corresponding small letters/ 



The Cytological Basis of Mendelism. Having before us some of the 

 principal facts of Mendelism and Mendel's interpretation of them, we 



