CYTOLOGY AND MENDELIAN HEREDITY 171 



themselves, the nvsultin^ /''•_. K^'neratioTi comprises phints of three visilily 

 different types: i)ure floiniiuiiits with crimson flo\v(M-s, hyl)ri{ls with 

 magenta flowers, and f)ure recessives with white flowei-s; these types 

 tend to occur in the ratio of 1:2:1. 



Two points should be emphasized before proceeding further. (1) It 

 should be understood that the above ratios indicate only the degree of 

 probability of obtaining the various types through random combination 

 of gametes. When the number of individuals is sufficiently large, the 

 ratios tend to be approached rather closely; sometimes they are equalled 

 exactly, even in small populations. The ratios, then, represent statistical 

 expectations. (2) One factor, or gene, is not wholly responsible for the 

 production of a given character. When one gene is singled out as the 

 gene for a character, it is either because it is the most influential one 

 known to affect that character, or because it is the only one of the influ- 

 ential genes that exists in both the dominant and recessive states in 

 the material studied. Thus it is the "differential" in a system of factors 

 otherwise uniform throughout the material, the character varying with 

 this one gene. In other strains of mateiial some other gene might be 

 the differential one. When two or more gene pairs are acting differ- 

 entially at the same time, the characters appear in ratios other than those 

 stated above. From such ratios the geneticist is able to infer the number 

 of genes concerned. 



Terminology. — The genotype is the entire assemblage of genetic 

 factors, or genes, which the organism actually possesses in its constitu- 

 tion, irrespective of how many of these may be expressed in externally 

 visible characters; or, it is a class of individuals with the same genetic 

 constitution. The phenotype is the aggregate of externally visible 

 characters, irrespective of any other factors, unexpressed in characters, 

 which ma}' be present in the organism; or, it is a class of outwardly 

 similar individuals. For illustration: In the case of the tall and dwarf 

 peas there are in the second generation (F-i) three genotypes (with 

 respect to the single character pair discussed) : T T, T t, and / /, repre- 

 sented, respectively, by pure tall plants, tall hybrids, and dwarfs; but 

 there are only two phenotypes: tall and dwarf, because 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 constitutions, 

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

 the four-o'clocks described above, there are represented in the F^ genera- 

 tion not only three genotypes, but also three phenotypes, since incomplete 

 dominance rendei-s the hybrids externally unlike either of the pure forms. 

 Practically, a phenotype is a class of individuals that look alike, and a 

 genotype is a class of individuals that breed alike (Castle). 



