290 INTRODUCTION TO CYTOLOGY 



members of each pair behaving toward each other in a manner similar to 

 that described above for tallness and dwarfness. He further observed 

 that the seven pairs were entirely independent of each other in inheritance. 

 For instance, smoothness and wrinkledness of seeds were allelomorphic 

 characters, and either of them was transmitted as often with tallness as 

 with dwarfness in crosses involving both pairs of characters. This 

 independence of the various pairs of factors was set forth in Mendel's 

 second law. The law held for the seven principal pairs studied by 

 Mendel, but it has since been learned that many pairs are not thus 

 independent, as will be pointed out below. 



Terminology. — We may here introduce certain terms prominent in the 

 literature of genetics. The genotype is the entire assemblage of genetic 

 factors, or genes, which the organism actually possesses in its constitution, 

 irrespective of how many of these may be expressed in externally visible 

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

 tion. The phenotype is the aggregate of externally visible characters, 

 irrespective of any other factors, unexpressed in characters, which may 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 {F2) three genotypes (with respect to the single char- 

 acter 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 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 Mirabilis, however, there are represented 

 in the F^ generation not only three genotypes but also three phenotypes, 

 since incomplete dominance renders the hybrids externally unlike either 

 of the pure forms. Practically, a phenotype is a class of individuals which 

 look alike, and a genotype is a class of individuals which breed alike 

 (Castle). 



An individual is said to be homozygous for a given allelomorphic 

 factor pair if it has received the same type of factor from the two parents — 

 a pea, for example, with the constitution TT or tt. If it has unlike 

 members in 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 

 factors. Thus an organism with the genotypic constitution AABbcc is 

 homozygous for the factors AA and cc and heterozygous for Bh. It is 

 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 b. The phenotypic 

 appearance of the organism is here determined by the dominant factors A 

 and B and the recessive c. It is a common practice to represent dominant 



