304 THE CONTINUITY OF THE RACE 



stages, some in later stages; but in all cases, such lethals are recessive and 

 show little or no harmful influence when present in a heterozygous state. 1 



Multiple effects of a single gene. It is also now recognized that 

 many genes are not limited to a single effect. We usually contrast (and 

 name) the most striking or immediately evident character that differen- 

 tiates the phenotypic expression of a dominant gene and its allelomorphic 

 recessive, but closer scrutiny will usually show that several or numerous 

 differences are involved, all attributable to the same pair of genes. For 

 instance, in Drosophila, the genes WW and ww are spoken of as the genes 

 for red and white eyes, respectively, but the white-eyed recessive also 

 has a colorless testicular sheath and shows a definitely lower vitality 

 than a red-eyed dominant. Many others of the several hundred known 

 recessive genes in Drosophila are correlated with various unnamed 

 structural differences in addition to the more conspicuous phenotypic 

 effects for which they are named. It is worth noting that in many in- 

 stances the several effects attributed to the phenotypic expression of a 

 single gene (or pair of genes) show no evident structural or physiological 

 relationship and give no clue as to why the single gene should produce the 

 particular cluster of effects that is observed. 



Genotype and phenotype. Mendel did not have any reason to dif- 

 ferentiate between the visible quality that was inherited and the factor 

 or gene that was transmitted through the gametes and that caused the 

 quality to reappear. Soon after 1900, however, the discovery of other 

 and much more complicated types of inheritance made it extremely 

 helpful to distinguish clearly between the genetic constitution of an 

 organism and its outward appearance. The word genotype was coined 

 to designate the hereditary-factor make-up of an organism, 2 and the 

 word phenotype to refer to the outward visible expression of these factors. 

 We have already seen that the phenotype of round and yellow seeds in 

 the pea may be due to a variety of different genotypes — GG WW, GG 

 Ww, Gg WW, or Gg Ww — and that only breeding tests can distinguish 

 among them. It is well to keep in mind that a gene cannot be seen or 

 directly demonstrated, and in all actual genetic experiments the genotype 

 must be assumed from data concerning the appearance and proportion of 

 various phenotypes in certain sequences of generations. 



THE INTERACTION OF GENES 



So far we have been concerned with what have been termed unit charac- 

 ters. Here each gene or pair of genes appears to produce a distinct and 

 evident phenotype, and, conversely, each phenotype appears to be due 

 to one gene or one pair of genes. Soon after 1900, however, numerous 

 cases began to be found in which a given phenotype was dependent upon 



1 Compare with the sex-linked condition in man known as hemophilia. 



2 The word gene was then coined to refer to a unit of the genotype. 



