Genotype

Genotype, a term introduced by Johannsen and denoting the aggregate of hereditary factors of an organism. These factors determine the formation of the phenotype (apparent type), i.e., the aggregate of all features of an individual accessible to direct analysis or observation. It is not possible to directly judge the genotype, and it is determined only by how it is manifested in the phenotype. However, in the formation of the phenotype, not only the genotype but also another, very variable factor—the external environment—plays a role. The genotype determines a certain norm of reaction to external conditions, i.e., under identical external developmental conditions, individuals with the same genotypic composition (isogenic) will also be phenotypically uniform (isophenous). On the other hand, due to the fact that the genotype gives different reactions depending on the nature of the environment, individuals that are genotypically uniform may be phenotypically dissimilar if they develop under different conditions. This is especially evident in the most variable traits influenced by external conditions, such as size, weight, etc. When raised under different conditions, the opposite phenomenon can also be obtained: phenotypic similarity with genotypic difference. Thus, individuals of a larger race under poor nutritional conditions will be of the same size as individuals of a smaller race under more favorable nutritional conditions, or even smaller. Thus, one of the conditions for judging the similarity or difference of genotypes based on phenotype is the development of the compared individuals under identical external conditions. However, this condition alone is not sufficient, since certain genotypes that differ from each other may react identically under certain conditions. There are races of corn, one of which gives red cobs under all conditions, while the other only in the light; there exists a race of fruit fly (Drosophila), sharply differing from the normal one by the abnormal structure of the abdomen. If this race is raised with a deficiency of moisture and food, it will not differ from the normal one raised under the same conditions. Thus, even when raised under identical conditions, in the first case in the light, in the second with a deficiency of moisture and food, we will not notice a difference between the races. Finally, genotypically dissimilar individuals may be similar under any rearing conditions. This occurs in the case of heterozygosity with complete dominance (see). Then heterozygous individuals will be completely similar to homozygous ones for the dominant trait. In Mendel's experiments on crossing peas, in the second generation, three yellow peas were obtained to one green; however, despite phenotypic similarity under any developmental conditions, the yellow peas are genotypically heterogeneous. One third of them gives plants only with yellow peas, and two thirds with yellow and green. The former are homozygous for the dominant trait (yellow color), the latter are heterozygous (in the genotype there are factors of yellow and green coloring). Thus, one can judge the genotype based on phenotype only after verification by rearing under different conditions and after studying the offspring. Cases of complete genotypic uniformity among dioecious species are unlikely due to the possibility of constant crossing between genotypically dissimilar individuals. Thus, in these species, phenotypic differences between individuals depend on both the influence of the environment and the genotypic composition. Only among self-pollinating plants (beans, wheat), originating from a single individual—so-called pure lines—can one find genetically uniform individuals. However, pure lines remain genotypically uniform only until changes occur in the genotype, which does not always remain constant but is capable of changing—giving mutations. The genotype consists of hereditary units—genes (see), being, however, not a simple sum of them, but an aggregate, a whole; each phenotypic trait develops under the influence of all genes, and conversely—each gene participates in the realization of all traits of the organism. When we speak of genes of individual traits, it is only because the change of a certain gene particularly sharply affects a certain trait. Many authors (among Russians, Filipchenko) use the terms 'genotype' and 'phenotype' in a somewhat different sense than proposed by Johannsen. They speak not of the type of a particular individual, but of the type common to a group of individuals. Thus, by genotype they mean the community of genotypically uniform individuals—a biotype according to Johannsen, and by phenotype they mean the community of phenotypically uniform individuals. Such an interpretation of these terms is completely incorrect and only confuses the terminology, all the more so since all these authors do not specify that they have changed Johannsen's terminology.