Genotype versus phenotype


The characteristics of an organism emerge through complex interactions between its genetic makeup and the effects of the environment. This article discusses this terminological distinction in genetics – that of genotype, the inherited material, or DNA, contained in genes and passed from one generation to the next, versus the phenotype, physical and behavioral traits of an organism. The genotype-phenotype distinction is one of the most fundamental concepts in biology and is crucial for the theory of evolution.

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Here we will discuss how these terms were first introduced in biology at the beginning of the 20th century by the Danish botanist Wilhelm Johannsen (1857-1927). Since their first introduction, the meanings of genotype and phenotype have changed over time. Biologists have rejected genetic reductionism, insisting that there is no causal effect between a single gene and a phenotypic trait, for example. Although a simple cause and effect relationship may not exist, progress has been made in mapping genotypes with phenotypic characteristics.

Wilhelm Johannsen and the genotypic conception of inheritance

In 1911, Wilhelm Johannsen coined the terms genotype and phenotype when he proposed “The genotype design of inheritance”. The two categories form two fundamentally different levels of biological abstraction. The genotype refers to the genetic material transmitted between generations and the phenotype corresponds to the observable characteristics or traits of an organism.

While Johannsen produced new and innovative research in genetics and stressed the need to accompany new terminology, the American geneticist Alfred Henry Sturtevant (1891-1910) constructed a genetic map of the chromosome, having located specific genes of the fruit fly. Drosophila fruit.

Genotype versus phenotype: a fundamental distinction

The combination of alleles possessed by an individual for a specific gene gives us the genotype. The combination of alleles possessed by an individual for a specific gene gives us the genotype. For example, considering eye color, if a person inherits a dominant brown (B) and recessive blue (b) allele, they will be genetically heterozygous for these characteristics. Since one allele is dominant, the person will have brown eyes.

The phenotype, however, relates to the observable traits of an organism. Unlike the genotype, it is not inherited from its parents and is influenced by the genotype and several other factors:

  • Environment, p. ex. nutrition, temperature, humidity and stress
  • Epigenetic factors

Once the distinction between these terms was established, research sought to understand the precise relationship between the two. How do genotypes correspond to phenotypes? Pursuing this line of inquiry has not been an easy task, often related to how their meanings have changed over time.

Conceptual confusion

Confusion regarding concepts can easily arise. Take the notion of “phenotypic plasticity,” an adaptive so-called rapid response mechanism that allows an organism to change in response to environmental stimuli – to produce different phenotypes. Although this mechanism excludes the genome, changes in gene expression are part of the process. Considering that some definitions of phenotypic plasticity refer to the environmental sensitivity of a genotype, defining terminology can clearly become difficult.

The use of the genotype is confusing here – environmental sensitivity does not mean that the genome itself responds directly to the environment. Rather, phenotypic plasticity involves a change in phenotype without a permanent genetic change in the individual. It is defined as the range of phenotypes that an organism can express depending on its environment and is the resulting component of the GxE (genotype by environment) interaction.

No simple causal link

There is no clear causal link between genotype and phenotype, and genome-wide association studies – an approach used in genetic research to see if a variant is associated with a trait – have made this even more obvious. Human genetic association studies of single nucleotide polymorphisms (SNPs) explain only a small proportion of the evident phenotypic variation, and this may be due to:

  • Rare SNPs
  • Structural and epigenetic variants
  • Synergistic genetic interactions
  • Multiple alleles with additive effects
  • Multiple alleles with additive effects

The emphasis on multiple genetic interactions with many environmental variables is problematic in assessing the genotype-phenotype relationship.

Recently, researchers have called for a return to the emphasis on the genotype-phenotype relationship as the link between differences at two different biological levels, as Johannsen originally argued. The differential view of this relationship will be a useful explanatory framework in the context of pleiotropy, epistasis and environmental effects.

The genotype-phenotype distinction in medical genetics

Understanding the link between genotype and phenotype is essential for several avenues of research, including medical approaches. Rapid genome sequencing methods and whole genome transcription profiling offer the possibility of predicting the phenotype from a genotype.

DNA sequence changes have been linked to phenotypic differences at the individual and species level in eukaryotic organisms using these methods. In humans, the OMIM catalog (Online Mendelian Inheritance of Man) is a compilation of the genetic determinants of phenotypes linked to the disease.

The references

Further reading


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