The characteristics of an organism emerge through complex interactions of its genetic make-up and the effects of the environment. This article discusses this terminological distinction in genetics - that of the genotype, the heredity material, or DNA, contained within genes and passed from one generation to the next, versus the phenotype, the physical and behavioral traits of an organism. The genotype-phenotype distinction is one of the most fundamental concepts in biology and is crucially crucial to evolutionary theory.
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Here we’ll discuss how these terms were first introduced into biology, in the early part of the twentieth century, by the Danish botanist Wilhelm Johannsen (1857-1927). Since their first introduction, the meanings of genotype and phenotype have altered over time. Biologists have dismissed genetic reductionism, insisting 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 to phenotypic characteristics.
Wilhelm Johannsen and the genotype conception of heredity
In 1911 Wilhelm Johannsen coined the terms genotype and phenotype when he proposed “The Genotype Conception of Heredity.” The two categories form two fundamentally different levels of biological abstraction. The genotype refers to the genetic material passed between generations, and the phenotype is observable characteristics or traits of an organism.
While Johannsen produced novel and innovative research in genetics and highlighted the need for accompanying new terminology, the American geneticist Alfred Henry Sturtevant (1891-1910) constructed a genetics map of the chromosome, having located specific genes of the fruit fly Drosophila.
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, when considering eye color, if a person inherits a dominant brown allele (B) and a recessive blue (b), they’ll be genetically heterozygous for those characteristics. As 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:
- Environmental, e.g., nutrition, temperature, humidity, and stress
- Epigenetic factors
Once the distinction between these terms was established, research has sought to understand the precise relationship between the two. How do genotypes map onto phenotypes? Pursuing this line of inquiry has not been an easy task, often linked to how their meanings have changed over time.
Conceptual Confusion
Confusion concerning the concepts can easily occur. Take the notion of 'phenotypic plasticity', a so-called rapid response adaptive 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 a part of the process. Considering that some definitions of phenotypic plasticity refer to a genotype's environmental sensitivity, defining terminology clearly can become difficult.
The use of genotype is confusing here - environmental sensitivity does not mean the genome itself responds directly to the environment. Rather phenotypic plasticity involves a change in the phenotype without a permanent genetic change in the individual. It is defined as the range of phenotypes that an organism can express as a function of its environment and is the resultant component of the GxE (genotype by environment) interaction.
No Straightforward Causal Connection
The is no clear causal connection between the genotype and the phenotype, and Genome-Wide Association Studies - an approach used in genetics research to see if any variant is associated with a trait - have made this ever more apparent. Human genetic association studies of single nucleotide polymorphisms (SNPs) explain only a small proportion of manifest phenotypic variation, and this may be due to:
- Rare SNP's
- Structural and epigenetic variants
- Synergistic genetic interactions
- Multiple alleles with additive effects
- Multiple alleles with additive effects
The emphasis placed on multiple genetic interactions with numerous environmental variables is problematic for assessing the genotype-phenotype relation.
Recently, researchers have called for a return to emphasizing the genotype-phenotype relationship as a connection between differences at two different biological levels, as originally argued by Johannsen. 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 connection between genotype and phenotype is essential for several research avenues, including medical approaches. Rapid genome-sequencing methods and whole-genome transcription profiling offer the prospect of predicting 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 genetic determinants of disease-related phenotypes.
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