In this blog post, we will look at how important genes are in shaping the phenotype and whether their influence is maintained in the interaction with the environment.
We use the term “gene” very often in our daily lives. For example, there are countless expressions that refer to genes, such as genes for height, genes for good grades, and genes for obesity. Although genes are mentioned in many situations, few people know clearly what the definition of “gene” is, how much genes affect the phenotype, and whether genes contain information alone. Let’s take a closer look at genes, the foundation of life, from the perspective of evolutionary biology.
When asked, “What is a gene?”, not many people, even among biologists, can explain it simply and clearly. From a molecular biology perspective, a gene is a DNA chain that encodes a protein. However, the gene we commonly refer to only accounts for a very small portion of the entire DNA chain. If we introduce a molecular concept, only the exon and regulatory region can be said to be a gene. Although the way genes are defined varies from one scholar to another, they can be easily explained from an evolutionary perspective. For example, the eyeless gene in Drosophila induces the development of eyes during the process of embryogenesis. If the eyeless gene is not expressed, the fruit fly is born without eyes, which shows that genes have a decisive influence on the phenotype. In this way, the DNA chain that affects the phenotype can be called a gene.
We now know that the main body of a gene is a DNA chain that encodes a protein. Genes determine the phenotype by regulating the types of proteins and the timing of their expression. There is, however, some controversy over the extent to which genes play a role in determining the phenotype. There are two schools of thought: “gene selectionism,” which holds that genes play a leading role, and “multilevel selectionism,” which holds that not only genes but also the surrounding environment play an equal role. I believe that the mainstream view of evolutionary biology, which is based on the theory of gene selection, has been correct for over 100 years and has not faced any major crisis, and I want to emphasize the powerful role of genes.
Some scholars question the idea that genes determine the phenotype. They argue that genes interact with the surrounding environment to form the phenotype, and the “environment” here includes not only the ecological environment of the individual but also the molecular environment inside the cell. In other words, genes do not play a leading role, but both genes and the environment have a significant impact on the phenotype. This is based on the fact that identical twins can have different phenotypes depending on the environment even if they have the same genes. However, this argument can also be resolved by defining genes from an evolutionary perspective. Even if a certain gene is expressed in an optimized environment, the phenotype will not appear without that gene. Ultimately, genes play a decisive role in determining the phenotype.
If genes determine the phenotype, what is the relationship between genes and the phenotype? In his research, Professor Richard Charles Lewontin distinguished four modes of reaction in which the phenotype changes depending on genetic or environmental changes: genetic determinism, environmental determinism, epigenetic interaction, and non-epigenetic interaction. Genetic determinism refers to the case where the same phenotype is expressed in any environment, and environmental determinism is the theory that different genes express the same phenotype in the same environment. However, both theories have been proven false in modern biology because they completely exclude the influence of genes or the environment. Therefore, there is no need to mention them any further. Today, most biologists agree that both genes and the environment affect the phenotype. However, there is disagreement as to which of the two, genes or the environment, plays a more dominant role.
A pleiotropic interaction is when different genes show different phenotypes depending on the environment, and this difference is maintained. A non-pleiotropic interaction means that the phenotypes of two genes can be reversed depending on the environment. Professor Lewontin argues that people usually only consider pleiotropic interactions, but in reality, non-pleiotropic interactions are dominant. For example, changes in the height of milfoil depending on the altitude above sea level or changes in the G1 survival rate depending on temperature during the development of fruit flies are such cases. However, according to the results of the studies conducted so far, it is difficult to conclude that non-additive interactions are dominant in the real world. Even if additive interactions are more common, it does not mean that they ignore the influence of the environment in determining the phenotype. Nevertheless, the phenotype is still greatly influenced by genes.
Some scholars argue that genetic information is not unique in determining the phenotype, as it is not only found in DNA chains. For example, in the maternal inheritance effect, maternal proteins or ribosomes contained in the egg affect early development, so genetic information is not only found in DNA. In addition, the expression of genes is regulated by the degree of acetylation of histone proteins, and some of these histone proteins are also inherited. DNA methylation itself also regulates gene expression, and this methylation information is also passed on to daughter cells during cell division.
However, these cases do not support the claim that genetic information is not only in DNA, and that genes do not play a unique role in determining the phenotype. Even if there is an environment that optimizes gene expression during the development of an individual, the phenotype will not appear without the genes. Rather, the fact that there are various levels of genetic information means that the interactions between genes are very important. For example, the Pax6 gene in vertebrates and the eyeless gene in fruit flies play the same role, so injecting the eyeless gene of fruit flies into mouse embryos results in the normal development of the eyes. As such, interactions between genes are very crucial in the process of development.
Some scholars who are wary of genetic determinism do not accept that genes play a leading role in determining phenotypes. They argue that a new research perspective that takes into account the influence of the environment is needed. However, gene-centric biology is expected to continue to have infinite research areas, and since the current paradigm has not yet experienced a major crisis, it is considered unnecessary to try a new perspective.
