In this blog post, we will examine how crucial genes are in shaping phenotype and whether their influence is maintained through interactions with the environment.
We use the biological term “gene” very frequently in everyday life. For example, there are countless expressions referring to genes, such as “genes for height,” “genes for academic success,” and “obesity genes.” Although we mention genes in so many contexts, few people clearly understand what the definition of a “gene” actually is, how much genes influence the phenotype, or whether genes are the only carriers of genetic information. Let’s take a closer look at genes—the foundation of life—from the perspective of evolutionary biology.
Even among biologists, few can provide a simple and clear explanation when asked, “What is a gene?” From a molecular biological perspective, a gene refers to a DNA sequence that encodes a protein. However, what we commonly refer to as a “gene” constitutes only a very small portion of the entire DNA sequence. If we adopt a molecular concept, only exons and regulatory regions can be considered genes. While scholars define genes differently, understanding them from an evolutionary perspective makes it easier to explain. For example, the eyeless gene in fruit flies induces the development of eyes during the embryonic process. If the eyeless gene is not expressed, the fruit fly is born without eyes, demonstrating that genes have a decisive influence on the phenotype. In this way, DNA sequences that influence the phenotype can be called genes.
We now know that the essence of a gene is a DNA sequence that encodes proteins. Genes determine the phenotype by regulating the types of proteins produced and the timing of their expression. However, there is debate regarding the extent to which genes determine the phenotype. There are two main theories: “gene selection theory,” which posits that genes play a dominant role, and “multilevel selection theory,” which argues that the surrounding environment plays an equally significant role alongside genes. I believe that the theory of gene selection—the mainstream perspective in evolutionary biology that has faced no major challenges for over 100 years—is correct, and I wish to emphasize the powerful role of genes.
Some scholars question whether genes determine the phenotype. They argue that genes interact with the surrounding environment to shape the phenotype, and here, “environment” is said to include not only the organism’s ecological environment but also the molecular environment inside the cell. In other words, it is not that genes play a leading role, but rather that both genes and the environment exert a significant influence on the phenotype. This argument is based on the fact that even identical twins, who share the same genes, can exhibit different phenotypes depending on their environment. However, this argument can be resolved by defining genes from an evolutionary perspective. Even if a specific gene is expressed in an optimized environment, the corresponding phenotype will not appear without that gene. Ultimately, genes play a decisive role in determining the phenotype.
If we say that genes determine the phenotype, what is the relationship between genes and the phenotype? In his research, Professor Richard Charles Lewontin classified how phenotypes change in response to genetic or environmental changes into four response patterns: genetic determinism, environmental determinism, additive interaction, and non-additive interaction. Genetic determinism refers to cases where the same phenotype is expressed regardless of the environment, while environmental determinism is the theory that different genes produce the same phenotype in the same environment. However, since both theories completely exclude the influence of either genes or the environment, they have been proven false in modern biology. Therefore, there is no longer any need to discuss them. Today, most biologists agree that both genes and the environment influence the phenotype. However, there is a difference of opinion regarding which of the two—genes or the environment—plays the more dominant role.
Additive interaction refers to cases where different genes express different phenotypes depending on the environment, and this difference is maintained. Non-additive interaction means that the phenotypes expressed by two genes can be reversed depending on the environment. Professor Lewontin argues that while people generally consider only additive interactions, non-additive interactions are actually predominant. Examples include changes in the height of milfoil with altitude and changes in G1 survival rates during fruit fly development depending on temperature. However, based on research conducted to date, it is difficult to conclusively state that non-additive interactions are dominant in real-world environments. Even if additive interactions are more common, this does not mean they ignore the influence of the environment on phenotype determination. Nevertheless, phenotypes are still significantly influenced by genes.
Some scholars argue that genes do not exert a dominant influence on phenotype determination, pointing out that genetic information is not confined solely to the DNA strand. For example, in maternal genetic effects, maternal proteins or ribosomes contained in the egg influence early development, indicating that genetic information is not limited to DNA. Furthermore, gene expression is regulated by the degree of acetylation of histone proteins, and these histone proteins are also partially inherited. Methylation of DNA 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 genes do not play a dominant role in determining the phenotype simply because genetic information is not confined to DNA. Even if there is an environment that optimizes gene expression during an organism’s development, the phenotype will not emerge without genes. Rather, the existence of genetic information at various levels implies that interactions between genes are extremely important. For example, because the Pax6 gene in vertebrates and the eyeless gene in fruit flies perform the same function, injecting the fruit fly’s eyeless gene into a mouse embryo results in normal eye development. As such, gene interactions are also highly decisive during the developmental process.
Some scholars who are wary of genetic determinism do not accept that genes play a leading role in determining the phenotype. They argue that a new research perspective that takes environmental influences into account is necessary. However, gene-centered biology is expected to have infinite scope for future research, and since the current paradigm has not yet faced a major crisis, attempts at new perspectives are considered unnecessary.
