# Chapter 6: How the environment can affect our genes

At the beginning of the 20th Century, scientists did not yet have an exact idea of how genes cause certain phenotypes. While the existence of DNA was already known, it was not yet linked to the task of transporting genetic information, and it was debated whether proteins fulfilled this task. Later, after the discovery of the gene as the basic unit of heredity, there were efforts to describe its function, but this could only be done on the basis of the analysis of phenotypes. The result of these observations was the conclusion that genes do not tend to act in isolation, but that they perform their task in harmony with other genes and, moreover, in the context of the environment surrounding them. Thus, the scientists realized that the external appearance of an individual - the phenotype - is created by the interaction between the genotype and the environment.

There are traits encoded by a gene that manifest themselves at the phenotype level with relatively low variability (i.e., a low number of possible manifestations). In this case we talk about qualitative traits, which includes, for example, a person's blood type or a hereditary disease, the occurrence of which can be assumed based on the presence of the disease in the family, such as the blood disease haemophilia A. However, there is also a second and very numerous group of genes, the so-called polygenes. Here, the effect of a single gene on the phenotype is too small to be observed, but when acting with other genes, together they produce an effect that is seen. The phenotypic manifestation is dependent on this group of genes, characteristic of the quantitative type of inheritance, since every single allele contributes to the formation of the phenotype. As a result, continuous phenotypic variability can be observed, e.g. in the case of human height. All the people of the world are not divided into two or three height categories, but there is a wide range of different heights, while the resulting height of each of us depends on the number and types of alleles of individual genes, each with a small effect. In addition, the overall phenotypic manifestation of a trait is also influenced by the environment in which the organism lives, which is why the inheritance of these traits is sometimes referred to as multifactorial inheritance. If a person has a predisposition to be tall, but did not have sufficient nutrition throughout childhood, then their height potential cannot be manifested to its full extent. Quantitative genetics studies the influence of specific polygenes and the influence of the environment on the formation of the phenotype. The distribution of phenotypic classes of quantitative traits in the population is characterized by the so-called Gaussian curve (Figure 6.1), with the highest number of average individuals and the lowest number of individuals with extreme phenotypes (too short or too tall individuals).