The phenotype is the outwardly visible appearance of an organism with its various characteristics. Both genetic makeup (genotype) and environment influence the expression of the phenotype.
What is the phenotype?
The phenotype is the outwardly visible appearance of an organism with its various characteristics. The visible expressions of an organism, but also the behavior and physiological characteristics make up the phenotype. The term is derived from the ancient Greek “phaino” and means “shape”. The genetic makeup of an individual, the so-called “genotype” determines the expression of the phenotype. In addition to genes, the environment also influences the extent to which a particular phenotype is expressed. The extent to which an organism is phenotypically changeable by environmental influences is also related to its genotype. This environmental variability of the phenotype is the so-called reaction norm. This norm can be very broad and result in very different phenotypes. However, it can also be small and as a consequence the phenotypic variants that occur are all very similar. Normally, very basic traits in the phenotype that are important for the survival of an organism tend to have a low response norm because changes in them often have negative consequences for the individual.
Function and task
The genetic makeup of an organism always determines its appearance. This also applies to humans, in which more than 20,000 genes form the genotype and thus determine the phenotype. Depending on how strongly the genes determine the appearance and how high the influence of environmental factors is, one speaks of phenotypic plasticity. Traits with a high phenotypic plasticity, such as a person’s behavior, are strongly shaped by environmental influences. Traits with a low phenotypic plasticity, such as a person’s eye color, are virtually unchangeable by external influences. The phenotypic expression of certain traits over several generations within families make it possible to draw conclusions for the descendants. This is particularly true for certain hereditary diseases, whose probability of occurrence can be predicted relatively reliably. The genes for the occurrence of certain diseases can be basically dominant or recessive in their effect. Dominant genes provide a very high probability of expression in the phenotype, whereas with recessive genes the probability of phenotypic occurrence is significantly lower. For example, in the case of a dominantly inherited disease, the probability of phenotypic occurrence in the offspring is at least 50 percent if one parent has the disease. If both parents phenotypically show the appearance of a dominantly inherited disease, the probability of the disease in the children is 100 percent. In contrast, recessively inherited diseases are much less likely to be visible in the phenotype than dominantly inherited diseases. If one parent phenotypically has such a disease, then the probability of occurrence in the offspring is at most 50 percent. In the case of these diseases, it is also possible that there is no phenotypic expression at all, although a recessive gene is present. The particular variant of a phenotype is not always passed on by inheritance within several generations. There is also the possibility of a spontaneous mutation in the genotype, so that an altered phenotype with new characteristics suddenly appears for the first time in one generation. This explains why phenotypically divergent individuals with new traits repeatedly appear in families. If these mutations in the genotype with their effect in the phenotype did not exist, ultimately an extinction of the species would be the result. This is because only by keeping the genotype flexible and allowing new phenotypes to emerge again and again is it possible to adapt to changing environmental conditions. This is a fundamental principle of evolution and is also known as variability.
Diseases and ailments
Current research suggests that environmental influences on phenotype play a role in more diseases and dissociations than previously thought. For example, a person’s early development is thought to determine whether he or she will develop toward obesity or remain lean.Certain genetic regulatory programs are probably responsible for one or the other expression of the phenotype. With this knowledge, researchers hope to develop new drugs and therapies in the future that could act before the phenotype is expressed. Phenotypic expression of certain inherited diseases can be predicted to occur in offspring, making early and effective treatment possible. Some rare phenotypes based on mutations in the genotype occur in all classes of organisms. One example is albinism. In this mutation, affected individuals lack pigment in the skin, hair and eyes and are very sensitive to sunlight. This particular expression of the phenotype exists in both humans and animals. To date, the phenotypic expression of genetically determined traits can only be influenced medically to a limited extent, but the possibilities of changing a phenotype before it is expressed are increasing. Research fields such as epigenetics contribute to this and also provide for the development of new drugs and therapies. Accordingly, in the future, a certain genotype will not necessarily lead to the formation of a certain phenotype. This is a particularly promising prospect with regard to genotype-related diseases.
