The term phenotype literally means "the form shown", and can be defined as the set of visible characteristics of an organism that are the result of the expression of its genes and its interaction with the environment that surrounds it.
According to Manher and Kary in 1997, the phenotype of an organism is simply a set of all the types of traits or characters that it or one of its subsystems possesses. It refers to any type of physical, physiological, biochemical, ecological or even behavioral characteristic.
Phenotypic variation in human eye color (Source: LeuschteLampe via Wikimedia Commons)
This author considers, then, that any phenotype is the result of the expression of a subset within the genotype of an organism that develops in a particular environment.
Considered the “father of genetics”, Gregor Mendel, more than 150 years ago, was the first to study and describe the heritable characteristics of organisms, only without coining the modern terms that are used today.
It was in the first decade of the 1900s that Wilhelm Johannsen introduced the fundamental concepts of phenotype and genotype to science. Since then, these have been the subject of much debate, since different authors use them for different purposes and some texts present certain inconsistencies regarding their use.
Phenotypic characteristics
From the point of view of some authors, the phenotype is the physical expression of a character in an individual and is genetically determined. Most phenotypes are produced by the concerted action of more than one gene, and the same gene can participate in the establishment of more than one specific phenotype.
The phenotypic characteristics can be contemplated at various levels, since one can speak of a species, a population, an individual, a system within said individual, the cells of any of their organs and even proteins and organelles internal cells of a given cell.
If, for example, we speak of a species of bird, numerous phenotypic characteristics can be defined: plumage color, song sound, ethology (behavior), ecology, etc., and these and other traits can be distinguished in any population of this species.
Thus, it is easy to ensure that an individual of this hypothetical bird species will also possess phenotypic characteristics that will make it visibly and quantifiably different from other individuals in the same population, both at a macro and microscopic level.
This is applicable for all living organisms: unicellular or multicellular, animals or plants, fungi, bacteria and archaea, since there are no two identical individuals, although they share the same DNA sequences.
Phenotypic differences
Two individuals may have similar phenotypic characteristics that do not result from the expression of the same genes. However, even if two individuals come from an organism whose reproduction is asexual ("clones"), these two will never be phenotypically identical.
This fact is due to the fact that there are multiple mechanisms that regulate the phenotypic characteristics of an organism that do not depend on the modification of the genomic DNA sequence; that is, they participate in the regulation of the expression of genes that will dictate a certain phenotype.
These mechanisms are known as epigenetic mechanisms ("epi" from the Greek prefix "on" or "in"); and generally they have to do with methylation (addition of a methyl group (CH3) to the cytosine base of DNA) or with the modification of chromatin (the complex of proteins histones and DNA that makes up chromosomes).
The genotype contains all the genetic instructions necessary for the construction of all types of tissues in an animal or a plant, but it is epigenetics that determines which instructions are “read” and carried out in each case, giving rise to the observable phenotype of each individual.
Epigenetic mechanisms are frequently controlled by the environmental factors that an individual is constantly subjected to during their life cycle. However, these mechanisms can pass from one generation to another regardless of whether the initial stimulus has been removed.
Thus, although many phenotypic differences have to do with the presence of a different underlying genotype, epigenetics also play an important role in regulating the expression of the genes contained therein.
Differences with the genotype
The phenotype refers to any characteristic that is expressed in an organism that inhabits a certain environment as a result of the expression of a set of genes within it. On the other hand, the genotype has to do with the compendium of inherited genes that an organism possesses, whether they are expressed or not.
The genotype is an invariable characteristic, since the set of genes that an organism inherits is basically the same from conception to death. The phenotype, on the other hand, can and does change continuously throughout the life of individuals. Thus, genotype stability does not imply an invariant phenotype.
Despite these differences and despite the great environmental influence that exists, it is possible to infer a phenotype by analyzing its genotype, since this is, in the first instance, the one that determines the phenotype. In short, the genotype is what determines the potential for the development of the phenotype.
Examples
A good example of the influence of the environmental environment on the establishment of a phenotype is the one that occurs in identical twins (monozygotic) who share all of their DNA, such as the uterus, the family and the home; and yet they show diametrically opposite phenotypic characteristics in behavior, personality, diseases, IQ, and others.
Bacteria are another classic example of environment-related phenotypic variation, as they have complex mechanisms to respond to rapidly and continuously changing environmental conditions. For this reason, it is possible to find stable subpopulations that present different phenotypes in the same bacterial population.
Plants can be considered as the organisms that most exploit epigenetic mechanisms for phenotype control: a plant that grows in a humid and hot environment exhibits different traits (phenotype) than those that the same plant will exhibit in a cold and dry environment, for example.
An example of phenotype is also the shape and color of the flowers in plants, the size and shape of the wings in insects, the color of the eyes in humans, the color of the coat of dogs, the size and shape of stature of humans, color of fish, etc.
References
- Griffiths, A., Wessler, S., Lewontin, R., Gelbart, W., Suzuki, D., & Miller, J. (2005). An Introduction to Genetic Analysis (8th ed.). Freeman, WH & Company.
- Klug, W., Cummings, M., & Spencer, C. (2006). Concepts of Genetics (8th ed.). New Jersey: Pearson Education.
- Mahner, M., & Kary, M. (1997). What Exactly Are Genomes, Genotypes and Phenotypes? And What About Phenomes? J. Theor. Biol., 186, 55-63.
- Pierce, B. (2012). Genetics: A Conceptual Approach. Freeman, WH & Company.
- Rodden, T. (2010). Genetics For Dummies (2nd ed.). Indianapolis: Wiley Publishing, Inc.
- Smits, WK, Kuipers, OP, & Veening, J. (2006). Phenotypic variation in bacteria: the role of feedback regulation. Nature Reviews Microbiology, 4, 259–271.
- Szyf, M., Weaver, I., & Meaney, M. (2007). Maternal care, the epigenome and phenotypic differences in behavior. Reproductive Toxicology, 24, 9–19.
- Wong, AHC, Gottesman, II, & Petronis, A. (2005). Phenotypic differences in genetically identical organisms: the epigenetic perspective. Human Molecular Genetics, 14 (1), 11–18.