Phenotypes are all the observable characteristics of an organism.
For example, size, hair color, mating behavior and pattern of movement are all traits of a particular phenotype. Phenotypes can change as a result of environmental factors, or their traits can change as an organism evolves or adapts.
The phenotype of groups of organisms can change together if their food supply, type of food or kind of predator changes.
While environmental influences play a role and can affect a phenotype, an organism's observable traits are based on their DNA or genetic code. Traits result from the presence of one or several genes in the DNA. If the genes are expressed, meaning copied and used to synthesize proteins, then the corresponding characteristics appear in the organism.
The interplay between phenotype and genotype can be very complex.
The genotype is at the root of the phenotype traits, but the traits affect the organism's appearance and its observable behavior.
As a result, the phenotype largely determines how successful the organism is at survival and at mating. The organism's success allows it to have many descendants, but it passes on its genotype, not its phenotype.
The phenotype/genotype interaction can produce organisms better adapted to their environment.
Phenotypes Depend on Many Factors
The set of genes in the DNA of an organism forms the basis of the organism's phenotype, but there are many other influences at work. All cells in an organism have the same DNA, but many of the cells are different.
The differences depend on what parts of the DNA the cell uses in a process called gene expression. Gene expression can be influenced by environmental factors, and influences from the environment can further affect the phenotype in other ways.
The main things that can influence phenotype are:
- Genotype: The phenotype is limited by the genotype. An organism can't display a characteristic if there is no gene for it.
- Epigenetics: Epigenetics affects the expression of genes. If a gene is present but not expressed, the organism can't display the corresponding characteristic.
- Environment: The environment can affect traits directly by changing the behavior or appearance of an organism while the corresponding gene is unchanged. Environmental factors can also affect gene expression.
Genes and Gene Variants of the Genotype Determine the Possible Phenotype Traits
While the presence of a gene in the DNA genetic code makes it possible that a phenotype might include the corresponding trait, the trait itself could vary substantially. Organisms that reproduce sexually receive one set of genes from each parent. Their genetic makeup will contain two slightly different sets of genes, and a gene in each set could be dominant or recessive.
Because the two genes for a trait are always slightly different, having two dominant genes or a dominant and a recessive gene means the organism's possible trait is the one produced by the dominant variation of the gene.
An organism with two recessive gene copies displays the trait produced by the recessive gene variant. The two gene variants result in slightly different proteins being produced and can give rise to different phenotypes.
For example, humans have several genes that influence eye color. Gene variants resulting in dark eye color are dominant, and light eye-color gene variants are recessive. A person with two dominant dark eye-color gene sets or with one dominant dark and one recessive light eye-color set will have dark eyes.
People with two light eye-color gene sets will have light-colored eyes. The same gene with two different variants results in two phenotypes.
Epigenetics Helps Determine Which of the Possible Phenotypes Are Displayed
The organism's genotype determines the possible traits of the phenotype, but the corresponding genes have to be active for the trait to appear. Epigenetics studies gene expression in cells, and many genes are not active.
Different factors such as available nutrients, organism age and signals sent by other cells determine whether a cell expresses a gene or not.
To express a gene, a cell has to first make a copy of the gene from the original DNA code in the cell nucleus. The genetic code is copied to messenger RNA, which exits the nucleus and finds a cell ribosome to synthesize the corresponding protein from the coded sequence.
The protein gives the cell the characteristic, feature or capability that leads to the phenotype trait in the organism. The cell can block or regulate this process to make more, less or no protein.
The process of gene expression means that a phenotype such as hair color can change during the life of an organism even though the genetic code remains the same. The original gene set for a specific hair color remains in place, but some of the genes in the set are expressed more or less strongly as the cell regulates expression of a gene up or down.
For hair color, the gene in question may directly influence dark hair color, or it may cause a hormone or enzyme to be produced that affects the color of hair.
Environmental Factors Influence Phenotypes Directly or Through Gene Expression
The environment can affect the appearance and the behavior of organisms and change their phenotype. For example, some fur-bearing animals such as Siamese cats have temperature-sensitive skin. Colder skin grows dark-colored fur while warmer skin grows light fur. When the temperature of their environment changes, their fur color and phenotype may change as well.
In addition to changing phenotypes directly, environmental factors can affect traits by influencing gene expression. Availability of nutrients and other cell-related raw materials can further or prevent the expression of certain genes.
Making copies of genes and synthesizing proteins takes energy, which cells derive from food the organism digests. If there are not enough nutrients, gene expression may slow, and the traits may become less pronounced.
Phenotypes and Genotypes Both Affect Organism Development
While the genotype is the blueprint for the organism, the phenotype reflects how the coding is translated into reality. Depending on environmental factors and the life-experience of the organism, some parts of the genetic code may not be needed, and other parts may be expressed more or less strongly. The phenotype describes what is actually happening with the organism.
For example, an individual may have genes that predispose the organism to develop a specific type of disease. For the disease to develop, additional factors caused by environmental influences have to be present. The individual either has to engage in harmful behavior or to be inadvertently exposed to harmful factors.
The phenotype for someone susceptible to a disease may include obesity or high blood pressure. Behavioral factors may include cigarette smoking or excessive alcohol consumption. To trigger the disease, the individual may have to be exposed to toxic chemicals or normally harmless amounts of radiation. In each case, the genetic disposition is present, but if the individual does not eat, drink or smoke too much, the disease will not likely be triggered.
When variations in phenotypes influence natural selection, the successful phenotype can affect the genotype distribution for the organism.
For example, if organisms with a recessive gene are better adapted to the environment than those with a dominant gene, organisms with the recessive gene will become more common. They will have descendants with two recessive genes, and the population will consist mainly of genotypes with the two recessive genes. In this way, phenotypes can respond to environmental factors and affect the genotype distribution of a group of organisms.