A dominant gene is one responsible for defining the "dominant" characteristics of the phenotype in individuals. The term "phenotype" corresponds to the set of all the characteristics that can be observed, measured and quantified in a living organism. The characteristic expressed from a dominant gene will be the one that can be observed most frequently in a given population.
For example, in grizzly bear populations, dark brown fur is derived from the expression of a dominant gene, while reddish fur is derived from the expression of a recessive gene. Therefore, it is much more common to observe individuals with brown fur than reddish in color in bear populations.
Example of dominant genes: the dark brown fur of a Grizzly bear (Source: Gregory "Slobirdr" Smith via Wikimedia Commons)
The term "dominant" was used for the first time, in the context of the classification of phenotypes, by the monk Gregor Mendel in 1856, in the description of his work with pea plants. Mendel is known as the father of modern genetics.
Mendel determined that the purple phenotype in pea flowers was dominant over the white phenotype. This he observed when making crosses of purple-flowered pea plants with white-flowered plants.
What Mendel could not determine was that this dominant purple phenotype was due to being derived from a dominant gene.
Genetic principles
Mendel, in his experiments, observed that phenotypes were transmitted by "factors" that were found in pairs in each individual. These "factors" are now known as genes, which can be dominant or recessive.
Genes are the fundamental units of heredity. Before our time, the word "gene" was used to refer to a segment of DNA that contained the information necessary to encode a protein. However, today it is known that it is much more than that.
In Mendel's experiments, one of the plants that acted as a parent carried two dominant genes, while the other plant with which it crossed had two recessive genes; in other words, Mendel worked with dominant and recessive homozygous (homo = equal) plants.
When this researcher made the crosses of the parents and obtained the first generation (F1), all the resulting plants were heterozygous (hetero = different), that is, each individual inherited one gene from each type of the parents, one dominant and one recessive..
However, all the plants belonging to the F1 population had purple flowers, which today is known to be due to the dominance of purple over white.
This phenomenon of "dominance" was interpreted by Gregor Mendel as that the expression of one of the determining "factors" of the phenotype masked the expression of the other.
Study methods
Currently, the method of studying dominant genes consists of making crosses between individuals of the same species, since, following Mendel's laws of inheritance, genes can present alternative forms that influence the phenotype.
Mendel called the alternative forms of a gene (for each morphological character) " alleles." Alleles can configure the color of flowers, the shape of the seeds, the shapes of the leaves, the color of a grizzly bear's fur, and even the color of the eyes in people (as well as many other characteristics that we can't see.).
In humans and most animals, each trait transmitted through inheritance is controlled by two alleles, since they are diploid organisms. The diploid condition is that all cells have two sets of autosomal chromosomes.
Chromosomes are structures of protein and nucleic acids where most of the genetic information of individuals is found. These are highly organized structures and are only seen clearly defined during cell mitosis (division).
The individuals that reproduce in a population act as "vehicles" that "perpetuate" the different alleles (dominant and recessive genes) that can be found on the chromosomes of that population.
Factors influencing genetic dominance
Not all traits that depend on dominant genes exactly follow the pattern of inheritance discovered by Mendel. Many genes present incomplete dominance, this means that in heterozygous individuals with these genes the derived phenotype is intermediate.
An example of this is carnations. Carnations that have two genes for the color white express the color white. However, the carnations that carry the genes for the color white and for the color red, express a color derived from both alleles, that is, they are pink.
An example of incomplete dominance (Source: Sciencia58 via Wikimedia Commons)
Another very frequent variation is genetic codominance. When an individual is heterozygous (possessing a recessive gene and a dominant gene) he expresses the traits derived from both genes.
Such is the case with blood groups in humans. Genes for blood type O are recessive, genes for blood type A and B are codominant. Therefore, the A and B genes are dominant over the type O gene.
Thus, a person who inherits alleles of A and alleles of B has a type AB blood group.
Examples
Generally, the phenotype product of the dominant genes is twice more frequent than the phenotypes of the recessive genes, since, when analyzing the phenotypic traits as a single gene, we obtain that:
Dominant gene + Dominant gene = Dominant phenotype
Dominant gene + Recessive gene = Dominant phenotype
Recessive gene + Recessive gene = Recessive phenotype
However, recessive genes can be present in a population with very high frequencies.
Eye color is an example of dominant and recessive genes. People with a light-eyed phenotype are the product of recessive genes, while people with a dark-eyed phenotype are the product of dominant genes.
In Scandinavia, most people have light eyes, so we say then that the recessive genes for light eyes are much more frequent and common than the dominant genes for dark eye color.
Dominant alleles are no better than recessive alleles, but these may have implications on the fitness (reproductive effectiveness) of individuals.
References
- Anreiter, I., Sokolowski, HM, & Sokolowski, MB (2018). Gene – environment interplay and individual differences in behavior. Mind, Brain, and Education, 12 (4), 200-211.
- Griffiths, AJ, Miller, JH, Suzuki, DT, Lewontin, RC, & Gelbart, WM (2000). Mendel's experiments. In An Introduction to Genetic Analysis. 7th edition. WH Freeman.
- Herrera ‐ Estrella, L., De Block, M., Messens, EHJP, Hernalsteens, JP, Van Montagu, M., & Schell, J. (1983). Chimeric genes as dominant selectable markers in plant cells. The EMBO journal, 2 (6), 987-995.
- Mendel, G. (2015). Experiments in a monastery garden. American Zoologist, 26 (3), 749-752.
- Nakagawa, Y., & Yanagishima, N. (1981). Recessive and dominant genes controlling inducible sexual agglutinability in Saccharomyces cerevisiae. Molecular and General Genetics MGG, 183 (3), 459-462