WHAT IS POLYGENIC INHERITANCE? (WITH EXAMPLES) - BIOLOGY - 2025

The polygenic inheritance is the transmission of characters whose manifestation depends on several genes. In monogenic inheritance, a trait manifests itself from the expression of a single gene; in the dignified, two. In polygenic inheritance we generally speak of the participation of two, if not three, or more genes.

In reality, very few characters depend on the manifestation of just one gene or two genes. However, the simplicity of the analysis of traits that depend on few genes greatly helped Mendel's work.

Later studies by other researchers revealed that biological inheritance, in general, is a bit more complex than that.

When we talk about the inheritance of a character that depends on several genes, we say that they interact with each other to confer that character. In these interactions these genes complement or supplement each other.

One gene may do one part of the work, while others do another. The set of their actions is finally observed in the character of whose manifestation they participate.

In other heritages, each gene with similar function contributes a little each to the final manifestation of the character. In this kind of polygenic inheritance an additive effect is always observed. Furthermore, the variation in character manifestation is continuous, not discrete.

Finally, the absence of expression of a supplementary gene does not necessarily determine a phenotype loss due to absence, lack or nullity.

Examples of polygenic characters

In the simplest manifestation traits, the phenotype is all or nothing. That is, whether or not such activity, trait or characteristic is present. In other cases, there are two alternatives: green or yellow, for example.

Height

But there are other characters that manifest themselves in a broader way. For example, height. Obviously we all have stature. Depending on it, they classify us in a certain way: high or low.

But if we analyze a population well, we will realize that there is a very wide range of heights - with extremes on both sides of a normal distribution. Height depends on the manifestation of many different genes.

It also depends on other factors and that is why height is a case of polygenic and multifactorial inheritance. Because many genes are measurable and involved, the powerful tools of quantitative genetics are used for their analysis. Particularly in the analysis of quantitative trait loci (QTL, for its acronym in English).

Animal fur

Other characters that are generally polygenic include the manifestation of the color of the coat in some animals, or the shape of the fruit in plants.

In general, for any character whose manifestation shows a range of continuous variation in the population, polygenic inheritance can be suspected.

Diseases

In medicine, studying the genetic basis of diseases is very important in order to understand them and find ways to alleviate it. In polygenic epidemiology an attempt is made, for example, to determine how many different genes contribute to the manifestation of a disease.

Based on this, strategies can be proposed for detecting each gene, or for treating the deficiency of one or more of them.

Some polygenic inherited diseases in humans include asthma, schizophrenia, some autoimmune diseases, diabetes, hypertension, bipolar disorder, depression, skin color, etc.

Complementary genes

The experience and evidence accumulated over the years indicate that many genes participate in the manifestation of characters with multiple phenotypes.

In the case of complementary gene interactions between alleles of genes at different loci, these can be epistatic or non-epistatic.

Epistatic interactions

In epistatic interactions, the expression of the allele of a gene from one locus masks the expression of another from a different locus. It is the most common interaction between different genes that code for the same character.

For example, it is possible that for a character to manifest, it depends on two genes (A / a and B / b). This means that the products of genes A and B must be involved for the trait to manifest.

This is known as double dominant epistasis. In a case of recessive epistasis of a on B, on the contrary, the lack of manifestation of the trait encoded by A prevents that of the expression of B. There are a large number of different cases of epistasis.

Non-epistatic interactions between complementary genes

Depending on how they are defined, there are other interactions between genes that complement each other that are not epistatic. Take for example the definition of plumage color in birds.

It has been seen that the biosynthetic pathway that leads to the production of a pigment (eg yellow) is independent of that of another color (eg blue).

Both in the pathway of the manifestation of the color yellow and blue, which are independent of each other, the gene interactions are epistatic for each color.

However, if we consider the bird's coat color as a whole, the contribution of yellow is independent of the contribution of blue. Therefore, the manifestation of one color is not epistatic about the other.

In addition, there are other genes that determine the pattern in which the colors of skin, hair and feathers appear (or do not appear). However, the characters of color, and of the coloring pattern, complement each other in the coloration shown by the individual.

On the other hand, at least twelve different genes participate in the coloration of the skin in humans. It is easy to understand then how humans vary so much in color if we also add other non-genetic factors. For example, sun exposure (or artificial sources of "tan"), availability of vitamin D, etc.

Supplementary genes

There are cases in which the action of a gene allows the manifestation of a character to be observed to a greater degree. It is even possible that there is no gene to define a biological characteristic that is actually the sum of many independent activities.

For example, height, milk production, seed production, etc. Many activities, functions, or capabilities add up to provide such phenotypes.

These phenotypes are generally said to be the parts that account for the manifestation of a whole that reflects the performance of an individual, a lineage, an animal race, a plant variety, etc.

The action of the supplementary genes also implies the existence of a range of phenotypes defined almost always by a normal distribution. It is sometimes very difficult to separate or distinguish the complementary from the supplementary effect of a gene in complex phenotypes.

Some examples of supplementary genes

Action and reaction to certain drugs, for example, have been shown to depend on the activity of many different genes.

Generally, these genes also have many alleles in the population, which is why the diversity of responses increases. A similar case occurs in other cases in which one person gains weight when consuming the same food, compared to which another does not experience significant changes.

Finally, it should be added that in addition to the additive effects that some genes have, there are those that suppress the manifestation of others.

In these cases, a gene unrelated to the manifestation of another can lead to the inactivation of the first by both genetic and epigenetic interactions.

References

1. Delmore, KE, Toews, DP, Germain, RR, Owens, GL, Irwin, DE (2016) The genetics of seasonal migration and plumage color. Current Biology, 26: 2167-2173.
2. Dudbridge, F. (2016) Polygenic epidemiology. Genetic Epidemiology, 4: 268-272.
3. Quillen, EE, Norton, HL, Parra, EJ, Lona-Durazo, F., Ang, KC, Illiescu, FM, Pearson, LN, Shriver, MD, Lasisi, T., Gokcumen, O., Starr, I., Lin., YL, Martin, AR, Jablonski, N. G. (2018) Shades of complexity: New perspectives on the evolution and genetic architecture of human skin. American Journal of Physical Anthropology, doi: 10.1002 / ajpa.23737.
4. Maurer, MJ, Sutardja, L., Pinel, D., Bauer, S., Muehlbauer, AL, Ames, TD, Skerker, JM, Arkin, AP (2017) Quantitative Trait Loci (QTL) -guided metabolic engineering of a complex trait. ACS Synthetic Biology, 6: 566-581.
5. Sasaki, A., Ashikari, M., Ueguchi-Tanaka, M., Itoh, H., Nishimura, A., Swapan, D.,
6. Tomita, M., Ishii, K. (2017) Genetic performance of the semidwarfing allele sd1 derived from a Japonica rice cultivar and minimum requirements to detect its single-nucleotide polymorphism by miSeq whole-genome Ssequencing. BioMed Research International.