POLYGENIA: WHAT IT CONSISTS OF AND EXAMPLES - BIOLOGY - 2025

The polygeny is an inheritance pattern in which multiple genes involved in determining a single phenotypic characteristic. In these cases, it is difficult to distinguish the participation and effect of each gene separately.

This mode of inheritance is applicable to most of the complex traits that we observe in the phenotype of humans and other animals. In these cases, inheritance cannot be studied from the “simplified and discrete” point of view described by Mendel's laws, since we are dealing with a multifactorial modality.

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The opposite concept to polygeny is pleiotropy, where the action of one gene affects multiple characteristics. This phenomenon is common. For example, there is an allele that when present in a homozygous recessive condition causes blue eyes, fair skin, mental retardation, and a medical condition called phenylketonuria.

Also, the term polygeny should not be confused with polygyny. The latter derives from the Greek roots that it literally translates as "several women or wives" and describes the pattern of mating choice where males copulate with several females. The concept also applies to human societies.

What is polygeny?

We say that inheritance is of the polygenic type when a phenotypic characteristic is the result of the joint action of multiple genes. A gene is a region of genetic material that encodes a functional unit, be it a protein or an RNA.

Although it is possible to detect a single gene involved in a certain trait, it is very likely to detect the "modifying" influence of other genes as well.

Discrete and continuous features

When we refer to traits that are inherited following the Mendelian proportions, we say that they are discrete or discontinuous characteristics since the phenotypes do not overlap and we can classify them into well-defined categories. A classic example is the color of peas: green or yellow. There are no intermediates.

However, there are traits that exhibit a wide range of phenotype expressions, in the form of degraded series.

As we will see later, one of the most cited examples of this inheritance pattern in humans is skin color. We are aware that there are no two colors: black and white - this would be a discreet feature. There are multiple shades and variations in colors, since they are controlled by several genes.

Variable expressiveness and incomplete penetrant

For some traits, individuals with the same genotype may have different phenotypes, even for traits controlled by a single gene. In the case of individuals with some genetic pathology, each one may have unique symptoms - more severe or milder. This is variable expressiveness.

Incomplete penetrance, on the other hand, refers to organisms with an identical genotype but which may or may not develop the condition associated with said genotype. In the case of genetic pathology, individuals may have the symptoms or never develop the disorder.

The explanation for these two phenomena are the action of the environment and the influence of other genes that can suppress or accentuate the effect.

Environmental action

Normally, phenotypic characteristics are not only influenced by genes - be it one or several. They are also modified by the environment that surrounds the organism in question.

There is a concept called "reaction norm", where a single genotype in interaction with its environment is capable of generating a different range of phenotypes. In this situation, the final product (the phenotype) will be the result of the interaction of the genotype with environmental conditions.

When a continuous trait falls into the category of polygenic and is also influenced by environmental factors, the trait is called multifactorial - since there are several factors that contribute to the phenotype.

Examples

Eye color in humans

Generally, it is quite difficult to attribute a particular phenotypic characteristic to a single gene.

For example, when we evaluate a couple where he has green eyes and she has brown eyes, we try to predict the likely eye color of the offspring. Furthermore, we may try to apply Mendelian concepts to solve this question.

We would use the concepts of dominant and recessive gene within our prediction and we would surely conclude that the child has a high probability of presenting brown eyes.

Our prediction may be correct. However, our reasoning is an oversimplification of what is happening in the cell, as this trait is of polygenic inheritance.

Although it may seem complex, each allele (variants or forms in which a gene can occur) at each locus (physical location of the gene on the chromosome) follows Mendel's principles. However, since several genes participate, we cannot observe the characteristic Mendelian proportions.

It is worth mentioning that there are traits in humans that follow the traditional Mendelian heritage, such as blood groups.

Skin color in humans

We are witnesses of the multiple skin tones that our species exhibits. One of the determining factors of skin color is the amount of melanin. Melanin is a pigment that is produced by skin cells. Its main function is protective.

Melanin production depends on different loci and some have already been identified. Each locus can possess at least two codominant alleles. Thus, there will be multiple loci and alleles involved, so there will be numerous ways in which alleles can be combined, influencing skin color.

If a person inherits 11 alleles that code for maximum pigmentation and only one that codes for low melanin production, their skin will be quite dark. Similarly, an individual who inherits the majority of alleles related to low melanin production will have a fair complexion.

This occurs because this polygenic system has an additive effect on the gene products involved in heredity. Each allele that codes for low melanin production will contribute to fair skin.

Furthermore, a well conserved gene with two alleles that contribute disproportionately to pigmentation has been demonstrated.

References

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