Polygenic inheritance

Essence of polymeric interaction

Non-allelic genes can affect completely different traits as well as a single shared trait. Polymery (or polymeric gene action) refers to the second option. This is a genetic mechanism in which several non-allelic genes influence a single trait, gradually enhancing its manifestation. As a result, quantitative traits are formed that can vary smoothly over a wide range.

Place of polymery among other interactions

In addition to polymery, genetics distinguishes other types of non-allelic gene interaction:

• Complementary interaction: genes complement each other. For example, the inheritance of hearing in humans is determined by two dominant genes from different pairs (one determines the development of the auditory nerve, the other the cochlea). Retinoblastoma and nephroblastoma are also encoded by two pairs of non-allelic genes.
• Epistasis: a gene from one allelic pair (inhibitor I or suppressor Su) suppresses the action of a gene from another pair. A striking example of recessive epistasis is the Bombay phenotype (the recessive homozygote hh does not form substance H, preventing the synthesis of antigens A and B, resulting in blood group O). An example of dominant epistasis is enzymopathies (phenylketonuria, galactosemia).
• Position effect: the interaction is due to the physical location of the gene in the genotype. For example, the inheritance of the Rhesus factor depends on the arrangement of genes C, D, and E in homologous chromosomes.
• Regulatory interactions: carried out via specific and general transcription factors (TF). In the human genome, 10% of genes encode TF. General factors bind to the promoter's TATA box, while specific factors bind to enhancers, form a DNA loop, and interact with RNA polymerase II to ensure uniqueness of expression.

Inheritance of skin color

A classic example of polymery is the genetic determination of skin color in humans. This trait is determined by the interaction of at least three pairs of non-allelic genes.

The final color depends on the ratio of three main pigments:

1. Melanin — provides brown color.
2. Carotene — provides yellow color.
3. Hemoglobin — provides red color.

The combination of parental genes forms the offspring's phenotype. Inheritance patterns can be traced in the table:

Examples of polymeric traits in humans

Polymeric gene action is characteristic of many vital quantitative parameters. In humans, such traits include:

• Growth
• Hair color
• Skin color
• Blood pressure levels
• Cognitive abilities
• Muscle strength

Polymery and evolutionary factors

Polymery is closely related to evolutionary processes. According to Ronald Fisher's theory (1930), the external manifestation of any gene depends on its genetic background, which is shaped by polymery and pleiotropy.

The role of recombination lies in creating new gene combinations. This forms a background that allows a hidden mutation to manifest in the phenotype, after which it becomes subject to natural selection. Selection eliminates combinations that hinder survival and reproduction, but preserves neutral and advantageous ones.

In this context, it is important to distinguish between organisms:

• Mutants — individuals in which the mutation has already manifested phenotypically.
• Mutation carriers — heterozygotes for a recessive mutant allele. In them, the mutation does not manifest and is not subjected to selection.

There are also selectively neutral mutations, which are not significantly affected by selection. Evolutionary models always consider only germline mutations, while genetic mosaics are not taken into account.

Class and exam questions

What else to study on this topic

• Complementary gene interaction: inheritance of hearing, retinoblastoma, and nephroblastoma.
• Epistasis: molecular mechanisms of the Bombay phenotype and enzymopathies.
• Position effect: features of Rhesus factor inheritance and C, D, E antigens.
• Transcription factors: the role of general and specific factors in regulating gene expression.
• Elementary evolutionary factors: Fisher's theory, mutation types, and carrier status.

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.