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Diagnosis of multifactorial diseases
Last reviewed: 05.07.2025
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Many of the phenotypic traits of a person are controlled by a large number of genes. Each of these genes acts independently of the others. The probability that an individual will receive many genes acting in the same direction is small. Environmental factors make a certain contribution to the normal distribution of genes. In most cases, the variability of phenotypic traits in a population reflects the combined action of a set of genes and environmental factors. It has long been known about the existence of a "family" predisposition to many common diseases, such as atherosclerosis, coronary heart disease, diabetes mellitus, malignant tumors, bronchial asthma, peptic ulcer, arterial hypertension, etc., but their genetic component is not inherited in accordance with Mendel's laws. These diseases develop as a result of the interaction of a number of genes with numerous environmental factors. This type of inheritance is called multifactorial.
Multifactorial genetic diseases always have a polygenic component, consisting of a sequence of genes that interact cumulatively with each other. An individual who has inherited the appropriate combination of these genes crosses a “risk threshold,” and from that point on, the environmental component determines whether the person will develop the disease and how severe it will be.
Variability of hereditary predisposition to diseases is due to the phenomenon of genetic polymorphism. Genes that are represented in the population by several varieties - alleles - are called polymorphic. Differences between alleles of the same gene, as a rule, consist of minor variations in its genetic code, and the latter may or may not be reflected at the phenotypic level (up to clinical manifestations). With an unfavorable combination of certain alleles, the risk of developing various diseases may increase. These associations can be either direct, if allelic polymorphism affects the function of the gene, or have a "marker" nature, that is, manifest as a result of the linkage of any allele with an unfavorable variant of the true "disease gene".
Polymorphism of nucleotide sequences is found in all structural elements of the genome: exons, introns, regulatory regions, etc. Variations that directly affect coding fragments of the gene (exons) and affect the amino acid sequence of their products are observed relatively rarely. Most cases of polymorphism are expressed either in the replacement of one nucleotide or in the variation in the number of repeating fragments.
It should be noted that at present, data on the relationship between diseases and certain genetic markers for multifactorial diseases are very contradictory.
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