Diagnosis of multifactorial diseases
Many of the phenotypic signs 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 one direction is not large. External factors contribute to the normal distribution of genes. In most cases, the variability of phenotypic traits in a population reflects the joint action of a combination of genes and environmental factors. It has long been known that there are "family" predispositions to many common diseases, such as atherosclerosis, IHD, diabetes, malignant tumors, bronchial asthma, peptic ulcer, hypertension, etc., but their genetic component is not inherited according to 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 multi-factorial.
In multifactorial genetic diseases, there is always a polygenic component consisting of a sequence of genes cumulatively interacting with each other. The individual who inherits the corresponding combination of these genes passes the "threshold of risk", and from that moment the environmental component already determines whether the person will develop the disease and how much it will be expressed.
The variability of the genetic predisposition to diseases is due to the phenomenon of genetic polymorphism. Polymorphic genes are called, which are represented in the population by several varieties - alleles. Differences between alleles of the same gene, as a rule, consist in insignificant variations of its genetic code, and the latter can both be not reflected, and reflected at the phenotypic level( up to the clinical manifestations).If an unfavorable combination of certain alleles can increase the risk of various diseases. These associations can be either direct, if the allelic polymorphism affects the function of the gene, and have a "marker" nature, that is, manifest as a result of the coupling of an allele with an unfavorable version of the true "disease gene".
Polymorphism of nucleotide sequences is found in all structural elements of the genome: exons, introns, regulatory sites, etc. Variations affecting directly the coding fragments of the gene( exons) and reflecting on the amino acid sequence of their products are relatively rare. Most cases of polymorphism are expressed either in substitutions of one nucleotide, or in variations in the number of repeating fragments.
It should be noted that at present, the data on the relationship of diseases with certain genetic markers for multifactorial diseases are highly contradictory.