Genetic polymorphism: variation and clinical significance

The human genome and the development of genetic research

The international Human Genome Project, completed in 2003, produced a high-quality reference sequence covering most of the genome. It was the work of many research groups, rather than a complete interpretation of all DNA functions under one person’s leadership. Difficult regions remained unfinished and were investigated later.

Humans have approximately 20,000 protein-coding genes, alongside noncoding RNA genes and numerous regulatory regions. Genes contribute to development and function but do not independently determine every disease, physical feature or behavior. Interactions between genes, environment and life experience matter.

Advances in technology allow DNA variants to be detected, some inherited diseases to be diagnosed and certain risks to be assessed. However, the capabilities of a particular test depend on the variants it detects and the evidence for their clinical significance.

What polymorphism means

Alleles are sequence versions at the same genomic location. Genetic polymorphism means that different variants occur sufficiently often in a population. A frequency of at least 1% has historically been a common threshold; it is a conventional criterion that depends on the definition and population studied. Frequency does not automatically determine whether a variant is beneficial, harmful or neutral.

A single-nucleotide variant (SNV) is a difference at one nucleotide; a common form is called a single-nucleotide polymorphism (SNP). Other forms of variation include insertions and deletions (indels), differences in repeat number, copy-number changes and larger structural rearrangements.

Changes occur in coding and noncoding regions. Exons remain in mature RNA, but not every exon region encodes protein. Introns are usually removed during splicing but can contain regulatory information; they are not necessarily irrelevant. Promoters and other regulatory elements influence gene expression.

How variation arises

New variants arise through DNA changes. Heritable changes may pass between generations, with frequencies shaped by selection, genetic drift, migration and population history. No single variant frequency applies to everyone, and a common allele is not necessarily “normal” or a rare one “disease-causing.”

MTHFR C677T, for example, is a common variant whose frequency differs between populations. Its presence is not equivalent to severe inherited MTHFR deficiency. Percentages from a particular sample cannot be applied to populations in Europe, the United States or Armenia without specifying the sample and methods.

Human genomes are highly similar but differ at many locations. The exact number of known variants depends on the database, technology, population sample and inclusion of structural variation. The outdated fixed figure of “12 million European polymorphisms” does not represent current knowledge. Most variants do not cause disease; some influence traits or gene function.

Single-gene disease and susceptibility

In some single-gene disorders, pathogenic variants in a particular gene have substantial diagnostic value. Examples include cystic fibrosis, phenylketonuria, some muscular dystrophies and familial Mediterranean fever. Such variants may be single-nucleotide or structural changes; they are not all common, and not every disorder presents at birth. Interpretation considers inheritance, the combination of variants and clinical findings.

Other variants increase disease probability without making disease inevitable. Pathogenic BRCA1/BRCA2 variants, for example, confer substantial susceptibility to certain cancers. This differs from small research associations between an ordinary SNP and hypertension, cardiovascular disease, hair loss or pregnancy complications.

Many common diseases are multifactorial, involving numerous variants, age, diet, smoking, accompanying diseases and other circumstances. “Polygenic” describes contributions from many genes, while “multifactorial” describes genetic and nongenetic causes together; the terms are related but not identical.

Limits of prediction

Intelligence, temperament, creativity and athletic ability are complex traits with genetic and environmental components. A research association does not mean an ordinary genetic panel can reliably predict an individual’s IQ, work capacity, personality or suitable sport. Such promises should not be presented as established diagnostic capabilities.

Medical use requires demonstrated analytical accuracy and usefulness in the particular setting. Even polygenic risk estimates require validation for the relevant population and purpose. A positive variant does not always mean disease, and a negative limited panel does not exclude every cause. Surveillance, prevention and treatment are chosen from the overall evidence, with genetic counseling when needed, rather than from one polymorphism.