Deoxyribonucleic acid, or DNA, is often called the blueprint of life, a complex molecule containing the complete set of instructions for building and operating a human being. This genetic code is organized into approximately 20,000 to 25,000 genes within the nucleus of nearly every cell. A DNA sample holds a unique record of an individual’s identity, biology, and history. Because this information is passed down through generations, the genome is also a shared family history.
Physical Characteristics and Ancestral Heritage
DNA testing provides detailed insights into an individual’s physical appearance and geographic origins by analyzing specific points of variation in the genome. These variations, known as single nucleotide polymorphisms (SNPs), are single-letter differences in the DNA sequence that occur across the population. Scientists use panels of these SNPs to predict externally visible characteristics. For instance, predicting traits like eye color, hair color, and skin tone relies on analyzing multiple SNPs located in genes that control melanin pigment production. While DNA can accurately predict traits like blue eyes, the prediction of complex traits like height is less precise, as it is influenced by hundreds of genetic variants alongside environmental factors. Specific sets of ancestry-informative SNPs (AIMs) are used to map a person’s lineage back to distinct geographic populations and ethnic groups. By comparing an individual’s genetic markers to global reference populations, testing companies estimate the percentage of DNA inherited from ancestors in different world regions.
Disease Risk and Health Predispositions
The analysis of a person’s genome can reveal extensive information about their current health and future disease susceptibility. This medical data covers single-gene disorders, complex polygenic conditions, and drug metabolism. Direct-to-consumer tests often screen for carrier status for specific single-gene disorders, such as a variant of the CFTR gene linked to cystic fibrosis. They may also test for pathogenic variants in high-risk genes, like the BRCA1 and BRCA2 genes associated with an increased lifetime risk for breast and ovarian cancers.
For common conditions like Type 2 diabetes or late-onset Alzheimer’s disease, a polygenic risk score (PRS) is calculated. This score aggregates the risk contributions from thousands of common genetic variants to estimate an individual’s overall genetic predisposition. A PRS is a measure of genetic risk, not a diagnosis, and disease onset is heavily influenced by lifestyle and environment.
Another application is pharmacogenetics, which analyzes variants in genes like CYP2D6 and CYP2C19 that code for liver enzymes responsible for breaking down most prescription drugs. Knowing a person’s genetic profile helps physicians determine the most effective dosage for medications like antidepressants or warfarin, minimizing the risk of adverse drug reactions or treatment failure.
Complex Traits and Behavioral Indicators
Beyond health and ancestry, DNA analysis explores the genetic underpinnings of abstract and complex human traits. Traits such as intelligence, memory function, and longevity are highly polygenic, meaning they are influenced by thousands of genes, each contributing a very small effect. While studies show that inherited DNA differences account for a substantial portion of the variance in intelligence scores, current science can only identify a small fraction of the specific genetic variants involved.
Genetic research has also identified links between certain genetic variants and the “Big Five” personality traits, such as neuroticism or conscientiousness. These associations are probabilistic, suggesting a tendency rather than a deterministic outcome, and they are heavily moderated by environmental factors and personal experience. Research has found significant genetic overlap between these personality traits and certain mental health conditions, like the connection between neuroticism and a predisposition for depression.
External Use and Privacy Implications
The highly personal nature of genetic data raises serious concerns about data ownership and privacy once a sample is submitted for testing. One primary risk involves law enforcement accessing public genealogy databases to identify suspects through familial searching. By identifying distant relatives in the database, investigators can narrow down a search to a specific family line, which has led to the solving of cold cases but bypasses the need for a warrant on the individual.
Furthermore, the data is susceptible to breaches and misuse by third parties, depending on the terms of service agreed to with the testing company. In the United States, the Genetic Information Nondiscrimination Act (GINA) prohibits the use of genetic information in health insurance and employment decisions. However, GINA has significant limitations, as it does not prevent discrimination in other areas, such as the underwriting of life insurance, disability insurance, or long-term care insurance. These policies may legally ask for genetic test results as part of the application process, allowing the information to be used to set rates or deny coverage.