Genetic Testing Explained: Types, Uses, and What Results Mean

Types of Genetic Tests

Diagnostic testing confirms or rules out a suspected genetic condition in a person who already shows symptoms. If a child presents with symptoms consistent with cystic fibrosis, for example, diagnostic testing analyzes the CFTR gene for known disease-causing mutations. A positive result confirms the diagnosis and can guide treatment planning.

Predictive and presymptomatic testing identifies mutations in people who have no current symptoms but may develop a condition later in life. BRCA1 and BRCA2 testing identifies women at high risk for breast and ovarian cancer. Huntington disease testing can reveal whether a person carries the expanded CAG repeat that inevitably causes the disease. These tests carry significant psychological implications and are typically accompanied by genetic counseling.

Carrier testing identifies individuals who carry one copy of a recessive disease allele without being affected themselves. This is particularly relevant for family planning: if both parents are carriers of the same recessive condition, each pregnancy carries a 25 percent chance of producing an affected child. Population-based carrier screening panels now test for dozens to hundreds of recessive conditions simultaneously.

Pharmacogenomic testing determines how a person genetic makeup affects their response to specific medications. Variants in genes encoding drug-metabolizing enzymes (such as CYP2D6 and CYP2C19) can make standard drug doses either ineffective or dangerously toxic. Testing before prescribing allows doctors to select the right drug and dose from the start, avoiding adverse reactions and treatment failures.

Prenatal and newborn screening tests for genetic conditions in fetuses and infants. Non-invasive prenatal testing (NIPT) analyzes fetal DNA circulating in maternal blood to screen for chromosomal abnormalities like Down syndrome with high accuracy and no risk to the pregnancy. Newborn screening programs test blood spots from virtually all newborns for dozens of treatable genetic conditions, enabling early intervention before symptoms develop.

How Genetic Tests Work

Most genetic tests require only a blood sample or cheek swab to obtain DNA. The specific laboratory method depends on what type of change is being sought. Targeted genotyping looks for known specific mutations at defined positions. Gene sequencing reads the entire coding region of one or more genes to find any mutations, including novel ones not previously reported. Chromosomal microarray detects deletions and duplications too small to see under a microscope.

Whole-exome sequencing reads the protein-coding portions of all 20,000+ genes simultaneously (about 1.5 percent of the genome). Whole-genome sequencing reads the entire genome including non-coding regions. These comprehensive approaches are particularly useful when the genetic cause of a condition is unknown, as they can identify unexpected mutations that targeted tests would miss.

Direct-to-consumer genetic testing, offered by companies like 23andMe and AncestryDNA, typically uses SNP genotyping chips that test hundreds of thousands to millions of genetic variants simultaneously. These tests provide information about ancestry composition, certain health predispositions, carrier status for some conditions, and pharmacogenomic variants. However, they are less comprehensive than clinical genetic testing and may miss important variants.

Interpreting Results

Genetic test results are not always straightforward. A positive result confirms the presence of a mutation but may not predict exactly when or how severely a disease will manifest. Many genetic conditions show variable expressivity (different severity in different people with the same mutation) and incomplete penetrance (not everyone with the mutation develops the condition). Environmental factors, other genes, and chance all influence outcomes.

Variants of uncertain significance (VUS) are genetic changes whose impact on health is currently unknown. As databases grow and more research is conducted, many VUS are eventually reclassified as either benign or pathogenic. In the meantime, VUS results can cause anxiety and confusion, which is why genetic counseling is recommended both before and after testing.

A negative result does not always guarantee that a person is unaffected or not at risk. Negative results mean that the specific mutations tested for were not found, but other untested mutations in the same gene, or mutations in different genes entirely, could still be present. The sensitivity of a test (its ability to detect all relevant mutations) varies depending on the technology used and the condition being tested.

Genetic Counseling

Genetic counselors are healthcare professionals trained to help individuals understand and adapt to the medical, psychological, and familial implications of genetic test results. They help patients understand what a test can and cannot reveal, interpret complex results, assess risk for family members, and make informed decisions about screening, prevention, and treatment options.

Pre-test counseling helps patients understand the potential outcomes of testing, including the possibility of finding unexpected results (such as non-paternity or unexpected ancestry) or variants of uncertain significance. Post-test counseling helps patients understand their results in context, connect with appropriate specialists, and communicate risk information to family members who might benefit from their own testing.

Privacy and Ethical Considerations

Genetic information is uniquely sensitive because it reveals information not only about the tested individual but also about their biological relatives who did not consent to testing. In the United States, the Genetic Information Nondiscrimination Act (GINA) prohibits health insurers and employers from discriminating based on genetic information, but does not cover life insurance, disability insurance, or long-term care insurance.

Law enforcement use of genetic genealogy databases (matching crime scene DNA to distant relatives in consumer databases to identify suspects) has raised privacy concerns. Several jurisdictions now require warrants for law enforcement access to genetic databases. The balance between public safety and genetic privacy remains an active policy debate.