Why This Glossary Exists
Conversations about genetic health — with a doctor, a genetic counselor, or even a consumer DNA report — arrive packed with unfamiliar words. Terms like penetrance or polygenic risk score can make an otherwise clear discussion feel opaque. This reference is designed to close that gap.
The definitions below are organized to build on one another, starting with foundational biology and moving toward the risk language you are most likely to encounter in a clinical or screening context. For a broader look at how these concepts connect, see our end-to-end resource on genetics and chronic disease.
This article is for general informational and educational purposes only. It is not a substitute for professional medical or genetic counseling advice.
Gene
A segment of DNA that carries instructions for making a protein or regulating a biological process. Humans have roughly 20,000–25,000 protein-coding genes, though this figure continues to be refined by ongoing research.
Allele
One of two or more versions of the same gene. You inherit one allele from each biological parent, so every gene position holds a pair. Which alleles you carry influences traits and, in some cases, disease susceptibility.
Variant
A difference in a DNA sequence compared with a reference sequence. Most variants are harmless; some are linked to increased or decreased disease risk. The older term 'mutation' is still used clinically but carries misleading connotations for everyday communication.
Genotype
The specific genetic makeup of an individual at one or more gene positions. A genotype describes which alleles are present, whereas phenotype describes the observable trait or health outcome those alleles may contribute to.
Phenotype
An observable characteristic — such as eye color, height, or the presence of a disease — that results from the interaction of genes and environment. The same genotype can produce different phenotypes depending on lifestyle, age, and other factors.
Dominant vs. Recessive
A dominant variant causes an effect when only one copy is present; a recessive variant typically requires two copies (one from each parent) to produce an effect. Many serious hereditary conditions follow one of these two inheritance patterns.
Penetrance
The proportion of people carrying a specific variant who actually develop the associated condition. High penetrance means most carriers show the trait; low penetrance means many carriers never do, often because other genetic or environmental factors intervene.
Polygenic Risk Score
A single number summarizing the combined effect of many small genetic variants across the genome on a person's likelihood of developing a particular condition, such as heart disease or type 2 diabetes. It represents a statistical estimate of relative risk, not a diagnosis.
Hereditary vs. Familial
'Hereditary' refers to conditions caused by variants that can be passed from parent to child through genes. 'Familial' describes conditions that occur more often in a family than expected by chance, which may reflect shared genes, shared environment, or both.
Carrier
A person who has one copy of a recessive disease-causing variant but typically does not show symptoms themselves. Carriers can pass the variant to children, who may develop the condition if they inherit a matching variant from the other parent.
Epigenetics
Changes in how genes are expressed — turned on or off — without altering the underlying DNA sequence. Factors such as diet, stress, and age can influence epigenetic patterns, affecting health in ways that are distinct from inherited DNA variants.
Genetic Counseling
A professional consultation in which a trained counselor helps individuals and families understand the medical, psychological, and familial implications of genetic information, including test results and inheritance risks.
Key Terms at a Glance
The quick-reference card below highlights the most-encountered data points in genetic health screening. These figures provide context for the definitions above and reflect broadly reported findings in human genetics research.
| Human protein-coding genes (estimated) | ~20,000–25,000 (Ensembl Human Genome Browser, ongoing updates) |
| DNA shared between any two humans | >99.9% (National Human Genome Research Institute) |
| Variants typically summarized in a polygenic risk score | Thousands to millions (Nature Reviews Genetics, general literature) |
| Proportion of common diseases influenced by multiple genes | Most (NIH National Human Genome Research Institute) |
| Recommended professional for interpreting genetic test results | Board-certified genetic counselor or physician (American College of Medical Genetics and Genomics) |
Understanding what these numbers mean in your own situation requires professional interpretation. A genetic counselor or your primary care physician can help translate a specific test result into a personally meaningful picture. For guidance on common pitfalls in reading genetic results, see where people often go wrong when reading genetic risk.
If you are exploring conditions that tend to cluster in families, the conditions that run in families reference guide pairs well with this glossary. And if you want to understand what genes can and cannot predict, what your genes actually tell you about disease risk is a logical next read.
Genetic Risk Is Not Genetic Destiny
A variant linked to higher disease risk raises the probability of a condition — it does not guarantee it. Lifestyle factors, healthcare access, and other genes all shape actual outcomes. Polygenic risk scores and similar tools are population-level estimates; they are most useful when reviewed alongside personal and family history with a qualified healthcare provider.
