DNA (Deoxyribonucleic Acid) | DNA Testing Glossary

DNA (deoxyribonucleic acid) is the molecule inside nearly every cell of your body that carries your complete genetic instructions. It has a double-helix shape — two strands wound around each other — and is made up of four chemical bases (adenine, thymine, guanine, and cytosine) arranged in pairs. The specific sequence of those base pairs determines everything from eye color to blood type. In DNA relationship testing, laboratories compare specific regions of DNA between two or more people to determine whether they share a biological relationship such as parent and child or siblings.

How DNA Is Structured

Each DNA molecule is a long chain of chemical units called nucleotides. Every nucleotide has three parts: a sugar (deoxyribose), a phosphate group, and one of four bases — adenine (A), thymine (T), guanine (G), or cytosine (C). Two chains twist around each other to form the double helix. Adenine always pairs with thymine and guanine always pairs with cytosine, which is why one strand can be used to reconstruct the other.

The human genome contains about 3 billion base pairs, organized into 23 pairs of chromosomes inside each cell nucleus. Twenty-two of those pairs are called autosomes and are the ones used in most relationship testing. The 23rd pair is the sex chromosomes (XX in most females, XY in most males).

How DNA Is Inherited

You inherit half of your DNA from your biological mother and half from your biological father. Each parent contributes one chromosome to every autosomal pair. That inheritance pattern is the foundation of relationship testing: if a man is a child's biological father, then for every location on the child's chromosomes tested, one of the two versions the child carries must match a version the alleged father carries. When that expected match is missing at multiple locations, the man is excluded as the biological father.

Because inheritance is random within each parent's pool of DNA, siblings share about 50 percent of their DNA on average but not the same 50 percent. That is why a sibling DNA test uses probability rather than a simple yes/no answer and is inherently less definitive than a direct paternity test.

Where DNA Lives in Your Body

Almost every cell in your body contains a full copy of your DNA. Red blood cells are the main exception — they lose their nucleus during development and carry no DNA. In relationship testing, the sample of choice is usually cells from the inside of the cheek because they are easy to collect with a swab, are rich in DNA, and do not require a needle. Blood, saliva, hair with the follicle attached, and other tissues can also work, but a buccal (cheek) swab is the standard for at-home kits.

There are two types of DNA inside most cells: nuclear DNA (the 23 chromosome pairs in the nucleus) and mitochondrial DNA (a small circular molecule inherited only from the mother). Standard paternity and kinship testing uses nuclear DNA. Mitochondrial DNA is used mainly in maternal-line ancestry research and specialized forensic cases and is not part of everyday relationship testing.

How DNA Is Used in Relationship Testing

Laboratories do not sequence your entire genome for a relationship test. Instead, they examine short, highly variable regions called short tandem repeats (STRs) at specific locations on the chromosomes. Because these regions vary so much from person to person, a small number of them is enough to identify or rule out biological relationships with a high degree of certainty.

USDC's home paternity test can analyze up to 28 genetic markers. The standard panel is 23 autosomal STR markers, which produces probabilities of paternity at or above 99.9999 percent for true inclusions and unambiguous 0 percent exclusions for true exclusions. For kinship tests such as grandparent, aunt/uncle, and sibling, the lab can add up to 5 more markers when the initial result falls in the inconclusive range, reaching up to 28 unique autosomal markers total.

For the full step-by-step of how the lab processes a sample and calculates a result, see Understanding DNA Testing.

What DNA Can and Cannot Reveal in a Relationship Test

What it can reveal: whether two people share the biological relationship in question (parent/child, siblings, grandparent/grandchild, aunt or uncle/niece or nephew, or identical vs. fraternal twins), and how strong the statistical support for that relationship is.

What it cannot reveal in a standard relationship test: ancestry breakdown or ethnicity estimates, disease risk or carrier status, personality or behavior traits, or non-paternity events beyond the pair being tested. A paternity test tells you about the two people whose DNA is in the tube — nothing else.

Common Questions About DNA

Does your DNA change over time? No. The DNA in your cells stays the same from conception to death. Small errors called mutations can accumulate in individual cells over a lifetime, but the DNA used for relationship testing does not change.

Can two unrelated people have the same DNA? Only identical twins share the same nuclear DNA. Any two unrelated people will differ at many locations, which is why a small STR panel is enough to distinguish them.

Can food, medication, or a recent blood transfusion affect a DNA test? Diet and medications do not change your DNA. A recent blood transfusion or a bone marrow transplant can temporarily introduce donor DNA into blood samples, which is one reason cheek swabs are preferred over blood for at-home kits.

Is the DNA in a cheek swab the same as the DNA in blood? Yes. Every cell with a nucleus carries the same complete copy of your DNA. Cheek cells are just easier and less invasive to collect.

Source

National Human Genome Research Institute — DNA Fact Sheet

Account

Your cart is empty