Genetics of DNA Paternity Testing

DNA paternity testing makes it possible to, with a very high degree of certainty, determine the identity of a child's biological father. Here's how.

What Is DNA?

Everyone, except identical twins, has a unique set of genetic instructions made of DNA (deoxyribonucleic acid). DNA is a nucleic acid polymer comprised of smaller molecules called nucleotide monomers. Individual nucleotides consist of three subunits:

• a pentose sugar
• one or more phosphate groups
• a cyclic nitrogenous base

Nucleotide Bases

The nucleotides of DNA each contain one of four possible nitrogenous bases:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

The specific base is what makes nucleotides differ from one another, and when nucleotides exist together in a nucleic acid, such as the DNA of our genome, the sequence of these bases is what makes each of us unique.

The Genetic Code

These four bases, specifically the combination in which they exist in along the DNA molecule, form the 'genetic code.' The base sequence within each person's DNA, occurs in a different order of bases for all people other than identical twins.

On the whole, human DNA is very similar from one individual to another (the human genome is 99.9% of the same thing), but the occasional differences in the order of bases is what make each of us unique. On average, any two people at random have a different base every thousand bases or so.

Sexual Reproduction & DNA

We each get half our DNA from our father and half from our mother. Since this DNA has been passed down through the generations of our family, an individual's DNA is more similar to that of his or her grandparents, cousins, aunts, uncles, and, most of all, parents, than it is to the DNA of a random stranger. Paternity tests use this similarity to figure out who the father of a child is.

How Is DNA Used to Establish Paternity?

When samples of DNA are taken from a baby, mother and possible father, each is shipped to a lab. At the laboratory, each sample is exposed to restriction enzymes; proteins which cut DNA into specific lengths according to the distinctive sequence of base pairs. This results in each individual's restriction fragments being a unique size...unique like a fingerprint.

After being cut up, the restriction fragments of each genome (mother, father and baby's) are separated using a technique called gel electrophoresis. Since people who are closely related have similar DNA, the DNA of more closely related individuals will show more similar length fragments of DNA.

Gel electrophoresis enables the lab technician to see which DNA fragments of the mother are shared by the child and, more importantly, if some of the child's DNA fragments are the same size as fragments found in the supposed father's DNA sample.

Sources

US Department of Energy, Human Genome Project. (2008) Gene Testing.

US Department of Health and Human Services. Understanding Gene Testing.