What Is a Paternity Test?

Before we were able to analyze DNA, blood tests were used to assess paternity. However, blood testing is limited, in that it can only rule out a possible father, not prove paternal identity.

Today, genetic technology is used, and DNA paternity testing makes it possible to, with a very high degree of certainty, determine the identity of a child’s biological father.

The Genetic Code

Everyone, except identical twins, has a unique set of genetic instructions made of DNA (deoxyribonucleic acid). DNA is a nucleic acid made of smaller molecules called nucleotide monomers, and each nucleotide is made of the same general ingredients:

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

The specific nitrogenous base is what makes one nucleotide differ from another, and nucleotides of DNA each contain one of four possible nitrogenous bases:

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

These four bases, particularly the combination in which they exist in along the DNA molecule, form the ‘genetic code’.

Restriction Fragment Analysis

Also called DNA fingerprinting, this type of genetic testing compares segments of DNA termed restriction fragment length polymorphisms (RFLPs). By using special restriction enzymes, the DNA molecule is cut at base sequences, also known as recognition sequences or restriction sites.

There are many different kinds of restriction enzymes. Each type will only cut the DNA when it encounters a specific combination of bases; A, G, T, and C. Different restriction enzymes cut DNA at different places—each has a unique sequence it recognizes. For example, the restriction enzyme named EcoRI cuts DNA when it encounters the sequence GAATTC and will cut only at that sequence, no other.

Gel Electrophoresis

After being cut up, the restriction fragments of each genome (mother, father and baby’s) are separated, according to length, 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.