DNA Testing and the Science Behind it
Paternity and Relationship DNA Testing Basics
Biology of DNA: The Building blocks
DNA, or Deoxyribonucleic Acid, was discovered in 1953 and is the genetic material that contains what is affectionately known as the “building blocks of all life”. The DNA in each cell of every person contains the same basic DNA code, but each person has a unique DNA sequence of code that can be used to identify them and their children. We inherit our unique DNA code from our biological parents in equal amounts. The building blocks of DNA code are made up of the four bases known as nucleotides (dNTPs): adenine, thymine, cytosine, and guanine, abbreviated as A, T, C, and G respectively. Nucleotides appear in DNA as letter pairs of AT and CG. The genetic code for humans comprises of over 3 billion of these letter pairs.
DNA nucleotides combine to create what is known as the DNA helix. This helix has the appearance of a twisted ladder or spiral staircase. Each step of this staircase is comprised of the four dNTPs (A, T, G, C) and are held together by hydrogen bonds that connect specific pairs of these molecules together (A to T and G to C). This DNA sequence spells out the instructions for all of the physical characteristics and body functions. These instructions are found in our genes.
In humans, most of our DNA takes the form of tightly coiled strands called chromosomes. Chromosomes are found inside of almost all cell nuclei. In a human cell there are 46 chromosomes arranged in 22 autosomal, the non-sex chromosome, pairs and two sex chromosomes (XY for males and XX for females). Were you to unwind each chromosome you would have a double-stranded DNA helix about 3 meters in total length. From just one tiny microscopic cell!
Extracting the DNA for testing: Breaking an egg
To simplify the idea of extracting DNA from a cell it is best to think of a cell like an egg, with the nucleus being the yolk where the DNA is stored. To get to the DNA you must break the egg and separate the yolk (DNA) from the other parts of the cell. Most labs use commonly accepted scientific methods to perform the DNA extraction along with the latest computer and robotic instruments.
PCR: The foundation for all DNA tests
Developed in the 1980s PCR, or Polymerase Chain Reaction, is a scientific technique that allows a laboratory to make millions of copies of a particular area or areas in a section of DNA. PCR harnesses and amplifies a cell’s ability to replicate DNA in a test tube or agar dish. PCR is a common tool used in medical and biological research labs to perform a number of different tests and is used for all paternity and relationship DNA testing.
Once the DNA is extracted, it is combined with chemicals to support a reaction including primers which are small pieces of synthetic DNA labeled with florescent tags that flank the region to analyze as well as the DNA building blocks (A, T, C, G) and an enzyme to support the replication of the DNA known as DNA polymerase.
The products of replication are two sets of double-stranded DNA molecules that have exactly the same sequence as the original. During cell division, each cell receives one set of DNA. In this way, all the cells of the body have the same DNA molecules. For an extensive run through of PCR and the steps involved see this handy online genome resource.
For paternity and relationship testing, laboratories use a multiplex PCR reaction that tests for multiple different locations (loci) in the DNA at once. This is accomplished using an instrument known as a thermocycler that heats and cools the DNA. Each round of heating and cooling doubles the amount of DNA at the target locations, so that after multiple cycles of heating and cooling over a million copies of the starting DNA are created.
STR Loci: accuracy in numbers
STR, or Short Tandem Repeat, loci are used in all relationship DNA analysis. This includes paternity, maternity, grandparent, and siblingship testing, etc. STR loci contain a repeating 4-letter DNA code. These repeats can vary in number and there are two copies of each STR loci. The combinations of STR alleles is unique to each person and create a DNA profile. These DNA fingerprints can be used to either include or exclude a person from a relationship when used for relationship testing.
For relationship testing a reputable laboratory will test for, examine, and report at least 23 different STR loci plus a locus that indicates sex/gender (X and Y markers). Be warned of companies who test for and report fewer than 23 STR loci, their test may be less accurate.
How to Read DNA test results
The information and patterns from the STR loci are incorporated into the results of the DNA relationship report. At every STR locus, the number of repeats detected is listed as a number. For example when a 12 is listed it means there are 12 four letter repeats present. Each of the numbers represents an allele at that location. Since each STR locus is assigned a chromosome position and chromosomes are always present in pairs, there are typically two numbers per person at each STR location. When only one number is shown at a locus, both chromosomes in the pair have the same number of repeats.
A parent and child must share a percentage of common alleles, the number at the location, to be considered a biological parent. Once the results of the 23 STR locations are evaluated, a calculation is performed to provide the numerical support for the tested biological relationship and is expressed in a percentage as the probability of relatedness.
DNA Results: Why are they written that way?
We understand that for many people this will be the first experience with any kind of DNA testing. Due to how DNA tests are used in a court of law, and even though they cannot be used in court it is present on informational DNA tests, the results are usually expressed in legal terms. Sometimes it is difficult to understand what the legal jargon at the bottom of the page of the DNA test results are actually saying. To understand them it may be helpful to look at what is calculated and what they say about the results.
Combined Paternity Index: Probability of Coincidence
The Combined Paternity Index or CPI, is a scientific algorithm used for DNA comparisons. When a given person’s DNA undergoes analysis the scientists examine the genetic Loci at very specific locations on the DNA. These loci help accurately confirm or exclude the genetic relationship between the people tested. The actual CPI of a test subject is the coincidence, or how similar, of a person’s genetic markers are in comparison with another. It is done by multiplying all 23 paternity indexes that are derived for every one of the 23 genetic loci a laboratory should test for. Using these values we can proceed to calculate the probability of paternity or relatedness. This figure expresses how many times more likely the alleged biological father is the real father of the child when compared with an untested male in the same ethnic group.
Probability of Paternity: the Results in simpler terms
The DNA test results will either include or exclude a given person from being biologically related to another person. In paternity testing, the tested alleged father is excluded from being the biological father when he does not share a sufficient number of genetic markers with the child. In case like this the probability of paternity reported will read as 0% in the Probability of Paternity box. When an alleged father and child share a sufficient number of genetic markers (STR loci), he is included as the biological father. In these cases the probability of paternity in this case reads as 99.9% or higher.