What is a DNA Test?
Testing for similarities between DNA (deoxyribonucleic acid) samples from two people allows family relationships to be established - or disproved - to an extraordinarily high degree of certainty. A common use for a DNA test is to establish if a man is the biological father of a child, a paternity test. However, there are other uses for the science of DNA testing (also called genotyping), these include forensic analysis of human DNA samples, and tracking relationships amongst domesticated animals.
It was realized in the 1940s that DNA molecules are passed down through the generations of a family. In 1953 Watson and Crick elucidated the chemical structure of this molecule as a twisted ladder (a 'helix') made of two strands. DNA occurs in all the cells of our body, it is our blueprint! The strands of DNA contain information in the form of a code, which in turn determines our individual traits and characteristics. This code, the genetic code, is the order of four types of DNA building block. When the two strands of DNA separate, each building block ('base') accurately templates a corresponding base on the newly made strand of DNA so that information is not lost but is instead duplicated and preserved.
The order in which the bases occur in DNA is referred to as the DNA sequence. Each person is unique and just as people differ in their fingerprints, they also have a unique and slightly different DNA sequence. Half of a person's DNA is received from their mother, and half is received from the father. However, while fingerprints have no value for establishing family relationships, the minor variations in DNA sequence are extraordinarily useful for this purpose. All cells of our body contain DNA, skin cells from the lining of the cheek provide a simple and convenient source of material.
DNA is purified from these cells and the minor variations are read out as a type of bar-code by a DNA sequencing machine. When the net DNA 'barcodes' from family members are lined up next to each other it becomes clear when a child is related to biological parents because half the stripes in the bar-code like signature will line up with those of the mother, and half will line up with those of the father. On the other hand, in the absence of a biological relationship, the DNA signatures from a child and from a potential parent are not found to have 50% in common. It may be appreciated that DNA testing is the most convenient and scientifically accurate method of determining relationships between people.
How to Interpret a DNA Paternity Test Report from International Biosciences
The DNA Test Report shows the results of laboratory DNA tests that provide evidence regarding the alleged family relationship. This is a brief explanation of the meaning of the numbers (that is to say, the statistical certainty of the result) and other items that appear in a DNA test report:
The laboratory analysis tests the DNA isolated from cheek swabs to locate certain regions of chromosomes that are known to vary in length between individuals. Twenty Four of these sites are tested; each site is called a "locus", ("loci" – plural). Analysis of these 24 sites in a large population has revealed many different sized versions associated with each site. Versions of a DNA sequence or a gene are called “alleles”. These are the genetic markers* used in parentage analysis.
Because each individual has two of each type of chromosome, one inherited from each parent, everyone has two alleles at each locus. These two alleles are sometimes identical (homozygous), but usually they are not the same size (heterozygous). During parentage DNA testing, the laboratory identifies the length of the two alleles found at each locus.
The DNA test report you will receive shows numbers (in the first column) that indicate each of the 24 loci involved in the DNA testing process.
The columns marked "allele" on the DNA test report contain numbers indicating the two alleles found at each locus (or one number if they are the same size).
If, for example, a child has two alleles that are designated 12.1 and 18, and if the mother has alleles 12.1 and 16, then the child inherited the 12.1 allele from the mother. The child has to have inherited the 18 allele from the father. The 18 allele is the "obligate paternal allele." Generally, the alleged father must have this allele if he is the biological father of the child.
23 different loci are used as genetic markers in the DNA tests, as well as one (Amelogenin) to confirm the gender of the person providing the DNA sample. If the alleged father does not have the matching allele at every tested locus, then he usually cannot be the biological parent. If he does have it, then he could be the father. A "relationship index" (called the "Direct Index" in the report) for each locus is calculated based on information including the portion of the male population that has the obligate paternal allele at that locus. This index is reported for each DNA locus.
A combined relationship (or "Direct") index for all of the tested alleles is then calculated and appears below the chart. This number is used to calculate the "probability of relationship," which is the percentage likelihood that a man with the alleles of the alleged father is the biological parent of the child, as compared to an untested, unrelated man of the same race. If the DNA of the alleged father is consistent (to a degree of mathematical certainty) with that of the child, then the report will conclude that the alleged father cannot be excluded as the biological father of the child. If the DNA is not consistent however, it will conclude that the alleged father can be excluded as the biological father of the child.
* Each locus used in DNA testing is composed of a variable number of repeating short sequences of the DNA building blocks or “bases”. These are referred to in short-hand as “A”, “C”, “T” and “G”. An example of a natural DNA sequence having such simple short repeats would be ACGACGACGACG, i.e. (ACG)4. However, each allele can have a different number of total repeats, for example (ACG)3 or (ACG)5 instead of (ACG)4, giving rise to a different fragment length when the DNA is purified and amplified in the test tube. In this example, (ACG)3, (ACG)4 and (ACG)5 would encode variant alleles with building block lengths of 3 x 3, 4 x 3, and 5 x 3, that is to say 9, 12, and 15 building block lengths. Since DNA analysis machines detect changes in length of just one unit, these differences in length are easily distinguished.
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