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DNA Test Results
Interpreting the results of a DNA test can be daunting, especially with numerous figures and unfamiliar terms, particularly in emotionally charged situations. Let’s break down the potential outcomes of a DNA paternity test:
- Exclusion: This outcome indicates that the person tested is NOT the biological parent. A paternity exclusion means that the tested individual is not the true biological father of the child. The report will show a minimum of two exclusions (i.e., non-matching alleles) at two different genetic markers. In cases of paternity exclusion, the probability of paternity is 0%.
- Inclusion: This outcome signifies that the person tested IS ACCEPTED as the biological parent, and the report states “NOT EXCLUDED.” If the report indicates that the tested individual is not excluded as the biological father of the child, a combined paternity index (CPI) of over 1000 is presented. A CPI exceeding 1000 indicates a probability of paternity greater than 99.9%. The CPI number reflects the number of individuals in the combined population who would need to be tested and might have a profile identical to the tested individual.
Combined Paternity Index (CPI)
The Combined Paternity Index (CPI) assesses the likelihood that the tested individual is the biological father of the child compared to a randomly selected individual from a validated population. Typically, paternity is affirmed if the CPI exceeds 1000.
If the DNA of the alleged father aligns (with a certain degree of mathematical certainty) with that of the child, the report will conclude that the alleged father cannot be excluded as the biological father. Conversely, if the DNA does not align, the report will indicate that the alleged father can be excluded as the biological father. It’s crucial to recognize that it’s never possible to definitively prove 100% that a man is the biological father of a child, as there’s always a small possibility that another man in the population shares DNA traits with the child.
It’s important to note that the inclusion of paternity does not rule out close blood relatives as potential fathers. Men who are not the father of the child may still share a few genetic markers by chance, similar to how a man and child may share certain physical traits like eye or hair color without being related. If the tested man does not match in more than one marker, however, he cannot be considered the biological father.
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Testing Process
Each of us possesses 22 pairs of chromosomes in our bodies. During embryo formation, one chromosome is inherited from the mother and the other from the father, resulting in a total of 46 chromosomes in the embryo. Additionally, there are sex chromosomes, either XX for a girl or XY for a boy.
To determine paternity, we examine specific locations on these chromosomes, known as markers, in the child. Subsequently, we search for matching markers in both the mother and the alleged father. These markers are assigned numbers corresponding to their size and matching markers are termed alleles.
If the tested man is not the biological father of the child, the genetic material inherited from the father will not match the profile of the tested man. This mismatch results in the exclusion of the tested man as the true biological father. This exclusion must occur in at least 2 of the 25 markers to definitively exclude the man as the father.
We analyse 25 markers and then calculate the result to determine if the alleged father is either excluded or not excluded as the father. When there is a match across all 25 markers, we conclude that there is a very high probability (greater than 99.9999%) of the alleged father being the biological father. This determination is based on calculating the probability of paternity, which represents the likelihood, expressed as a percentage, that a man with the alleles of the alleged father is the biological parent of the child compared to an untested, unrelated man of the same race.
Short Tandem Repeats
If you’ve observed a possible mutation in yourself or your child during genetic testing, there’s generally no need for concern. Short tandem repeats (STRs), the genetic markers used in our analysis, are short sequences of DNA that repeat multiple times in a row. These repeats are highly variable among individuals and are primarily found in non-coding regions of the genome, meaning they don’t contain instructions for making proteins.
While STRs are excellent for discriminating between unrelated individuals in paternity testing due to their high variability, they also have a higher mutation rate compared to other DNA regions, such as coding regions; however, mutations in noncoding regions like STRs typically do not cause disease.
During parentage testing, the child’s DNA is compared to that of the alleged parent, with each parent contributing one allele to the child. A direct match is expected at each tested locus. If a direct match isn’t observed, the locus is evaluated for a potential mutation. Inherited mutations from a true biological parent typically result in an increase or decrease of one repeat unit.
In cases where a potential mutation is detected between the alleged father and child, we make an assumption of a probable mutation and incorporate this into the calculation of the probability of paternity. Any such assumptions are clearly noted in the test report.
Overall, while mutations may occur in genetic testing, they are typically accounted for and do not significantly impact the reliability of paternity results.