- 1 Introduction
- 2 Paternity Index
DNA Paternity testing is the use of DNA profiles to determine whether an individual is the biological parent of another. It’s the most reliable method of paternity testing.
This technique Utilizes two main molecular pathology technologies: PCR and Restriction Fragment Length Polymorphism (RFLP)
All human beings (the 7 billion of us) like all life forms contain the genetic code. The genetic code contains the instructions that define all aspects of the organism.
This genetic material is in the form of Deoxyribonucleic Acid (DNA) and is contained within the cell’s nucleus. All somatic cells in the body have identical DNA. 99.9% of our DNA is identical to one another. It’s the 0.1% difference that we exploit in identity testing, of which paternity testing is part of the sample used depending on the specific circumstances eg postmortem, body remains, routine DNA from the child, mother, and alleged father(s) are extracted, amplified and identified.
Inheritance of genetic material
Half of the genetic material (alleles) comes from each of our parents. During fertilization, the ovum and sperm contribute half of the genetic material that combines in the zygote and develop into a fetus. The 0.1% of unique sequences are transmitted from the parents to their offspring. Analyzing these unique DNA sequences (called Short Tandem Repeats -STRs) in the mother or father, and comparing them to the child. Depending on the match, we can conclude with a high degree of accuracy whether the child is related to the mother /father. In the case of paternity, the alleged father or father’s DNA is analysed. The highly conserved STR sequences that are identical in both the father and child mean they could only have been inherited from that father.
Based on the distribution of the specific sequences in the population, the probability a random man could have been the father is computed against the alleged father(s). The result is expressed as a percentage and often is above 99.999% when the alleged father is the biological father
Let’s assume a child has a 10 & 11 allele for particular genetic loci. The mother is known to possess a 10 and 12. The mother must have contributed the 10 and 11 must be from the father. Any man who does not possess an 11 allele for this particular locus is excluded as a father. For the man not to be ruled out, the likelihood a randomly chosen man might provide the same allele to the child is determined by examining the allelic frequencies from relevant population databases
Obligate Paternal Allele
An obligate paternal allele refers to an allele the actual father must have because its in the child and the only parent who would have contributed it is the father.
Compare the child to the mom, The allele the mother doesn’t have is the obligate paternal allele. Increasing the number of loci examined –higher chances of finding unique alleles.
PI compares the likelihood that an alleged father passed an allele to the possibility that a randomly selected man of similar ethnic background could pass the allele to the child.
PI=xy=chance AF could transmit obligate allele, Chance another man of the same race transmitted the allele
X= assigned value of 1 if the alleged father is homozygous for the allele & 0.5 if heterozygous.The larger the ratio, the more likely the man is the biological father.
Combined Paternity Index
When multiple alleles are tested, a PI is calculated for each. Since alleles are inherited independently, a product of the PIs gives a Combined Paternity Index(CPI). CPI is an odds Ratio indicating how many more times the AF is the biological father than randomly selected unrelated men of similar ethnicity. Most Laboratories in Kenya test up to 15 different loci to increase the accuracy of the paternity testing.
The CPI is therefore a measure of the strength of genetic evidence of paternity.
Interpretation of CPI
Theoretically range of CPI is 0 -∞ (zero to Infinity)
- CPI = 1 – No information
- CPI< 1 Genetic evidence more consistent with non-paternity
- CPI> 1 Genetic evidence supports the assertion that the tested man is the father.
Probability of Paternity
The measure of the belief in the hypothesis tested man is the biological father. The correct probability is based on ALL the evidence of the case i.e,
- Non-genetic evidence
- Mothers Testimony
- Tested Man testimony
- Other Witnesses
Genetic evidence comes from DNA testing.
An assumption is made prior to testing the probability the AF is the true biological father. Prior probability of paternity is the strength of one’s belief the AF is the biological father based on non-genetic evidence. Typically PP= 0.5 is used assuming it’s a neutral unbiased value.
Probability of Paternity
To convert genetic evidence to a probability of paternity, the Bayesian theorem is used. CPI and Prior Probability (PP) are used.
When prior probability of 0.5 is used, our probability of paternity becomes:
Effect of Prior Probability
Suppose a defendant presents a powerful testimony e.g. mother is unreliable and has been caught telling lies before or he’s a prominent person and therefore a subject of blackmail. So we take his word for it and reduce the Prior Probability to a figure say 1/500 i.e 0.02
From the formula W= 99.5% at PP= 0.02 and 99.92% if we assume pp=0.5. This leads us to virtually the same verdict, the tested man is probably the biological father.
Put another way, the alleged father may have his say in court BUT it’s unlikely whatever he says will be significant compared to the force of DNA evidence!
There are three main outcomes of paternity testing.
The results indicated the tested man could be the child’s father. This is when the value of W above is 99% and above. In a typical result, this will indicate the results are conclusive the tested man is the father.
The results concluded the tested man could not possibly be the child’s father. The results are typically W value of zero.
Results from genetic testing were inconclusive and could not determine if the tested man is the biological father. This occurs when the probability of paternity falls below >0% and 99%. In such cases, additional testing is required to establish paternity.
Challenges in Paternity Testing.
Despite its accuracy, genetic testing is not perfect. Here are some situations where results may be inconclusive.
- Consanguinity. This is when the two parents are genetically close relatives.
- Identical Twins – This is obvious, that if the alleged fathers are identical twins, they possess identical genetic material, and are therefore impossible to tell them apart.
- Unavailable mother’s genetic profile. If the mother’s DNA was not analysed for whatever reason, it makes it harder to identify the paternal alleles and make the conclusion.
- Random mutations – Extremely rare, though theoretically can complicate analysis.
This brings us to the end of this discussion. I hope this will help you in understanding the process and results of paternity testing.
To help you decide on the best lab for paternity testing within Kenya and beyond, costs, and interpretation of results, get in touch with me for unbiased advice. As always, you can rely on us for top-notch confidentiality. Please leave your comment below or hit me up on my email at firstname.lastname@example.org.