About Paternity Testing
A paternity test is conducted to prove paternity, that is, whether a man is the biological father of another individual. This may be relevant in view of rights and duties of the father. Similarly, a maternity test can be carried out. This is less common, because at least during childbirth, except in the case of a pregnancy involving embryo transfer or egg donation, it is obvious who the mother is.
This by DNA analysis of the three individuals, although older methods have included ABO blood group typing, analysis of various other proteins and enzymes, or using HLA antigens. For the most part however, DNA has all but taken over all the other forms of testing.
The DNA of an individual is almost exactly the same in each and every somatic cell. Sexual reproduction brings the DNA of both parents together randomly to create a unique combination of genetic material in a new cell, so the genetic material of an individual is derived from the genetic material of their parents. This genetic material is known as the nuclear genome of the individual, because it is found in the nucleus.
Comparing the DNA sequence of an individual to that of another individual can show if one of them was derived from the other or not. Specific sequences are usually looked at to see if they were copied verbatim from one of the individuals genome to the other. If that was the case, then this proves that the genetic material of one individual was derived from that of the other (i.e.: one is the parent of the other). Besides the nuclear DNA in the nucleus, the mitochondria in the cells also have their own genetic material termed the mitochondrial genome. Mitochondrial DNA comes only from the mother, without any shuffling.
Proving a relationship based on comparison of the mitochondrial genome is much easier than that based on the nuclear genome. However, testing the mitochondrial genome can only prove if two individuals are related by common descent through maternal lines only from a common ancestor and is thus of limited value (for instance, it could not be used to test for paternity).
Obtaining news that a child either is or isn't one's own is something that a parent or erstwhile parent can undertake at their own expense, without necessarily informing anyone either about the test or its result. However this situation will change when the Human Tissue Act comes into force in the UK in April 2006. Section 45 states that it will be an offence to possess any bodily material intending that any human DNA in the material be analysed without qualifying consent
Tests can also be ordered by courts when proof of paternity is required. Source: Wikipedia.
The DNA paternity test is based on a segregation analysis of 15 Short Tandem Repeat Sequences (STRs) in the human genome. These STRs include the FBI's panel of DNA markers used for human identification and have been thoroughly tested and validated. Each individual will have two specific alleles for each marker tested. These alleles are constant for each individual regardless of the source of the DNA sample but may differ between individuals. Relatedness, therefore, is based on the number of shared alleles. A child will receive half of its DNA from the mother and half from the father; thus each biological parent must share one allele with the child for every marker. Since the test is exclusionary (explained below) there is still the possibility that an unrelated individual could have a genetic profile that would not be excluded (i.e. shares an allele at every marker). To evaluate this possibility, a cumulative paternity index and a probability of paternity is calculated based on published allelic frequency tables. Currently, paternity is accepted in most court jurisdictions with a minimum probability of 99%. The markers used in this test generally give probabilities greater than 99.9%.
If an individual does not share an allele with the child at a given marker, then that individual is excluded as the parent. However, based on paternity testing standards set by the American Association of Blood Banks (AABB), paternity exclusion requires mismatch at two or more markers. Exclusion at only a single marker must be declared “inconclusive”. The reason being that there is a possibility that the single marker mismatch may be due to a mutation event occurring in the alleged parent. A mutation event will not show in the alleged parent's profile but will be exhibited in the child's profile. The probability of two mutational events occurring in the marker system is so remote it is not considered.
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