The Paternity Test can be used to identify who the biological father of a child is by comparing the DNA of the alleged father to that of the child. If there is an adequate amount of common DNA, then the child is said to be his.
The similarity is tested by comparing a number of genetic markers, which each individual has a unique mix of. With this in mind, it can be tricky to carry out a paternity test between alleged fathers who are biologically related since parts of their DNA are similar. This will make it harder, but not impossible, to differentiate between the two and find out who the real father is.
The further away that a person is related from the immediate family, the easier it is to distinguish between them due to the differences in their genetic mixture. For example, two brothers will have more in common than two cousins. When two cousins are tested for paternity, the test has a higher probability of a defining result.
The possible fathers may also be related through a father-son relationship. The father will always pass 50% of his DNA to his child and the grandchild will receive 25% of this. This will give higher chances towards a false positive - when the test is incorrectly marked as positive. The laboratory carrying out the test needs to be informed of the biological relationship between the fathers so that further analysis is carried out to ensure better results. The participation of both fathers is always ideal.
If the alleged fathers are brothers, they each will have 50% of their DNA from their mother and the other 50% from their father. However, the DNA they receive from one parent may not be the same exact DNA profile that the brothers share between them.
To know for sure which of the brothers is the father of the child, it is recommended that both brothers are tested so as to confirm which of them shares the most genetic markers with the child. If only one brother is available or is consenting for testing, there is still a good chance to get an accurate result. The mother’s participation in the test is not compulsory, but recommended as her input will help analyse the genetic profile of the child much easier. Fraternal twins will have the same genetic mixture as normal siblings (50% father: 50% mother), therefore what applies to brothers applies here.
On the other hand, in the unique case of identical twins there is presently no method developed enough to distinguish between them as both their appearance and genetic profile is completely identical and therefore indistinguishable.
Whatever may be the case, it is important to inform the laboratory if the possible fathers are related, as by being informed of this they will be able to look at more genetic markers to give a more precise result.