Once we have determined which alleles are present in all of these markers, we bring them together in a special format known as the DNA profile. In each DNA profiling experiment, we will see two alleles at each marker tested i.e. one which has been inherited from the individual’s biological father and one which has been inherited from their biological mother.
Each person’s cells contain DNA; half of which comes from the biological mother and half of which comes from the biological father. The problem is, we do not know which half !
In tests involving the child’s biological mother, our analysis is able to identify the half of the DNA (alleles) in the tested child that comes from the mother. As such, the remaining alleles must have come from the biological father. If we get a match across all of the remaining 15 alleles, then the tested man is the biological father of the child and we can then go ahead and calculate a ‘probability of paternity’. Conversely, if there are three or more mis-matches between the tested man and child, then we can conclude, with 100% certainty, that the tested man cannot be the biological father of the child.
For a positive result, we expect a match with the tested child at all or in rare cases, all but one of the biological father’s DNA markers. Rarely, we may observe a small genetic change (mutation), which shows itself as a mismatch between the tested man and child in one of the markers examined. Sometimes this appears as an allele that does not occur in either parent. This genetic change is a normal, albeit in the wider context a rare, event in human evolution and if we observe this, we make allowances in our calculations. Please note however that this scenario is also observed in those cases where the tested man is shown not to be related to the child as its biological father but as a close male relative i.e. the true biological father of the child is in fact a brother or the biological father of the tested man.
For immigration purposes, we are often asked to prove maternity. For example, in some parts of the world, it is common for a child to be brought up by an aunt from birth and to consider her to be her mother.