To establish paternity, 23 highly
polymorphic (variable) genetic markers are tested. For each one of
these markers, a child will receive two different copies (alleles).
One allele is inherited from the biological mother and the other
from the biological father.
Paternity
'NOT EXCLUDED'
When a father matches at the markers tested,
a probability of paternity is calculated
that is determined by the frequency of each matched allele within
the population. Our paternity testing usually results in a
probability greater than or equal to 99.9%, in cases where both the
mother and father are tested. The probability of paternity is also
expressed as the Paternity Index (PI). The PI indicates how many
times more likely the tested man will match at all markers compared
to a person chosen at random from the general population. For
example, if the PI is 1000, the tested man is 1000 times more
likely to be the biological father than any man chosen at
random.
Paternity
'EXCLUDED'
The putative father is excluded from being
the biological father of the child when at least two or more of
the sites tested do not match, ie. if the child has alleles
which cannot be matched to either the mother or the putative
father. In this case the probability of
paternity would be 0% and the Paternity Index = 0. A
man tested with mismatches to the child is excluded as being the
biological father.
How do I
interpret my DNA test results?
Genetic factors are inherited equally from
both parents. This genetic information is contained in every cell
in the human body. The purpose of paternity testing is to compare
the genetic profiles of individuals to one another, to establish
the biological (genetic) relationship.
In the examples below, we have
studied the numbers at a particular section of DNA, and worked out
which numbers the mother, father and child have.
Case 1
In Case 1, the child has a 15 and an 18. The
mother has a 14 and a 15 - she has therefore given a copy of her 15
to the child. The father of this child must have given him / her a
copy of 18. Father A has a copy of 18 (and a 21), and therefore is
a match. If this situation occurs at the other 14 sections on the
DNA, we say that Father A is NOT EXCLUDED from being the father of
this child, i.e. that Father A IS the biological father of this
child.
| |
DNA Section |
| Mother |
14,
15 |
| Child |
15,
18 |
| Father A |
18,
21 |
Case 2
In Case 2, the father must give an 18 as the
child and the mother share a 15. Father B has a 13 and a 21 - that
is Father B does not have a copy of 18, and therefore is a
mismatch. Simplistically, he cannot be the father of this child,
and we say Father B is EXCLUDED from being the biological
father.
| |
DNA Section |
| Mother |
14,
15 |
| Child |
15,
18 |
| Father B |
13,
21 |
Case 3
In Case 3, the child has only one number - a
15. It would seem the child has a number missing. In actual fact,
the child has two copies of 15 - one from the mother and one from
the father. The child is really a 15,15 (we call this homozygous),
and we say this is a match and therefore Father C is NOT EXCLUDED
from being the biological father of this child.
| |
DNA Section |
| Mother |
14,
15 |
| Child |
15 |
| Father C
|
15,
16 |