THE EDITOR: I was quite interested in the column by Eurico Jardim entitled “DNA testing unreliable?” in the Catholic News dated September 4 (paper edition).  

So many of us know very little about DNA testing, but it seems quite miraculous – just test two people's DNA and you will know if they are related.

It's not quite so easy as that, as I recently discovered. The situation described in the television programme reviewed by Mr Jardim is, I am sure quite unusual. Perhaps we should look at a more common hypothetical situation:

Let's say that there is a child – a girl – born out of wedlock. Her father dies without any will. He had no legal wife. If he has a child, then she is his next of kin. If he did not, his mother is his next of kin. His mother claims that her son had no children. 

So, the man's mother and the child have a DNA test done. It shows no relationship between the woman and the child. So, the child is not the daughter of the dead man? Not necessarily so.

First we should ask what type of DNA testing should be done. Who should be tested? If a Mitochondria (mtDNA) Test was done, this was in fact the wrong test! MtDNA testing is a maternal lineage test, and would not do anything to prove that the girl was related to her paternal grandmother! 

Mitochondria DNA is only passed from a woman to her girl children, not to her sons. The woman in the hypothetical situation could, indeed, be the child's grandmother, but this test would not prove, or disprove, this since the child's father would not have received mitochondria DNA from his mother!

In a case like the hypothetical situation I am using, if both paternal grandparents are alive, then three persons should be tested – the child and both paternal grandparents. 

If the paternal grandfather is not alive, then what is needed is a “reconstruction” test. This test would require the paternal grandmother, a full sibling of the deceased father (brother or sister having the same parents as the deceased), the child and the child's biological mother. 

So, you see, we need much more than just DNA from two people to establish that there is a relationship, if it is not a mother and daughter.

So, actually DNA testing is not unreliable, but people requiring DNA testing do not always know enough to determine who should be included in the test to establish the relationships.

Joan Moore, Carenage.