Mr Williams of Dunedin asks :-

I was adopted as a baby. A few years ago I found a younger brother who, also as a baby, had been adopted by another family. Could DNA testing tell if we had the same father?

Russell Poulter, a biochemist at Otago University's School of Medical Science, responded.

DNA testing could either establish with certainty that the two boys definitely have different fathers or indicate (with a certain probability) that they might have the same father. It can’t prove they had the same father. Each boy will have a Y chromosome passed to them by their father. If the boys have the same father their Y chromosome will be identical in DNA sequence. If they have different fathers their Y chromosomes are very likely to be different in sequence (but could be the same). The Y chromosome is passed down through the male line for many generations with minimal change. As a result many men with the same surname (especially less common names like Poulter) are quite likely to have the same Y chromosome. It follows that having the same Y chromosome means the boys may (or may not) have the same father. It would be possible to use the sequence of the remainder of the genetic material (the Y chromosome is only a small part) to further confirm the analysis but this would involve Mendel’s laws and might get a bit complicated. The Y chromosome analysis could be done without the knowledge or consent of the mother. However such things should always be approached carefully to avoid causing possible distress.

This type of analysis was nicely illustrated in 1998 by the investigation of the claim that President Thomas Jefferson was the father of his slave Sally Hemings’ children. The study, which tested Y-chromosomal DNA samples from living male-line descendants of Field Jefferson (Thomas Jefferson's paternal uncle) and living male-line descendants of Eston Hemings, (one of Sally’s sons), indicated that the Jefferson and Hemings Y chromosomes were apparently identical, proving a genetic link between the Jefferson and Hemings descendants. The results established that an individual carrying the same Y chromosome as Thomas Jefferson fathered Eston Hemings (born 1808). There were however approximately 25 adult male Jeffersons (all presumably carrying this chromosome) living in Virginia at that time. There were probably many other men (including slaves) with different surnames who carried the same Y chromosome. The study's authors, however, concluded "the simplest and most probable" conclusion was that Thomas Jefferson had fathered Eston Hemings.

John Withal’s Dictionary of 1553 includes the proverb ‘Wise sonnes they be in very deede, That knowe their Parents who did them breede.’