Forensic Topic: DNA Analysis

I'm so pleased to have forensic expert Amryn Cross back with a question from a reader.

Welcome back, Amryn!

S.H. posed the following question about DNA analysis:

In my book, an investigator has a DNA test done on some samples and two profiles are found. The profiles are for half siblings, a man and a woman, who share the same father but different mothers.

What I need to know is this: Could they tell from just the brother and sister's DNA that they are half siblings? I know they could probably tell that they're related, but how clear would the match be? Would it be possible to say they share the same father? Or would they need to take a sample from dad to indicate that both were likely to be his children?

This is a great question about what can and can’t be gained from DNA testing. The tests performed in most crime labs will look at a set of 13 markers plus an additional marker to determine sex (amelogenin). For each of these 13 markers, a person will have two numbers. For instance, at marker number one, person A might have the numbers 10 and 13. We would say that their profile at that marker is a 10,13.

Using basic genetics, we know that a person inherits one of these numbers from the mother and one from the dad. In the above example, person A’s mother might have been 10,11 and the father might have been 11, 13. In that case, person A inherited the 10 from the mother and the 13 from the father. As you can imagine, this gets quite complicated when you have to look at several sets of these numbers which make up a DNA profile.

If you compared the profiles of a mother and her son or daughter, they would have at least one number in common at each of these 13 locations. If you also had the profile of the father, you could see that the child would also share at least one number with him as well. But for a brother and sister, things get more complicated. Look at this example:

Mother: 10, 11

Father: 13, 14

Son: 10, 13

Daughter: 11, 14

As you can see, each child received one number from their parents, but the brother and sister don’t share any numbers. Therefore, it is possible that we wouldn’t suspect they were related based on their DNA profiles. If you throw half-siblings in the mix, it becomes even less likely that the connection would be recognized if you didn’t have a reason to suspect it in the first place. It is possible that half-siblings would share no markers or at least not any more than unrelated people.

If you had the father's DNA profile you would likely be able to say that he's possibly the father of both, but not definitely. There are statistical calculations that can be done to help determine the degree of relatedness. Special testing of the Y chromosome would be able to tell that the father and son are definitively related but wouldn’t be helpful with the daughter.

As a side note, many crime labs won't do paternity testing, and if your investigator didn't already suspect these two people to be siblings, he would have a hard time getting a warrant for the father's DNA profile. Of course, if the father willingly provided it, a comparison could potentially be made.

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Amryn Cross is a full-time forensic scientist and author of romantic suspense novels. Her first novel, Learning to Die, will be released in September. In her spare time, she enjoys college football, reading, watching movies, and researching her next novel. You can connect with Amryn via her website, Twitter and Facebook.