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Sunday, July 4, 2010

Identifying half-siblings by genetic tests

I have received four related questions from different people. They all concern the issue of half-siblings, and the utility of genetic tests for establishing such a relationship.

1) Is it absolutely possible to tell if two females have the same father? These supposedly half sisters do not share the same mother. The father in question is deceased. Will our DNA reveal, certainly, that we share the same father? How do we find a reliable U.S. company to do this for us?

Information from a company like 23andme will provide you with enough markers to be completely sure. I'm not entirely sure how their interface works, so I can't tell you how easy the interpretation will be, but there will be enough data for you to be sure, and you will be able to get help with the interpretation. The reason is that two women who share a father will have an entire X chromosome in common. That is extremely unlikely to occur otherwise.

2) My sister donated a kidney to me. We were told that we matched 5 out of 6 genetic markers. From that information, is it possible to make an "educated guess" as to whether we are more likely to be full or half sisters? We believed we were half-sisters but some interesting coincidences lead me to believe that we may be full-sisters.

The six markers used for this test are not conclusive. As you know, even half-siblings can be a perfect match. A conclusive test would require many markers. Fortunately, companies like 23andme and Navigenetics provide information about many markers (about 450,000), and those tests could tell you definitively. If you are full siblings, then there will be parts of the genome (about one-fourth of the total) where you are identical for long stretches. That would be extremely unlikely if you are only half siblings. However, a small number of markers (less than 200 or so) would make it harder to make a definitive distinction between being half siblings and being full siblings. Six is definitely too few.

3) I wonder if you could help me. In trying to find my biological father, I came up with what could be two half siblings. The parents in both cases are deceased. I have been given a price of $500 for the three of us to test by saliva. Do you think without any parents, this could prove half siblings or would it be a waste of money?

You will share one of your two alleles with a half-sibling at about half of the sites in your genome. So, the answer is that with enough markers (thousands) the answer will be absolutely clear. The source of DNA (saliva, cheek swab, blood) does not matter much. 23andme will do about 450,000 markers for $400 (per person) and give you lots of additional information. The technology is pretty standard so other firms are probably OK. Just make sure that there are many markers (more than 100,000) and that you get access to the data (not just their interpretation of the data). Once you get your results you'll want to look for large regions of the genome where you and the putative half-sibling share markers. Of course, your putative half-siblings will have to agree to this analysis.

4) I heard that a recent study proved that men don't have half children but any children by the same man are full brothers and sisters irregardless of all different birth mothers. Is there a genetic truth to this?

Two children with the same father and different mothers are referred to as half-siblings. What you are referring to is almost certainly a legal or cultural distinction, not the sort of thing that can be proved by a study.

As usual, I invite additional answers in the form of comments. We are looking for answers from people with some expertise, and you will be asked to log in so that we know who you are (no anonymous answers).


  1. Hi Steve--I could send 23andMe images that illustrate both half and full siblings if you'd like to post them.

    Linda Avey, co-founder 23andMe

  2. Yah........Answers to questions on gene gives alots of information to me. thank you so much...bioinformatics training

  3. regarding question 4 I'd like to give an input although absolutely not something professional. My father had 3 children with 3 different women. Therefor I have 2 half-brothers. None of us look alike and neither do we look like our father (we all look much more like our respective mothers). On my part, ma mother's family has a rare genetic mutation of the fingernails that is past down from generation to generation regardless of sex. My grand-mother has them, my mother and her brother have them and both me and my cousin have it as well. That means I most likely have more of my mother's genes then my father's since I look a lot like her and she past down the "claws" to me (I will also most likely pass them down to my children).

  4. "Is it absolutely possible to tell if two females have the same father? These supposedly half sisters do not share the same mother. The father in question is deceased. Will our DNA reveal, certainly, that we share the same father? How do we find a reliable U.S. company to do this for us?"

    Paternal half-sisters will be half-identical along the entire X chromosome. Maternal half-sisters, in contrast, will be half-identical along parts of the X chromosome but most likely not the entire X.

  5. I was told by my father (now deceased) that he was not my father. My mother is also deceased, can I test one of my sisters and myself to verify if we have the same father?

  6. Yes, you can tell, for sure, if you and your sister(s) had the same father. I'm familiar with 23andMe, but any "ancestry" testing service that probes thousands of autosomal markers can give you reliable results.

  7. Be sure to look at this later post:

  8. I was wondering if my half brothers cousin is related to me

  9. The main focus concerns the diagnosis of hereditary diseases
    Every paternity establishes a special relationship. This not only means a lifelong emotional bond, but also includes extensive and ongoing legal obligations. As large as the scope of that relationship is, the questions can be just as excruciating if there are any doubts about the Paternity Test, DNA Relationship Test and Identity Proofs. Unlike the mother, as a general rule, the father is naturally often left with his suspicions unless he decides to take a paternity test.

  10. The sister of a man that is possibly my biological father could possibly be his half sister or full sister. He is deceased. Can she still be tested for a siblings DNA test even if she is half?

  11. Genetic tests with many markers (such as Family Tree or 23andMe) can reliably distinguish an aunt who is your father's full sister from one who is your father's half sister; this is based on the assumption that you and she can both submit samples.

  12. Hi Steve, Since I think I am from the same mother as my sister, but think we don't share the same father, and both patents have passed.Where is the cheapest place this test can b done to c if we are really sisters or half sisters. It's been a thorn in my side. I don't have much money to spend on this.I looked like my mother but nothing like my sister. What kind of test am I looking to have done to prove my suspicion?

  13. I strongly recommend 23andme. If you submit samples from both of you to 23andMe, there will be no doubt about the result.
    They use hundreds of thousands of markers. If she is your half-sister, you will share some DNA on most chromosomes. If you are full siblings, it will be considerably more and there will be regions where you share both chromosomes. If you have the same father, you will share the entire X chromosome. There will be no question about the answer, even without a sample from either parent.
    (Half-sisters could look like niece/aunt or grandmother/granddaughter, and 23andMe might label you as such, especially if your ages are quite different, but half-sisters could never be confused with full sisters).
    Also, if the father of one of you is unknown, that fact that there are one million people on the site will help you to figure out who the real father is, should you wish to do that.
    The expense is perhaps higher than some other tests, but you get so much more information than some of the tests I've seen (less than 100 markers!) it's worth it. Also, there are forums to help you understand your results.

  14. Could i swab both of my kids to determine if they are full blooded brothers (same dad) without having the man present?