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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.

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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).

31 comments:

  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

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

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  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).

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  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.

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  5. so how do my sibiling and i figure out if we share the same father and how much does this cost us dahoodangelz@gmail.com

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  6. My best friend and I each have a child we think are from the same guy... How do we test the children

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  7. My best friend and I each have a child we think are from the same guy... How do we test the children

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  8. 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?

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  9. 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.

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  10. Be sure to look at this later post:
    http://ongenes.blogspot.com/2011/02/half-siblings-vs-full-siblings.html

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

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

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  13. 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.

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  14. 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?

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  15. 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.

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  16. I am a female and it has recently come to light my father had a son during an extra martial affair. I have no way of finding the woman's name he had the affair with since he is deceased. If I did 23 and me would it match genetic markers. Since I am XX and he would be XY

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  17. I had a question about a home DNA test that was just done on my son. Instead of his alleged father swabbing his own mouth, he swabbed the mouth of his other son (which would be my sons half-brother). The DNA test came back that he was not the father( 0.00% assuming prior probabilty being 0.5) So with that being the conclusion would it be safe to say that they are not related at all? They matched numbers on 3 of the markers that were tested but i also had the same number.

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  18. 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?

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  19. I strongly recommend 23andme. If you submit samples from both of you to 23andMe, there will be no doubt about the result. https://www.23andme.com/
    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.

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    1. Hi Steve. I am reading your writing here with interest as I just received results from a sibling test today that have me somewhat confused. The results (probability factor) suggested high for full siblingship ( 33,776.) and low for half siblingship ( 6478.). There is one father and two potential siblings that were tested. The father is deceased and I am the mother of only one of the girls. I am confused by the high number on the full sibling and low on the half. You wrote above that half-sisters could never be confused with full sisters. So now I am unsure as to trust the results I have recieied. Do you have any recommendations? Thank you in advance.

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    2. Tina,
      Write to me directly, at ongenetic@gmail.com, and tell me what test you had done (what company and how many markers). I don't know what those "probability factors" are. My statement that full and half siblings could not be mistaken is based on using a service with hundreds of thousands of markers that can be used to reliably calculate the fraction of the genome identical by descent.
      Steve

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

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  21. My name is Deborah. My mother has been deceased for 37 years and out of the blue, one of my sisters claims that Prophets in her church have told her we share the father but not a mother. She is reluctant to do a DNA test because she says it is only a paternity and not maternity test. Is there a test that can be done to prove that we shared a mother?

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  22. If you have a test by 23andMe or a comparable company (in which you are given the results from hundreds of thousands of variants across the genome), and you have access to the tools to analyze them (such as 23andMe's DNAview in DNA relatives), then the result will be clear. Full sisters are clearly different from half-sisters. Two half-sisters who share a father will share their entire X chromosome while two half-sisters that share a mother will not (they will share about half; the X will look like other chromosomes). Both of you have to submit your DNA for testing.

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  23. If I find my 1/2 aunt on my father's side... will we share an entire x chromosome?

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  24. Is it possible to share an entire x chromosome with a 1/2 aunt (related on father side)

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  25. I am an adoptee, but have been able to determine the identity of my biological mother through adoption paperwork. I have located other children of my biological mother, and one sister agreed to DNA testing via AncestryDNA. When compared, we share 2,576 centimorgans shared across 83 DNA segments. Am I correct in thinking that sharing 55% of our DNA makes us full siblings?

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  26. Is it possible to share an entire X chromosome with your father's sister? Yes. It means that your father's entire X was shared with his sister, and that is possible.

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  27. Yes, the numbers sound right for your and your sister being full siblings. You should share half of your genome (identical by descent) with your full sister, and 3/4 of the genome should be covered (that's about 2800 cM, but the amount will vary - I share with my own, full, sister over 2376 cM). However, you would also share half of your genome with a half-sibling as well, but a different measure (half-IBD, covering about 1860 cM, but with wide variation).

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  28. My half brother and I want to be tested to see if we have the same father. We know nothing about our father. We have the same mother, she is still alive. She does not want to share any information.

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  29. I answered this question above (for two sisters, but the answer is nearly identical).
    If you have a test by 23andMe or a comparable company (in which you are given the results from hundreds of thousands of variants across the genome), and you have access to the tools to analyze them (such as 23andMe's DNAview in DNA relatives), then the result will be clear. Both of you have to submit your DNA for testing. Also, the large database of users will help you to find the father (or fathers), and you can also upload your data to other sites. Also, you can share your DNA on the 23andMe DNA Relatives site anonymously.

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