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I couldn’t find anything on Wikipedia. 3/4th siblings are 37.5% related while full siblings are 50% related. Is there anything in between?

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    $\begingroup$ You may be confusing averages with the actual (genomic) relatedness of two given siblings. Two given siblings can in theory be anywhere from 0% to 100% related, but on average will be 50% related. $\endgroup$
    – Armand
    Aug 14 at 22:06
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The International Society of Genetic Genealogy Wiki provides the following situations for which siblings may be considered $3/4$ siblings (a coefficient of relationship of 37.5):

  • a man has children with each of two sisters (the children are related as half-siblings and first cousins)
  • a woman has children by each of two brothers (the children are related as half-siblings and first cousins)
  • a woman has children with both a man and his father (the children are related as half-siblings and half-aunt or half-uncle and half-niece or half-nephew)
  • a man has children with both a woman and her daughter (the children are related as half-siblings and half-aunt or half-uncle and half-niece or half-nephew)

Note that the above situations assume that all grandparents are unrelated. If you consider a situation where either or both sets of grandparents are full siblings, then you have a coefficient of relationship in the sibling grandchildren that is intermediate between 37.5 and 50.

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  • $\begingroup$ I edited my question, what I wrote was not what I meant. $\endgroup$
    – The Mamba
    Aug 14 at 16:15
  • $\begingroup$ I've extended my answer $\endgroup$
    – acvill
    Aug 14 at 16:42

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