Title is pretty self explanatory, but how much DNA would half-siblings share if they had the same mother ? I'm asking for a very specific reason. I'm writing a book in which some boxes can only be opened by people sharing at least 50% of their DNA. Like a father and son, for example (I'm no scientist lol, this is fiction, trying to keep things a bit simple but with a solid base). Would a half-sibling be able to open these boxes too ? If someone has any kind of info, wide or precise, I'd be very interested to know what you have to tell :) Thanks !

  • $\begingroup$ @Halfthawed Indeed, I was talking about the "small fraction of human DNA which isn't consistent in every human" : ) Let's say they have common ancestors (great aunts for instance) too. Would that reach 50% ? $\endgroup$ – FJB Feb 16 at 21:51
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    $\begingroup$ This question is not about a hypothetical world at all. In general, FJB, we encourage real world questions to go to the Stack Exchange for that question where you'll find more experts on the specific topic. Save the speculative questions for us! In this case, check out biology.stackexchange.com $\endgroup$ – SRM Feb 17 at 2:11

If we assume you mean 'have identical sequences' by 'share DNA', then half-blooded siblings share over 99% of their DNA. This is because DNA is responsible for everything responsible to make us human, and most of the differences between the various races of human are in under 1% of all DNA.

If, on the other hand, you're only referring to that small fraction of human DNA which isn't consistent in every human, than it's as follows: Full blooded siblings share, on average, 50% of that DNA, assuming that their parents aren't related at all. Half blooded siblings share, on average 25% of that DNA, assuming the parents aren't related at all.

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    $\begingroup$ Except if the half-siblings are brother and sister — even if they are full siblings — the single chromosome determining child gender caps the genetic similarity at 98%. So 99% of 98% for siblings of different genders. $\endgroup$ – EDL Feb 16 at 21:42

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