I am working on a fiction/mystery scene where blood is found in a particular character's residence but the character themselves is missing. I would like to understand whether the law enforcement/forensics team are realistically able to identify if the blood belongs to the missing character, without having a prior blood sample or DNA of this missing character? If not what details are they able to learn from the sample eg. gender, age, physical characteristics etc?

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    $\begingroup$ I'll add a full answer later, but sex, some physical characteristics (e.g. hair, eye, skin colour) and ethnicity can be determined (not perfectly) using DNA from blood. $\endgroup$ – user438383 May 6 at 11:20
  • $\begingroup$ @user438383 thank you very much, looking forward to your full answer. $\endgroup$ – FrontEnd May 6 at 11:35
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    $\begingroup$ The forensics team would also use other sources of DNA found around the house, like hairs and skin cells, to confirm (or not!) that the blood is from the person who lives there. You can also take samples from this character's relatives to establish if the blood came from someone related to them. $\endgroup$ – MattDMo May 6 at 23:51
  • $\begingroup$ @MattDMo very helpful, but if there's other people (or even one person) who visit(s) on a regular basis then their hair, skin cells etc would also be there? $\endgroup$ – FrontEnd May 7 at 6:06
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    $\begingroup$ @FrontEnd there would, but the majority would likely be from the resident. They would do things like look at the hair under the microscope to see if it matches the occupant, take DNA samples from the bed, a toothbrush, comb, or towel, that kind of thing. It wouldn't be just one sample, they'd take dozens. If one of the visitors is a romantic partner who sleeps over regularly, then it might be a little more difficult, but forensics people have lots of tricks up their sleeves. There are lots of accessible sites out there that explain aspects of criminal forensics, you might want to look them up. $\endgroup$ – MattDMo May 7 at 16:41

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