Dear Mat: Since even a minute quantity of somebody’s blood can be analyzed to determine the person’s DNA, what happens when somebody’s entire blood has been replaced by someone else’s blood during a transfusion? — Motocom, e-mail land

Dear Sir: Is DNA right-handed or left-handed? I’ve seen it drawn both ways. Please don’t answer “Yes.” — Larry James, Hillcrest

But — but — the answer really is yes, Larry. The right-handed DNA double helix (like the threading in a standard screw) is the norm, but ten years ago scientists discovered that segments of DNA can take on a left-handed twist. Southpaw DNA has a different chemical action from the same material in a right-handed configuration. There’s also supercoiled DNA and a dozen or so other variations on Crick and Watson’s original, pristine right-handed model.

As for Motocom’s scenario, if some inventive thug committed a crime, then somehow managed to wangle a full-volume blood transfusion to cover his tracks, it actually would work for a while. The DNA in his donor blood would be a perfect alibi. But that would eventually unravel because cells are continuously dying and being replaced. His bone marrow would still be churning out blood cells with his own incriminating DNA, and at some point all the donor cells would be gone. A partial transfusion would (temporarily) produce a DNA test with two different gene patterns.

And in the case of a bone marrow transplant, the recipient’s blood would forever have the donor’s DNA pattern, different from his other body cells. This actually came up during a paternity lawsuit. “Dad,” it turns out, was a bone marrow recipient, so the original DNA testing of his blood eliminated him from contention. The truth came out, though, when they retested cells scraped from the inside of his cheek and his own gene pattern emerged. Likewise, a transplanted liver, heart, whatever, will always bear the donor’s DNA.