I was watching an old CSI the other day and there was a case of a man who raped and then strangled a woman that he had picked up hitchhiking. When they examined the body they discovered semen stains, but because the man had had a vasectomy there was no sperm associated with the stains. (At that point I turned off the TV to go shovel snow off my driveway).
Does the lack of sperm mean that DNA analysis of the semen stain could not be used to link the man to the crime scene? Or would it just be harder to do the DNA analysis? Or would it not matter if the man had had a vasectomy or not?
Mods: I didn’t post this in Cafe Society because I felt this would be a better place to get a scientific answer.
This. I’m pretty sure there are cells (and therefore DNA) present in just about any bodily secretion, even if a vasectomy has been done. They just won’t be sperm cells; they’ll be cells sloughed off of the various other surfaces inside the male reproductive tract.
I don’t recall the episode in question, but what typically happens on TV shows in these circumstances is that the person turns out to be one of the small percentage of the population who is a “non-secretor”, ie doesn’t have enough protein in their secretions to generate a usable genetic profile.
I get the impression that non-secretors are a lot more common in cop-show land than they are in reality.
Secretor status refers specifically to ABO blood type antigens in semen/saliva/other body fluids, and has nothing to do with DNA. In the days before DNA analysis, blood type matching provided exclusion criteria for unidentified bodily fluids.
Modern DNA techniques (including things like low copy number PCR) can be used to get DNA profiles from sperm-free semen (from blood and other cells in the semen), using Y chromosome markers to exclude intermixed non-male DNA. Other techniques can be used to generate probable profiles from mixed DNA samples.
I also wonder - It’s one thing to take a swab of saliva - exclusivey a fluid from one person usually pretty fresh. OTOH, a rape kit is looking for a few odd perp cells mixed in with tons of the victim’s cells. Not sure how good that technnology is. Plus, I asume the number of skin cells shed or abraded from the mouth, or the vaginal walls, is probably substantially more than the number shed by the urethra. You never hear of the perp identified from a pee sample DNA, do you?
There is a really funny scene from the NZ drama-comedy Outrageous Fortune that discusses this - they decide that urine contains PeeNA.
Articles like this indicate that urine can be a source of DNA for identification. However, male urine contains fewer nucleated cells than female urine, and thus less likelyhood of DNA. Appropriate procedures did provide DNA from both male and female samples, however.
As for mixed samples (as per a rape kit), isolating markers that only occur on the Y chromosome gives a means of identifying a male contributor. Mixed same gender samples rely on mathematical approaches. This is less definitive than single sample DNA analysis. New DNA sequencing approaches that can sequence a single complete strand of DNA will provide new tools for identifying DNA in mixed samples as well as more definitive DNA identification (using more loci).