Good morning.
A razor nick opens back up due to rubbing on a pillowcase and leaves a dime sized spot of blood. The pillowcase is laundered with detergent and fabric softener and then put into the drier with the rest of the linens.
The pillowcase, being used in the regular rotation of the bed linens, goes through this cycle a few times over the period af a few months.
Could DNA be extracted after four or five washings?
(No, I do not need this answer fast. Just random curiosity)
I think the only factual answer is ‘it depends’. While it’s theoretically possible that some DNA might persist under these conditions, I don’t think it’s very likely. There are a number of variables here, such as the detergent concentration and the dryer temperature.
While our ability to detect DNA improves over time, keep in mind that DNA degrades over time, even if you just leave it alone.
And how badly they want the DNA. DNA’s been isolated from mummies, but you wouldn’t believe how difficult that was to do. Every piece of equipment used in the procedure had to be exposed to intense UV light for days first, for instance. So if the DNA was really, REALLY important, there are techniques that wouldn’t normally be used that might increase the chance of recovery somewhat.
In short, probably not.
Red Blood Cells do not have nuclear DNA. Any DNA wold be recovered from the much smaller White Blood Cell fraction or any other cells. So there isn’t as much there to begin with as you might imagine.
Secondly, laundry detergent is very good at rupturing cell membranes. Each washing subjects the sample to conditions that would tend to cause the White Blood Cells to lyse and spill their contents. Any DNA released would get rinsed away in the wash.
So whatever you’re planning longhair75, you likely won’t get caught. 
Most US laundry detergents have enzymes specifically designed to dissolve and remove proteins so the chances of DNA proteins surviving a thorough machine wash that involved detergents is pretty poor.
DNA isn’t a protein and as such wouldn’t be affected by proteases
Interesting, I thought proteins were somehow critical to the integrity of DNA. Ignorance fought.
DNA is packaged by proteins called histones which comprise chromatin. They act as something like a spool around which DNA threads are wound.
A protease from laundry detergent will cleave histones. Freed DNA then tends to form threads that can precipitate in certain conditions forming gooey stringy strands.
You can extract DNA in your kitchen with basic household supplies.
Another problem I see is obtaining DNA from only the bleeding person. Unless there’s only one person in the household and they do all their own laundry, DNA picked up on the pillow could be from anyone who lives there.
Not interesting to mods here.
reported.
The dryer would also do a lot of damage. 5 washer/dryer cycles? Nothing’s going to be left.