[Lewis Thomson (OP)]

Paul asks:

I have just had a bowl of sour stewed plums (with yoghurt to try to sweeten it up a bit) for breakfast. If I send off a spit sample for DNA now, or if I have a curry meal for dinner this evening and then send off a spit sample, or if I go and brush my teeth and then send off a spit sample, what are the chances that the acid in the plums or the curry or something in the toothpaste may affect the DNA sample extracted from my spit?

What do you think?

[evan_au]

Most publicly-accessible DNA scans today do not look at the whole genome - they look for specific stretches of human DNA that vary a lot between human individuals or ancestral human populations. These tests will not be distracted by samples of plum DNA or goat DNA.

However, in whole-genome DNA reading, efforts must be made to avoid contamination with foreign DNA from bacteria or meals. First all DNA is chopped into shorter strands for reading, with any foreign DNA mixed in too. These short strands are then reassembled into chromosomes by comparing them to a human reference genome; bits of plum, bacterial or goat DNA do not fit onto this template, and can be rejected.

See: https://en.wikipedia.org/wiki/Genealogical_DNA_test

Information about prehistoric diets has been extracted from DNA trapped in dental plaque.
By regularly cleaning your teeth, you are depriving future generations of palaeontologists of important dietary data about our society.
https://www.nature.com/news/neanderthal-tooth-plaque-hints-at-meals-and-kisses-1.21593