Every band in a DNA mix needs a source
- The idea
- Predict
- See it
- Why
- Check
The idea
“If all of a suspect's bands show up in the mixed sample, that suspect could be the other contributor.”
When every band a suspect has turns up in the sample, it looks like a match. Any band left over still has to come from someone.
The short version
The mistake: If all of a suspect's bands show up in the mixed sample, that suspect could be the other contributor.
What's actually true: The contributors together must explain every band in the mix, and every band each one has must appear in it.
- At one locus, each chromosome copy gives a person one band, so most people show two. A mixed sample shows everyone's bands together.
- First, take away the known person's bands. The suspect has to cover every band that's left.
- Then check that every band the suspect has is in the mix. One band the mix lacks rules them out.
On the exam: Check both ways. Every mix band needs a source, and every band a source has must be in the mix.