Now that the OP is taking a moment to calm down and come back to reality, they might have some space to learn a little about how DNA tests work.
For starters, DNA testing is a well researched and well-established science, developed by professionals and performed by professionals.
No one takes a sample from a transplanted hand or leg. (This sounds like an old wive's tale or something.) Good sources of DNA are blood and saliva, but a very common source of good cells containing DNA is a cheek swab. Cheek cells are large and easily sloughed off. Also, not a lot of transplanted cheeks out there, so little chance of getting the wrong sample.
They don't get a single result from a single cell. They get many results from many cells. And they multiply those small samples, until they have millions of samples.
The test analyses take into account the possibility of multiple DNA sets in any given sample. This accounts for both accidental contamination as well as the exceedingly rare incident of the patient having a transplant or some other form of pollution of their DNA.
There are a wide range of checks and balances, including
- interviewing the patient beforehand, say, to check if they have someone else's leg,
- taking multiple samples from multiple locations,
- rigorous analysis of the results, ruling out weak hits from strong hits
- repeating tests in the rare case of insufficient sample size, indeterminate results or ambiguity.
DNA testing - especially direct paternal/maternal relationships - are typically accurate down to parts-in-a-billion or thereabouts. It is safe to say it is a little more accurate than asking your momma if daddy is really daddy.
This all happens reliably - whether or not momma, daddy, or child believe in a god or gods of their choosing. In other words, even heathens and infidels can find out who their father is.