Corrected entry: The scientists claim they procured blood samples on the Prison colony that allowed them to recreate Ripley. If you used blood to clone Ripley, then you'd get a clone of Ripley without a chest-burster inside her. The chestburster doesn't alter DNA.
Rlvlk
6th May 2006
Correction: 1) I don't recall any DNA blood tests being given in any of the previous movies, so you don't really know if the chest burster does or does not alter the DNA in any way. 2) In an inscet colony, any egg laid has the potential to be a queen. When a new queen is needed, certain eggs are treated differently to produce a queen instead of a worker. Since Ripley was implanted by a queen, maybe the intent was to produce another queen and who knows what happens to a person's DNA implanted with a queen.
Rlvlk
There are different theories about how the facehugger actually impregnates its host. It may implant an embryo, or it may inject a mutagenic liquid that causes host's own cells to transform into alien embryo. The latter would also explain why aliens inherit the traits of their host, and would also explain why the blood sample resulted in cloned Ripley with embryo inside.