If someone donates blood, and that blood is transfused into someone
who later commits a crime that leaves blood evidence at the scene,
will the DNA of the donor be at the crime scene or only the DNA of the
person who committed the crime?

Dear mr_j-ga,

Depending on the type of blood transfusion, it is possible that two
separate DNA samples may be left, but unlikely. The DNA of the donor
would not be the only DNA found.

This is how the Canadian National DNA Databank answers a similar question.
 
"Blood Transfusion/Bone Marrow Donations 
Q: If an offender has had a blood transfusion, will two DNA profiles
appear in any biological evidence he/she has left at a crime scene?

A: It is possible, but unlikely. It will partly depend on the type of
transfusion received. If a person has received only red blood cells
(which do not contain DNA), there will be no confusion. If the person
has received white blood cells or platelets (both of which contain
DNA), there could be a mixture of recipient DNA and donor DNA in the
sample. Upon analysis, both DNA profiles would be identified. They
will not "mix" to create one, new profile.

Keep in mind that the human body will replace all transfused blood
cells within three or four weeks. If the transfusion was minor, the
recipient's original DNA profile would be clearly prominent. If the
transfusion was major, the recipient will probably be too ill to
commit a serious crime before their body has replaced most of the
donor cells.

In those rare cases where two DNA profiles are discovered in a sample,
it is always noted and the analysis double checked. Investigators are
also notified for further follow up as required."
http://www.nddb-bndg.org/updates/n_5_e.htm


At a National Law Enforcement Summit on DNA Technology, held at
Washington, D.C.  one of the attendees, Dr Foreman, answered a similar
question.

"DR. FORMAN: There have been many studies that have been done on
whether or not blood transfusions change a person's DNA. You can
certainly identify when a person has a fresh transfusion, but their
own DNA profile does not get removed. As the days post-transfusion go
forward the transfused blood profile does get removed from the blood,
so there really would not be an extreme concern about this. You might
get a confused answer to begin with. You would see the person's own
profile and the transfused profile originally, but that fades fairly
quickly."
http://www.ojp.usdoj.gov/nij/dnasummit/trans-7.html


Using the search term below, it appears to be a standard question to
people when giving DNA samples, if they have had a transfusion in the
previous 90 days, as this may interfere with the results. For
instance, from a training manual for police officers.
"Finally, the crime lab will need to know whether the victim or
suspect received a recent blood transfusion (i.e., in the last 90
days) that might interfere with the DNA results."
(page 18)
http://www.denverda.org/legalResource/DNA%20Evidence%20&%20Issues.pdf

A similar answer is given on Straightdope pages. The author and their
qualifications are not provided.
http://www.straightdope.com/mailbag/mblood.html

A slightly different question on the death of a transfusion victim is
discussed here.
http://www.newton.dep.anl.gov/askasci/mole00/mole00351.htm


I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder

dna "blood transfusion" crime
://www.google.com/search?hl=en&lr=&ie=UTF-8&q=dna+%22blood+transfusion%22+crime
dna "blood transfusion" crime "90 days"
://www.google.com/search?hl=en&lr=&ie=UTF-8&q=dna+%22blood+transfusion%22+crime+%2290+days%22

Very thorough and answered something that I had always wondered about.

Another interesting aspect of this question would be people who are
chimeras or mosaics, that is, people with two distinct genetic lines.
It is theorized that these are people who were, as zygotes,
non-identical twins, with one twin being dissolved and absorbed by the
other. Calico cats are visible examples of chimeras, as are animals
(or people) with two different colored eyes! I have heard of
interesting criminal DNA results found on chimeras, such as the "perp"
showing one DNA pattern for hair, yet a completely distinct DNA
pattern for blood, as if they had come from different people! This is
very rare however, and usually goes undetected by the individual,
unless a need for genetic testing arises.

Dear mr_j-ga,
Thank you for the tip. I don't say this often, but once and a while I
research a question which I should have asked myself - this was one of
those. Thank you.
answerfinder-ga

It is my belief that there is a much simpler (and correct) answer to
this question. If an individual who has received donor blood sheds
blood at a crime scene, DNA Fingerprinting (by gel electrophoresis)
will show their DNA and their's ONLY.

This is because donor blood contains only red blood corpuscles and
blood plasma NOT leucocytes (white blood cells) which contain DNA. Red
blood corpuscles (erythrocytes) do not contain DNA because they are
produced in the bone marrow, they do not self replicate. This is
likely to be due, in part, to evolution favouring maximal space within
the erythrocyte for the transport of oxygen and waste carbon dioxide.

Yours evolutionary! M