I'm afraid I have no idea what kind of DNA they're using, Charlot. I just know that it was in 2005 that this information came out about the killer's heritage. The one paragraph I'm certain about is what the mainstream media have said about it:
DNA analysis has revealed that traces of blood (type A) found at the scene not belonging to the family suggests that the killer has a mother of European descent, possibly from a country near the Mediterranean or Adriatic Sea. Analysis of the Y-chromosome has revealed that the killer's father is of Asian descent, with the DNA appearing in 1 in 4 or 5 Koreans, 1 in 10 Chinese, and 1 in 13 Japanese. He is believed to be about 170 cms tall and of thin build.
Much depends on when they did ancestral DNA testing.
I looked up. In 2000, ancestral tests that were available were 11 markers of Y-chromosome STR test and mitochondrial HVR1 (part of modern full mito) test.
First use of autosomal DNA for ancestral tests was started by 23@me in 2007.
So if the first statement of Tokyo police about the murderer’s ethnicity was made before 2007, chances are, they tested Y and mitochondrial DNA.
The potential mistakes in interpreting the results in this case are obvious.
Both mitochondrial DNA and Y might be passed unchanged for generations and generations. Of course, they are subject to mutations, but when they happen, with what frequency, is random. So if mitochondrial DNA indicates that someone in the maternal line was from Adriatic region, it could have been 500 years ago.
Same with Y. Let us assume it is Asian. But it is passed from grandpa to father to son…looking at Y, you can not assume that the man is Asian. You can only say that one of direct male forbearers was Asian. Maybe 20 generations ago.
Now, if the statement about the man being European on maternal line and Asian on paternal was made later, closer to 2010, it becomes more interesting. It is possible that by that time, the researchers were able to study the man’s autosomal DNA.
Let us imagine, that autosomal ethnicity is 50/50 European/Asian (IRL, close to 52/48 in men as Y chromosome is very small). And X chromosome, that a man gets from his mother, is all European. And Y is Asian. Then, they are likely right. Mother European, father - Asian. If the European component is only ~ 25% (could be 18-30 or close to it), then, it was likely one grandparent, or maybe, two great-grandparents. (It becomes more complicated, as, after all, both parents could be half-Asian, half-European, but in principle, autosomal testing could tell more about maternal/paternal ethnicity.)
P.S. Sorry, I missed 2005. In 2005, chances are, they had only Y 11-STR and part of mitochondrial genome. I left the part about autosomal DNA, because it might still be relevant. Today, they can find out a lot more, from the same DNA )).