A question regarding DNA evidence that has seemed to me to exist relates to the way that such evidence is generally presented.

Typical testimony is based on a "match" of some half-dozen to a dozen genetic "markers" that match in two separate samples.

The "mathematics" cited is that only 300 persons in a population of 1,000,000,000 would have those same markers, so "odds that the two samples came from the same person are 1 in 3,000,000, which is likely to convince most juries.

People having similar ethnic origins, however, tend to have similar sets of "markers" at the level of common evidence testing, and the percentage of possible "matching markers" is additionally affected if only persons genetically related to the suspect(s) are considered.

So what happens if all of the 300 "possible matches" live in the same town where the crime occured. In a small town of perhaps 14,000 population, 300 possible matches gets about 1 in 50 persons who could be "identified" as the criminal.

Not very good odds.

LiK's obsession with her dead relatives has led her to look for some of mine, and her current results show I probably have a a direct genetic relationship to about 80% of the people who lived in the small Kansas town, population about 14,000 then, where I was born (even though I never actually lived there and most of those "relatives" have names I don't even recognize).

A recent report that takes a different line of argument, too long to quote here, suggests that at least a few people recognize a "trend" that should admit the possibility of "bias" that could affect the validity of some "DNA evidence" as now used:

Family DNA searches hold potential for racial bias, study says
Tia Ghose LiveScience
14 August 2013
Gaston De Cardenas / Reuters

The problem of limiting the evidence to valid matches is magnified with a "inbred" population - like cats, dogs, and/or race horses (or small town Lutherans, Mormons, et. al.?). Identifying a larger number of "markers" in each sample can reduce the odds of false identifications, but not all who testify use a really sufficient number of characteristics for the positive conclusions claimed - especially, as the article notes, - in "ethnicly/familialy segregated" locations.

John