  Y-chromosomal and mitochondrial DNA profiles have been used as evidence in
courts for decades, yet the problem of evaluating the weight of evidence has
not been adequately resolved. Both are lineage markers (inherited from just one
parent), which presents different interpretation challenges compared with
standard autosomal DNA profiles (inherited from both parents), for which
recombination increases profile diversity and weakens the effects of
relatedness. We review approaches to the evaluation of lineage marker profiles
for forensic identification, focussing on the key roles of profile mutation
rate and relatedness. Higher mutation rates imply fewer individuals matching
the profile of an alleged contributor, but they will be more closely related.
This makes it challenging to evaluate the possibility that one of these
matching individuals could be the true source, because relatedness may make
them more plausible alternative contributors than less-related individuals, and
they may not be well mixed in the population. These issues reduce the
usefulness of profile databases drawn from a broad population: the larger the
population, the lower the profile relative frequency because of lower
relatedness with the alleged contributor. Many evaluation methods do not
adequately take account of relatedness, but its effects have become more
pronounced with the latest generation of high-mutation-rate Y profiles.
