%0 Journal Article
%T Response to: DNA identification by pedigree likelihood ratio accommodating population substructure and mutations
%A Thore Egeland
%A A Philip Dawid
%A Julia Mortera
%A Petter Mostad
%A Andreas Tillmar
%J Investigative Genetics
%D 2011
%I BioMed Central
%R 10.1186/2041-2223-2-7
%X Please see related letter: http://www.investigativegenetics.com/content/2/1/8 webcite.In a paper in Investigative Genetics, Ge, Budowle and Chakraborty [1] consider DNA identification by pedigree likelihood ratio (LR). A mutation model 'to accommodate the possibility of false exclusion' is presented. The model is explained on page 5: 'The transmission probability of two identical allele (sic) is 1- ¦Ì. The probability of a mutation event with x step (sic) (x > 0) iswhere ¦Á is the probability of being a one-step mutation and ¦Ì is the mutation rate of the locus. Equal probabilities for gaining or losing repeats are assumed.'Apparently equation (8) does not define a probability distribution since summing over x givesBelow we interpret 'Equal probabilities for gaining or losing repeats are assumed' to mean that a scaling factor 1/2 should be inserted on the right hand side of equation (8) since this leads to a proper probability distribution summing to 1.There are several problems with this model. Most importantly, it allows for alleles with zero or negative repeat numbers which is not meaningful. Furthermore, this may also be a practical problem. For instance, using the mutation model for marker THO1 having allele value of three repeats leads to an allele with a value less than or equal to zero with probability 1.25 ¡Á 10-6. While this probability, based on parameter values ¦Á = 0.95 and ¦Ì = 0.001 suggested in [1], is small, it is certainly not negligible. Unreasonable results will occur if the model is applied to a sufficiently large number of cases. The model is, therefore, inconsistent both from a biological and practical point of view. There are several ways of overcoming these inconsistencies. However, reasonable modifications may well lead to models that have already been published and implemented. One example of a consistent formulation is summarized by equation (1) in [2]. This latter model is stationary and so population allele frequencies are not altered by the
%U http://www.investigativegenetics.com/content/2/1/7