Combining autosomal and Y chromosome match probabilities using coalescent theory

Walsh et al. [1] outlined a method for adjusting autosomal coancestry values, θA, to take account of the existence of a Y chromosome match, θA|Y. The framework established by Walsh et al. is flexible and allows an investigation of some real world effects such as family structure. It also allows the effect of a Y chromosome match to be placed within the construct of existing casework practice. Most notable is the ability to deal with an assigned value for the autosomal coancestry coefficient and the fact that most casework statistics report a value for unrelated individuals unless case circumstances suggest differently. The values of θA|Y are not much larger than θA and a coherent argument could be made that any adjustment is unnecessary.