Impact of theta correction in siblingship testing

Genetic relationship testing is dependent on appropriate population data, for which there has been a considerable amount of effort invested over the past three decades. This can be evidenced by the large number of population genetic studies that have presented short tandem repeat (STR) data for a wide range of population groups worldwide. However, comparatively little effort has been invested in the measurement of population substructure at various geographical levels and in the investigation of its effect on genetic relationship testing. The concept of population substructure is utilized as a proxy for co-ancestry in kinship calculations and is often corrected for by introducing the co-ancestry coefficient theta (θ). In practice, it is frequently assessed at the national level or for entire subpopulations. However, infrastructure, geographic and sociocultural factors have the potential to significantly impact the estimation of co-ancestry at the local level, leading to larger population substructure in smaller local communities, which may not be adequately addressed by large-scale population studies. Consequently, it can be challenging to accurately estimate the appropriate degree of co-ancestry for a specific genetic relationship testing case. In contrast to the calculations of DNA match probabilities in forensics, there is no conservative approach to account for this uncertainty in genetic kinship testing. In the present paper, we demonstrate that the incorrect choice of the theta correction factor for co-ancestry can have a substantial impact on the outcome of the calculations and therefore potentially on the life of the concerned individuals. The findings of this study have been used to formulate recommendations for the communication of results, with a particular focus on cases where private individuals are involved in administrative or legal proceedings facing state authorities, such as immigration cases.