When working with twin samples and PGS, what is the preferred approach for accounting for relatedness? Is it sufficient to model family structure using a random effect (e.g., 1 | FAMID), or is it preferable to account for relatedness using the full genetic relationship matrix (GRM)? If the latter is recommended, how is this typically implemented in practice, and which software is commonly used?
How I should model this in OpenMx using the Full GRM?
Are there any software packages that can directly incorporate full GRMs in this context?
Thank you for the previous answers in the chat.