# Twin Samples and PRS

**URL:** <https://isgw-forum.colorado.edu/t/twin-samples-and-prs/863>\
**Category:** Day 6\
**Tags:** gcta, r, openmx\
**Created:** [June 10, 2026, 1:31pm UTC](https://isgw-forum.colorado.edu/t/twin-samples-and-prs/863 "2026-06-10T13:31:46Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![anonymous](https://isgw-forum.colorado.edu/letter_avatar_proxy/v4/letter/a/8491ac/32.png) [@anonymous](https://isgw-forum.colorado.edu/u/anonymous)\
**Post date:** [June 10, 2026, 1:31pm UTC](https://isgw-forum.colorado.edu/t/twin-samples-and-prs/863/1 "2026-06-10T13:31:46Z")

</div>

When working with twin simples 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?

> Replying to “When working with twin simples and PGS, what is th…”:

At QIMR we use GCTA with a GRM and add the PRS as a quantitative covariate

> Replying to “When working with twin simples and PGS, what is th…”:

Or you can include a PRS in an OpenMx model or similar

> Replying to “When working with twin simples and PGS, what is th…”:

Thanks Sarah!

> Replying to “When working with twin simples and PGS, what is th…”:

If you post to the forum we’ll share some GCTA code

> Replying to “When working with twin simples and PGS, what is th…”:

Ok, I Will do it thanks

> Replying to “When working with twin simples and PGS, what is th…”:

If you’re interested in incremental-R^2, you don’t need to correct for the relatedness, btw. The incremental-R^2 is not biased, the standard error of the beta is

> Replying to “When working with twin simples and PGS, what is th…”:

I’ve used GEE (generalized estimating equations) correcting SEs for correlated observations

> Replying to “When working with twin simples and PGS, what is th…”:

(package in R)

> Replying to “When working with twin simples and PGS, what is th…”:

yes and that would be after calculating PRS, so in the regression step

> Replying to “When working with twin simples and PGS, what is th…”:

but does it allow to use the full GRM?

> Replying to “When working with twin simples and PGS, what is th…”:

it uses famid, not a grm

> Replying to “When working with twin simples and PGS, what is th…”:

it uses family ID and/or twin ID