Uses individual level genetic and exposure data to generate covariance matrices for estimated effects of individual genetic variants on each exposure. The function returns a number of covariance matrices equal to the number of SNPs, where SNP and row numbers reference ordered exposures.

## Arguments

- Gs
A matrix or dataframe containing genetic instrument measures. Columns should indicate genetic variant number, with rows representing an observed measure of the genetic variant.

- Xs
A matrix or dataframe containing exposure measures. Columns should indicate exposure number, with rows representing an observed measure for the given exposure.

## Value

A list of covariance matrices with respect to each genetic variant, retaining the ordering in `Gs`

## References

Sanderson, E., et al., An examination of multivariable Mendelian randomization in the single-sample and two-sample summary data settings. International Journal of Epidemiology, 2019, 48, 3, 713-727. doi:10.1093/ije/dyy262