Adjusting for effects of age and sex in sex-limitation models

Hi,

I think I have found another thread which answers my question above: http://openmx.psyc.virginia.edu/thread/3922.

But I still have the following questions:
1) Is the adjustment for sex effects only necessary when I have both male twins (MZM, DZM) and female twins (MZF, DZF) in the model? In other words, it's unnecessary to add sex as a covariate if there are just MZF and DZF twins in the model?

2) If using independent t-test (or equivalent non-parametric test), I find that there are no group differences in means and variances between MZM and MZF (and similarly, between DZM and DZF), does that mean I can combine MZM & MZF into an MZ group and DZM & DZF into a DZ group and run genetic analysis?

Thanks!
-Yi Ting