Hi all,

I am trying to fit a univariate model.

I fitted the saturated model first and I got correlations of 0.64 and 0.34 for MZ and DZ respectively.

However, when I fitted the ACE model, I got these results:

A=0.04

C=0.52

E=0.44

So, I decided to check the correlations from the ACE model and I got 0.56 and 0.54 for MZ and DZ respectively.

I know that correlations may change from the ACE to the saturated model. However, these differences are huge.

I have tried with different scripts, umx and with/without covariates and I get the same results.

My sample comprise 100 pairs and there are no missing data.

Do you know why there are so big differences between the saturated and the ACE model?

Thank you so much in advance.

I agree that the discrepancy between the saturated-model and ACE-model results seems too big to be real. I notice that the phenotypic correlations suggest a substantial

a² and a near-zeroc², but the ACE results are the opposite of that. Have you perhaps made an error such that some of your MZ twins are being treated as DZ twins, and vice versa?You might as well post your full script, preferably as an attachment. I would also like to see the model-expected MZ and DZ

covariancematrices from the saturated model.Hi Rob,

Thank you so much for your prompt response.

Please find attached the saturated and ACE scripts.

Here the model-expected covariance matrices from the saturated model.

> fit$MZ.covMZ

SymmMatrix 'covMZ'

$labels

[,1] [,2]

[1,] "vMZ1" "cMZ21"

[2,] "cMZ21" "vMZ2"

$values

[,1] [,2]

[1,] 0.057890691 0.043141926

[2,] 0.043141926 0.078377175

$free

[,1] [,2]

[1,] TRUE TRUE

[2,] TRUE TRUE

$lbound

[,1] [,2]

[1,] 1e-24 0e+00

[2,] 0e+00 1e-24

> fit$DZ.covDZ

SymmMatrix 'covDZ'

$labels

[,1] [,2]

[1,] "vDZ1" "cDZ21"

[2,] "cDZ21" "vDZ2"

$values

[,1] [,2]

[1,] 0.0333631015 0.0099068022

[2,] 0.0099068022 0.0260063531

$free

[,1] [,2]

[1,] TRUE TRUE

[2,] TRUE TRUE

$lbound

[,1] [,2]

[1,] 1e-24 0e+00

[2,] 0e+00 1e-24

I am using the same file for both scripts so I do not think some MZ twins are being treated as DZ twins.

Let me know if you need something else.

Thank you so much for your help.

The phenotypic variance among MZ twins is about twice that among DZ twins. Is there a reason for that? Anyhow, that's a pretty important piece of information. Keep in mind that the ACE model imposes equal phenotypic variances for both zygosity groups. I don't think you should try to interpret standardized parameter estimates from the ACE model with this dataset. In fact, I'm not sure the ACE model should be interpreted at all in this case. How does it compare to the non-biometrical models in terms of fit? What do the raw (i.e., unstandardized) variance components look like?

Hi Rob,

Thank you so much for your response. I do not know why the variance is bigger in MZ twins. The sample is not too big so maybe that is the problem.

Here you can find the outputs. Please let me know if you need something else.

1-Comparison

2-Saturado

3-ACE

I don't think it means very much for the saturated model to fit better than the ACE model. The saturated model is usually kind of "silly", at least in cases where the order of twins in a pair is arbitrary. How do your

`fitEMVO`

and`fitEMVZ`

compare to the ACE model?In small sample sizes, variance differences sometimes occur due to outliers. Are there perhaps outlier observations in the MZ data set? However, the variances of both MZ twin 1 and MZ twin 2 are both about twice those of DZ twins, so one might expect both members of the pair to be outliers. What is the phenotype? For some traits, we observe contrast effects - parents ratings of their children’s activity for example.

There are models for sibling interaction that predict different variances. Here the phenotypes of the twins directly influence each other. If there is genetic variation, the sibling interaction generates greater variance in MZ pairs than DZ. But I would inspect the data for outliers first.

Thank you Rob and Mike,

Here the comparisons:

I have removed 3 twin pairs that were outliers (all MZ and one of them with high means in both members of the twin pair)

Here the results without outliers:

And now the correlations are:

SATURATED: MZ=0.47 y DZ=0.34

ACE: MZ=0.43 y DZ=0.39

Comparison between ACE and fitEMVO/fitEMVZ

*The phenotype is an objective measure of the eye.

Thank you so much for your helpful comments.