About the Slide 8 intuition behind the problem from The Direct Symmetric Matrix Approach to Fitting Twin Models

Why the mean correlation between MZ is 0.5 and DZ is 0.25?
I thought that was 1 in MZ and 0.5 in DZ.

This slide is an example of a phenotype where MZ pairs correlate phenotypically about 0.5 and the DZ correlate about 0.25.

When we are fitting ACE models, then we are talking about the genetic similarity, and the MZ pairs correlate 1 and the DZ pairs correlate 0.5