twin data math, clarify Rijsdijk & Sham

[MonDie]

Analytic approaches to twin data using structural equation models.
Rijsdijk & Sham, 2002

http://bib.oxfordjournals.org/content/3/2/119.long

 

 

It's good that I found an authentic source, but I'm very confused.

 

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The 1st confusion arises from Falconer's formula.

 

Why are the equations set equal to the square of h, c, or e? For example:

[math]h^{2} = 2(r_{MZ} - r_{DZ})[/math]

 

I understand that the difference between the covariance of the mono- and di- zygotes equals the heritability estimate x0.5. What I don't get is why h is squared. This seems to recur later in the paper: "the heritability is given by a² / (a²+c²+e²)."

 

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The 2nd and 3rd confusions arise from Path analysis and structural equations, two topics I know nothing about.

 

Why is D (dominant or non-additive genetic influences) 0.25 for dizygotic twins rather than 0.5?

 

 

The genetic covariance between twin 1 and twin 2 is the product of the paths linking the trait scores via A (for MZ: a * 1 * a = a² ; for DZ: a * ½ * a = ½ a² ). The covariance due to C and D can be similarly derived by 'path tracing rules'.

 

This quote just doesn't make any sense to me. What are paths, and why does A occur twice along its path?

Edited November 13, 2014 by MonDie