| Classical twin study | Twins + siblings model |
---|---|---|
Genetic model | Additive and dominance terms only: VDZg = 1/2VA+1/4VD | Variable: VDZg = pDZgVg with 0 < = pDZg < = 1/2 |
Shared twin environments | Equal environments assumption: cMD = 1 | Variable: 1 < = cMD < = RMD cMD = RMD implies Vg = 0 |
Shared sibling environments | Siblings not included. | Variable: 0 < = cSD < = RSD Familial aggregation may be due to genes (cSD = 0) or environment (cSD = RSD). |
Gene-environment interactions | None | Variable: Vge = f2ge· Vg· Ve/r2t -ε/(1-ε)PAFe < = fge < = 1/PAFe |
Gene-environment correlations | None | None |
Method | Total phenoptypic variance given by: VP = Vg+Ve VP is input and a single solution for Ve and Vg calculated. Heritabilities are given by: H2 = Vg/VP h2 = VA/VP | Ve and ε are input and Vg and Vge calculated, for a chosen cMD and all possible values of fge and pDZg. Method is not valid if RSD = 1. |