Sex-limitation models Karri Silventoinen

Testing sex-specific genetic effect It is possible to test whether the size of genetic and environmental variations is similar in men and women using information on same- sex twins only However, this does not answer to the question whether there are the same genes affecting the trait in men and women If we have information on opposite-sex twin pairs, we can study sex- specific genetic component In practice we expect male or female specific genetic component, but it is not possible to actually test whether the sex-specific genetic effect is specific to male of female sex Usually we analyze sex-specific genetic effect, but it is possible to test instead whether there is sex specific common environmental component, i.e., environmental factors affecting one sex only  In practice it is impossible to separate sex-specific genetic and environmental effects

Opposite-sex DZ twins AE Twin male AmAm AE Twin female 0.5

Same-sex DZ twins AE Twin male AmAm AE AmAm 1 / 0.5

Sex-specific genetic factors If there exists sex-specific genetic factor, it should be seen as lower correlation within OSDZ than SSDZ twins Thus in practice, we test whether additive genetic correlation in OSDZ pairs is lower than 0.5 It is important to note that sex-limitation model tests how genetic factors affect variance but not mean This is important for all genetic modeling Results on the effect of genetic factors within groups cannot be generalized to between group comparisons

Example: height difference between men and women Men are taller than women Is this because of genetic differences?

Within pair correlations for same and opposite sex DZ twins DZ malesDZ femalesOS twins Australia Denmark Finland (old) Finland (young) Italy Netherlands Norway Source: Silventoinen et al., Twin Res 2000 and 2003

Example: height difference between men and women Men are taller than women Is this because of genetic differences? There is no significant difference between correlations within SSDZ and OSDZ twin pairs There is no sex-specific genetic factor affecting height Is this a paradox?  It is not because it just tells us that the genes affecting height differences between individuals are the same in men and women  Probably because of hormonal differences, the function of the genes is different in men and women leading to taller average height in men