List the problems of classical twin studies (for estimating heritability derived from comparison of monozygotic and dizygotic twins).
- Non-additive genetic variance: ASK
- Epistasis: The effect of a particular allele at a locus may depend on the allele expression at a different locus.
- Random X-chromosome inactivation in female twins: Although MZ twins are nominally identicvcal in their genomes, this will not be true at the level of expression, owing to the random nature of X-chromosome inactivation.
- Assortative mating: Standard model assumes random mating in a population. However, it is often not (eg. similar height people may marry). DZ twins may share more or less than 50% of genes because of this.
- Differences in similarities of environments: MZ twins often have a different environment to DZ twins (MZ twins tend to be closer).