The relative importance of genes vs the environment in determining phenotypes can be seen in twin studies where the presence of the trait in question is measured among identical twins and fraternal twins. Identical twins have identical genotypes and if a trait is influenced primarily by genes, then one would expect both identical twins to display that trait (high concordance) while the fraternal twins with different genotypes would have a low concordance for that trait. If the environment plays a major role in the appearance of the trait, then concordance between the identical and fraternal twins should be about the same. To analyze whether identical twins are significantly different in their concordances and discordances from fraternal twins, a chi-square test for independence can be performed. Click on any one of the conditions listed in the table at the right to perform this type of
