Any of a series
projects carried out since the nineteenth century work of Francis Galton that attempt to estimate the heritability
of a complex
phenotypic trait. Such studies operate under the assumption that all twin pairs
are of only two modalities (monozygotic twins, MZT, or dizygotic twins
, DZT), that twin zygosity
can always be correctly ascertained, and that the average proportion of genes
shared by MZT twins is 100% whereas for DZT twins it is 50%.