The classic twin design aims to quantify the roles of genetic and environmental causes of variation in traits and in disease susceptibility.
In 1918,in his mid-20s, a twin called R. A. Fisher famously showed how the correlation between relatives (r) relates to A, C and E:
rMZ = A + C
rDZ = 0.5 A + C
Heritability = % of variation explained by genes
H = A / (A + C + E)
H = 2(rMZ – rDZ), provided H < rMZ
This equation assumes that MZ and DZ pairs share – to exactly the same extent – the non-genetic (environmental) factors specific to the characteristic of interest (C).
If rMZ > rDZ , then genetics might play a role.
Advantages (not just heritability!)
Limitations of classic twin approach
Fisher, R. (1918). The Correlation Between Relatives On The Supposition Of Mendelian Inheritance. Transactions of the Royal Society of Edinburgh, 52, 399-433.
Hopper, J. L. (2005). Genetic Correlations and Covariances. Encyclopedia of Biostatistics.
Boyd, N. F., Dite, G. S., Stone, J., Gunasekara, A., English, D. R., McCredie M. R. E, Giles, G., Tritchler, D., Chiarelli, A., , Yaffe, M. J. and Hopper, J. L. (2002). The New England Journal of Medicine, 347(12), 886-94.