twin concordance studies
One in every 89 deliveries produces twins. The pair are concordant if they both show a discontinuous trait, and discordant if only one shows the trait. For continuous traits, the degree of the trait, for example, height, is directly compared.
The bias towards reporting concordant twins rather than discordant twins, and the effect of environmental influences, must both be considered. The latter can be partially compensated for by seeking out monozygotic twins that have been reared apart during infancy.
Allowing for similar environmental influence, the rate of monozygotic concordance will be 100% for single gene and chromosomal traits. However, the dizygotic rate will be less than this and equal to the rate in siblings where there are also one-half of all genes in common.
For discontinuous genetic traits with both genetic and environmental contributions, the rate in monozygotic twins will be greater than the rate in dizygotic twins.
Ref: Essential Medical Genetics, J.M. Connor & M.A. Ferguson-Smith, Blackwell's Publishers, 2nd edition.
Last reviewed 01/2018