Twin Concordance and Heritability

Heritability is formally defined as the proportion of phenotypic variation that is caused by genotypic variation in a population. Comparing concordance for monozygotic versus dizygotic twins allows an indirect estimate of the importance of genetic factors in producing the phenotype. Heritability estimates based on twin concordance studies can be simplistically viewed as:

*Variance = 1 - concordance rate Adapted from: Thompson and Thompson, 1991 CHECK IN NEWER EDITION

When a phenotype is determined primarily by genetic factors, heritability approaches 100%.

• For a single-gene disorder with complete penetrance such as Huntington disease (an autosomal dominant condition), monozygotic twins should be concordant 100% of the time. Dizygotic twins should be concordant less frequently -- 50% of the time in the case of Huntington disease based on simple Mendelian principles.
  

  Heritability =	.5 - 0	= 1 (100%)
  	.5

When a phenotype is determined primarily by environmental factors, heritability approaches 0%.

• For a condition that is completely determined by environmental factors, concordance should not be any greater for monozygotic twins than for dizygotic twins. The concordance rate for both groups should be essentially equal and will instead depend on shared environment. As a hypothetical example, 10% of monozygotic twins are concordant for death due to an accident and 10% of dizygotic twins are also concordant.
  

  Heritability =	.9 - .9	= 0 (0%)
  	.9

When a phenotype is determined by a combination of genetic and environmental factors, heritability will be somewhere between 0% and 100%.

• Twin concordance data for autism is shown below:
  

  Relationship	Concordance for classic autism	Concordance for broader austic spectrum phenotype
  Monozygotic twins	60%	92%
  Dizygotic twins	0%	10%