Capozzi et al., "Congenital heart disease in a population of dizygotic twins
: an echocardiographic study," International Journal of Cardiology, vol.
Preferential use of the upper limb is important in genetic theories of asymmetry, because of the distinction between monozygotic twins, which are actually genetically identical, and dizygotic twins
, who share on average 50% of genes.
For ASD, probandwise concordance for monozygotic twins was 77% for 45 male pairs and 50% for 9 female pairs; however, concordance rates for dizygotic twins
were 31% for 45 male pairs and 36% for 13 female pairs.
(64) Nichols conducted a metaanalysis of the twin literature, and averaged all of the correlations; he found that the DT scores of monozygotic twins were correlated at .11 more than those of dizygotic twins
, which was not statistically significant--and this was the smallest difference of all measures studied, smaller than general intelligence, memory, verbal fluency, and many other measures.
While resting myocardial blood flow wasn't different in any of the twin pairs, dizygotic twins
with major depression had a 14% lower coronary flow rate response to adenosine challenge compared to their brothers without depression.
In studies of male twins, monozygotic twins (single egg) are more likely than are dizygotic twins
(two eggs fertilized by two different sperm) to be concordant for homosexuality.
However, twin studies have consistently shown low rates of concordance (5-8%) in monozygotic and dizygotic twins
(Foltynie et al.
If major psychiatric disorders have a strong genetic component, monozygotic twins will be highly concordant for the disorder; dizygotic twins
will show some level of concordance but not to the same extent as the monozygotic twins.
There are two kinds of twins--monozygotic twins, who are identical, and dizygotic twins
, who are nonidentical and are no more alike than brother or sister.
No specific suicide gene has been identified. Studies involving twins found higher concordance rates for suicide in monozygotic twins than in dizygotic twins
, meaning that an identical twin would be more likely than a fraternal twin to engage in suicidal behavior if the co-twin committed suicide. However, no study to date has examined the concordance for suicide in monozygotic twins separated at birth and raised apart, a requirement necessary to be met as a means to indicate inheritance of psychiatric illness. Such a study could assess the effects that parental rearing style and familial environment have on suicidal behavior.
Host genetic factors play a major role in determining the outcome of mycobacterial infections in humans, as shown by racial variation in susceptibility to infection and higher concordance of tuberculosis and leprosy among monozygotic twins compared with dizygotic twins
and siblings (50,51).
Identical twins in humans (not the same as fraternal or dizygotic twins
) are natural clones.