To the best of our knowledge, there have been no previously reported cases of
dizygotic twins concordant for both CHD and DS.
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.
More than half of monozygotic multiple births are monochorionic, which may lead to a greater risk of adverse outcomes than for
dizygotic multiple births (Vaag and Poulsen 2007).
A genetic and environmental analysis of the vocational interests of monozygotic and
dizygotic twins reared apart.
Third, data from twin studies show significantly greater concordance in myopia rates among monozygotic compared to
dizygotic twins.21 Finally, genetic studies have identified several loci for certain pathological variants of myopia22.
(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.
"Our study provides evidence that the rate of concordance in
dizygotic twins may have been seriously underestimated in previous studies, and the influence of genetic factors on the susceptibility to develop autism overestimated.
There is a strong genetic tendency, 75 per cent of monozygotic twins are concordant for the disease, compared to 10 per cent of
dizygotic twins; 10 per cent of first degree family relatives of an index patient have celiac disease8.
Thus, although twin study data support a significant role for genetics in the etiology of sexual orientation (concordance rates for
dizygotic twins--who share 50% of their genes on average--were 22% for men and 16% for women), they also clearly indicate that factors other than genes are at play.
(1-3) Suggestions of a genetic association include studies that have revealed that the concordance of clubfoot is about 33% for monozygotic twins versus only 3% for
dizygotic twins.
With time it became clear that this second sculpture court was in fact laid out as a mirror image of the first; each sculpture in the first room had a
dizygotic twin in the second that had been placed in a bilaterally symmetrical relation to the implied seam bisecting the gallery.
* Similarity: members of a population can fall into one of the two similar pairs; for example, twins can be either
dizygotic (fraternal or nonidentical) or monozygotic (identical).
