Twins, monozygotic

Schizophrenia is not a neurodegenerative disease but there is some general neuropathology. There is also evidence for a genetic influence. In monozygotic twins with... 

MS probably has a genetic component there is a 5% risk for family members of MS patients. Monozygotic twins, who share the same genetic material, are at highest risk, with a 25% to 30% concordance rate.4 However, a straightforward inheritance pattern cannot fully explain the etiology of MS, because only a small proportion of patients have a known family member with MS.4... 

Results of family and twin studies suggest a genetic basis for bipolar disorder.4 The lifetime risk of bipolar disorder in relatives of a bipolar patient is 40% to 70% for a monozygotic twin and 5% to 10% for another first-degree relative. 

Studies conducted in twins show a threefold increased risk of psoriasis in monozygotic twins versus fraternal twins.7 In addition, based on a study in 3,095 families with psoriasis, the calculated lifetime risk of developing psoriasis if no parent, one parent, or both parents have psoriasis was found to be 0.04, 0.28, and 0.65, respectively. If there was already one affected child in the family, the risks were increased to 0.24, 0.51, and 0.83, respectively.7,9 As many as 71% of patients with psoriasis during childhood have some positive family history.1 Similarly, psoriatic arthritis is heritable, with a prevalence 19 times higher in first-degree relatives of patients with psoriatic arthritis than in the general population.7... 

Hammond, BR, Fuld, K, and Currancelentano, J, 1995. Macular pigment density in monozygotic twins. Invest Ophthalmol Vis Sci 36, 2531-2541. 

The concordance rate of mood disorders is 60-80% for monozygotic twins and 14-20% for dizygotic twins. Linkage studies suggest that certain loci on genes and the X chromosome may contribute to genetic susceptibility of bipolar disorder,... 

Figure 1. Path diagrams for (a) parallel form reliability, (b) monozygotic twins reared apart, (c) unrelated individuals reared together, and (d) monozygotic twins reared together.

The correlation is not squared. Figure lb illustrates the correlation for a trait, represented by the boxes, between monozygotic twins reared apart (MZAs). 

Figure Id shows the correlation between monozygotic twins reared together. We have simply added a latent variable to represent common or shared environmental influence (C). This model represents the obvious fact that monozygotic twins reared together (MZTs) can be similar for both genetic and environmental reasons and that the correlation is confounded. Thus the equation ...

Table 5. Intraclass correlations, confidence intervals, samples sizes, and test utilized for IQ in five studies of monozygotic twins reared apart...

Contrary to Staples claim, the evidence for genetic influence on IQ has become much stronger in recent years and a number of important discoveries have been made. I summarize the key findings in a few tables and figures. Table 5 shows the most direct evidence regarding the heritability of IQ, the correlations for monozygotic twins reared apart.2... 

Juel-Nielsen, N. (1980). Individual and Environment Monozygotic twins reared apart (revised edition of 1965 monograph). New York International Universities Press. 

Shields, J. (1962). Monozygotic twins Brought up apart and brought up together. London Oxford University Press. 

Odor thresholds in humans appear to be more influenced by the environment than genetics. The detection thresholds for acetic acid, isobutyric acid, and cyclohexanone varied as much in monozygotic twins as in fraternal twins. Instead, smoking and diabetes were related to lower olfactory sensitivity, and body fatness and alcohol consumption to greater sensitivity to the latter two compounds (Hubert eta/., 1980). 

Kuratomi, G., Iwamoto, K., Bundo, M., Kusumi, I., Kato, N., Iwata, N., Ozaki, N. and Kato, T. (2008) Aberrant DNA methylation associated with bipolar disorder identified from discordant monozygotic twins. Molecular Psychiatry, 13, 429-441. 

Carlsson, E, Poulsen, P., Vaag, A., Stephan, Z., Spector, T.D., Wu, Y.Z., Plass, C. and Esteller, M. (2005) Epigenetic differences arise during the lifetime of monozygotic twins. Proceedings of the Natioruil Academy of Sciences of the United States of America, 102, 10604-10609. 

Fig,1, An [ F]FDG-PET scan depicting glucose metabolism in a monozygotic twin pairand in a control subject. Note the clear reduction in [ F]FDG uptake especially in temporoparietal areas (thick arrows) in the cotwin having AD (on the left), and similar, but less pronounced reduction in same brain areas (thin arrows) in the cognitively healthy cotwin (in the middle) as compared with the uptake in a control subject (on the right), (See Colour Plate Section at the end of this book,)... 

T. Viljanen, K. Ahonen, J.O. Rinne, Regional cerebral glucose metabolism in monozygotic twins discordant for Alzheimer s disease. Dement. Geriatr. Cogn. Disord. 16 (2003) 245-252. 

McNeil, T.F., Cantor-Graae, E., and Weinberger, D.R. (2000) Relationship of obstetric complications and differences in size of brain structures in monozygotic twin pairs discordant for schizophrenia. Am J Psychiatry 157 203-212. 

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