On the Autistic Spectrum Disorder concordance rates of twins and non-twin siblings
Introduction
In studies of Autistic Spectrum Disorder (ASD) it is reasonable to assume, on the basis of genetics, that if the first born child in a family is autistic the probability that the second child is also autistic should be the same for fraternal twins as for non-twin siblings. In this paper we present our calculations of the Autistic Spectrum Disorder (ASD) concordance rates for twins and non-twin siblings. Data used in the preparation of this article were obtained from the Interactive Autism Network (IAN) Research Database at the Kennedy Krieger Institute and Johns Hopkins Medicine-Baltimore, sponsored by the Autism Speaks Foundation, version dated August 16, 2010. For up-to-date information see www.ianproject.org. After outlining our methodology, the results are set forth with accompanying explanations. Following that section, we discuss interpretations arising from our analyses and results.
Section snippets
Methodology
For clarity, we begin by setting down the mathematical formulations of several different definitions of concordance rates. Consider the following example for the concordance rate of non-twin male siblings: Suppose that N sets of parents each have a first-born son; then if 1% of these are affected by ASD the number of ASD children is n (x1) = 0.01N where x denotes ASD and the subscript refers to the birth order within a family. The number of unaffected males is n (o1) = 0.99N where o denotes
Results
Table 1 presents the concordance rates SA and SB, which we have calculated from the IAN data base for non-twin siblings and fraternal (dizygotic or DZ) twins for both genders together, for males only, and for females only, and for identical (monozygotic or MZ) twins for males and for females. The standard deviations in Table 1 were calculated using (Favro [3])where n = n (xx) and N = n (xx) + n (xo) for SA and for SB.
From genetic considerations
Discussion
The hypothesis set forth in the article by Bohm and Stewart [5] proposes the existence of a uterine environmental effect on MZ twin concordance rates. It differs from the effect we see in the present data in that it deals with a difference in concordance rates between MZ-monchorionic (MZ/MC) and the MZ-dichorionic (MZ/DC) cases, whereas the present data shows a concordance rate difference between fraternal twins (DZ) and non-twin siblings. But it is reasonable to suggest that an MC/DC
Conclusions
This paper presents analyses of statistical evidence that appears to indicate that the uterine environment may contribute to the autism concordance rates of siblings, both twin and non-twin. It also shows a decreasing concordance rate with increasing age difference between siblings.
Conflicts of intersest
There are no conflicts of interest.
Acknowledgements
It is a pleasure to thank Professor Lawrence D. Favro for the many fruitful discussions concerning data analysis, and to John H. Stewart for his substantial guidance in the adaptation of the IAN database to meet our needs.
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Deceased.