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Genetics and Mental Retardation Syndromes

Genetics and Mental Retardation Syndromes

A New Look at Behavior and Interventions
Authors: Elisabeth M. Dykens Ph.D., Robert M. Hodapp Ph.D., Brenda M. Finucane M.S.

ISBN: 978-1-55766-471-6
Pages: 336
Copyright: 2000
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New genetic causes of mental retardation are being discovered at rapid rates—every day more people are diagnosed with specific syndromes. This comprehensive and timely guide identifies characteristics of people who have these syndromes to help you understand their behaviors and their needs, so you'll be able to provide better interventions and create wider community inclusion.

You'll get easy-to-read, detailed discussions of four major syndromes—Down, Williams, fragile X, and Prader-Willi—on genetic causes and implications, prevalence, basic physical characteristics, cognitive profiles, speech and language, medical complications, and behaviors. Research-based intervention strategies are explained to help you with education, personal relationships, employment, and residential issues. Five other, less researched syndromes—velocardiofacial, Rubinstein-Taybi, Angelman, Smith-Magenis, and 5p- (cri du chat) are also featured, giving you in-depth information you won't find elsewhere.

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Review: Journal of Genetic Counseling
"The book is well organized and well written. . . . The intended audience of this work, as stated by the authors, are ';non-genetically trained workers: those behavioral scientists, practitioners and family members who desire to learn more about etiology-based behavioral findings and issues,' however, as a genetic counselor I found it thoroughly engrossing and would recommend it to anyone who has an interest in any of the syndromes showcased."
I. Introduction to Behavioral and Genetic Issues
  1. Toward Etiology-Based Work
  2. Applying the New Genetics
II. Genetic Disorders
  1. Down Syndrome
  2. Williams Syndrome
  3. Fragile X Syndrome
  4. Prader-Willi Syndrome
  5. Five Other Intriguing Syndromes
III. New Directions
  1. Next Steps for Research


Excerpted from chapter 1 of Genetics and Mental Retardation Syndromes: A New Look at Behavior and Interventions, by Elisabeth M. Dykens, Ph.D., Robert M. Hodapp, Ph.D., & Brenda M. Finucane, M.S.

Copyright © 2000 by Paul H. Brookes Publishing Co. All rights reserved.

Toward Etiology-Based Work

Over the past few decades, advances in human genetics have brought revolutionary changes to the mental retardation field. With new genetic causes of mental retardation discovered each year, more than 750 genetic mental retardation etiologies have now been identified (Opitz, 1996). And, given the success of the Human Genome Project, these rapid advances show no signs of slowing. Indeed, in the late 1990s geneticists estimated that the chemical structure of each of the 100,000 human genes would be known by the year 2003 (Goodman, 1998), leading to a better understanding of the connections between particular genes and specific human behaviors (i.e., "gene–behavior correlations"). These advances mark a new era in our understanding of and interventions for people who have mental retardation.

Looking back, it is startling to see how far we have come in so short a time. Until Watson and Crick's discoveries were reported in 1953, the world did not even know the basic structure of the human chromosome, how many chromosomes human beings possessed, or how genes operated. In 1959, Lejeune, Gautier, and Turpin discovered that most cases of Down syndrome are caused by an extra — or third — chromosome 21 (i.e., trisomy 21). A decade later Lubs (1969) first observed the pinched, "fragile" site of fragile X syndrome, and, over the past two decades, discoveries in fragile X syndrome, as well as in Prader-Willi syndrome and Angelman syndrome, led to new models of human disease. In short, these advances are promoting a new way of looking at a variety of human disorders and permanently changing the disability field as a result.

This book describes these remarkable genetic advances as they apply to behavioral issues within specific mental retardation syndromes. To promote understanding of the behavioral characteristics of several genetic mental retardation disorders, we provide in each chapter descriptions of causes, prevalence, and basic physical characteristics for each syndrome, along with more in-depth understandings of many different behavioral characteristics.


This book goes beyond mere description to advance a larger argument. Simply put, we argue that genetic disorders affect various aspects of behavior, from cognition and language to adaptive and maladaptive behaviors. More indirectly, the behaviors often exhibited by people with a particular genetic disorder also influence how others react. In the first few decades of the new millennium genetic disorders will also increasingly influence the choice of which specific psychological, educational, or other therapies are most effective for individuals with a given syndrome. To us, then, genetic disorders matter in understanding the behaviors of people with mental retardation.

The assertion that "genetic disorders matter" is obvious to some researchers, anathema to others. Indeed, to professionals in many disciplines, our argument will seem so obvious that it is hardly worth stating. To these people, the importance of genetic etiology shows itself every day. In other disciplines, however, workers cringe at the idea of etiology-based research or intervention. To these individuals, our arguments concerning the importance of genetic etiology on behavior seem obviously wrong, at odds with major philosophies in the field, and heretical to good science or practice. How, then, can we best reconcile such diametrically opposed views on the same topic? We first offer answers to several common criticisms of etiology-based approaches.


Common criticisms of etiology-based approaches typically fall into one or more of the following general categories.

"Syndromes Rarely Occur"

Many mental retardation workers dismiss genetic syndromes as unimportant because of the low prevalence estimates of such conditions. They note that disorders such as 5p- (cri-du-chat) syndrome occur in only 1 per 50,000 births, whereas Prader-Willi syndrome and Williams syndrome occur in about 1 per 15,000–20,000 births. Even Down syndrome, the most common genetic-chromosomal cause of mental retardation, occurs in only about 1 per 700 live births.

This view is consistent with the long-held belief that only 25% of people with mental retardation have known "organic" causes for their developmental delay. These pre, peri-, or postnatal insults are thought to occur mostly among those with severe and profound mental retardation. The remainder of people with mental retardation, up to 75% (and supposedly almost all with mild mental retardation), are thought to have "nonspecific" or "cultural-familial" causes related to their syndromes (e.g., Drash, 1992; Zigler, 1967).

Population studies, however, have revealed a gradual shift in these percentages. As of 2000, genetic disorders probably account for about one third of all occurences of mental retardation (Matalainen, Airaksinen, Mononen, Launiala, & Kaariainen, 1995). Furthermore, individuals with Down syndrome, fragile X syndrome, Prader-Willi syndrome, Williams syndrome, and other genetic etiologies compose from 10% to 50% of individuals with mild mental retardation (Rutter, Simonoff, & Plomin, 1996). At the very least, these percentages show that not all cases of mild mental retardation are nonspecific or "cultural-familial" in nature. These prevalence rates also do not include the many people whose mental retardation is caused by fetal alcohol syndrome, premature birth, and other "organic but not genetic" causes. Altogether, approximately 50% of people with mental retardation show some form of organic etiology (Matalainen et al., 1995; Zigler & Hodapp, 1986).

Although one can only estimate the percentages of people with genetic mental retardation syndromes, several points seem clear. First, with remarkable advances in molecular genetics, additional mental retardation syndromes are being discovered every year (Moser, 1992). The number of identified genetic conditions can be expected to increase in the years to come.

Second, improved genetic techniques now more accurately diagnose individuals with many of these syndromes. Whereas currently many individuals with certain genetic disorders were only diagnosed in the later childhood or adult years, diagnoses during infancy or early childhood should occur for increasing numbers of individuals over the next few decades. And, as more and more people are diagnosed with a more complete list of genetic disorders, such disorders will account for growing percentages of people with mental retardation. Workers critical of etiology-based approaches will find it increasingly difficult to disregard genetic syndromes as too rare to bother with.

"Syndromes Provide Only Unnecessary Labels"

People with mental retardation have historically been classified according to their level of cognitive and adaptive impairment — the distinction between mild, moderate, severe, or profound mental retardation found in most behavior studies in the field. Departing from traditional nomenclature, some workers no longer label the people with mental retardation themselves, preferring instead to label their needs for environmental supports (American Association on Mental Retardation, 1992). In this spirit, some fear that genetic diagnoses further stigmatize people already identified as having mental retardation (Goodman, 1990). To some, genetic diagnoses may even be seen as highlighting an immutable difference (i.e., one's chromosomes) relative to those without mental retardation.

In contrast, we argue that the behavioral effects of genetic disorders are not unchangeable and that diagnoses can help in intervention efforts. We note, for example, the effectiveness of many environmental interventions that are based on these diagnoses, such as the long-term use of a specialized diet to alleviate the genetically based disorder phenylketonuria (PKU). Children with PKU do not metabolize the amino acid phenylalanine. By following a specific, phenylalanine-free diet from infancy, however, the severe/profound mental retardation related to PKU can be circumvented, so only subtle cognitive and attention problems exist (Waisbren, Brown, de Sonneville, & Levy, 1994).

So, too, can other interventions partially alleviate the effects of many genetic mental retardation disorders. By utilizing characteristic behavioral profiles found in each syndrome, we can develop more targeted, fine-grained, and effective interventions. These interventions can be applied in many areas, including behavioral management, education, pharmacotherapy, psychotherapy, and even the transition from child to adult services. To the extent possible, examples are provided later in this book of syndrome-specific studies that guide treatment and shed new light on how and when to intervene.

"Syndromes Don't Matter for Inclusion or Other 'Practical' Concerns"

A related criticism of etiology-based approaches distinguishes between "basic" and "applied" issues within the mental retardation field. Several professionals thus suggest that phenotypic research, though it may describe basic aspects of behavioral functioning, is meaningless for more practical, applied concerns. In essence, these individuals argue that because the outcome (mental retardation) is more or less the same for everyone, why should subtle behavioral differences from one syndrome to another matter for education, group home placement, or supported employment?

However, as we are increasingly understanding, syndromes do matter for inclusion and other aspects of life for people with mental retardation. To cite only one example, parents of children with Prader-Willi syndrome versus with Down syndrome show many differences (as well as some similarities) in the educational services that they desire for their children. In general, parents of children with Prader-Willi syndrome desire more specialized services and services related to help in specific academic areas (e.g., arithmetic), whereas parents of children with Down syndrome desire placements that are more integrated with their child's age-mates or are in their own neighborhood school (Hodapp, Freeman, & Kasari, 1998). More ominously, the hyperphagia (i.e., excessive overeating) and food-seeking behavior associated with Prader-Willi syndrome make the complications of obesity the leading cause of death for people with this syndrome (Dykens & Cassidy, 1996). Thus, for schools, group homes, and other environments that serve one or more individuals with Prader-Willi syndrome, a tension exists between people's rights to self-determination and the need to stop them from literally eating themselves to death (Dykens, Goff, et al., 1997). In short, only by attending to etiology can one address these everyday, life-determining issues in people with mental retardation.

"People with Different Syndromes All Behave the Same"

The final criticism of etiology-based approaches is rooted in a longstanding belief that people with different etiologies behave similarly. Consider the following statements, spanning three decades:

  • "Rarely have behavioral differences characterized different etiological groups" (Ellis, 1969, p. 561)

  • "There is now considerable skepticism as to the usefulness of classifying mental retardation by form, due primarily to our current inability to separate biological and psychological forces" (MacMillan, 1982, p. 60).

  • "Most special educators do not believe that etiology is pertinent to their function" (Kahn, 1988, p. 550).

  • "Classification systems based on etiology or clinical types have little value in education" (Blackhurst & Birdine, 1993, p. 425).

  • "In general, the focus of educational programs varies according to the degree of the student's retardation" (Hallahan & Kauffman, 1997, p. 138).

These statements, concerning both research and intervention, highlight the most commonly mentioned argument against etiology-based work. For decades, the prevailing sentiment has been that genetic etiology simply does not matter. This view is a well-established tradition in the field — a legacy inherited from researchers and clinicians working during a time when we understood much less about genetic disorders and their effects than we do today. Spurred by genetic advances, then, there is no better time to question this legacy, to better understand how, when, and why genetic syndromes make a difference in research and intervention. Yet, most traditions die hard, in part because they become woven into the very fabric of a much larger culture.

Excerpted from chapter 1 of Genetics and Mental Retardation Syndromes: A New Look at Behavior and Interventions, by Elisabeth M. Dykens, Ph.D., Robert M. Hodapp, Ph.D., & Brenda M. Finucane, M.S.

Copyright © 2000 by Paul H. Brookes Publishing Co. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

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