Cypriot Journal of
Educational Sciences
Volume 09, Issue 1, (2014) 14-20
www.awer-center/cjes
Understanding Chromosome Disorders and their Implications
for Special Educators
Linda Gilmore*, Faculty of Education, Queensland University of Technology, Victoria Park Road Kelvin,
Australia.
Suggested Citation:
Gilmore, L. (2014). Understanding Chromosome Disorders and their Implications for Special
Educators. Cypriot Journal of Educational Sciences. 9(1), 14-20.
Received August 11, 2013; revised December 23, 2013; accepted February 08, 2014.
Selection and peer review under responsibility of Assoc. Prof. Dr. Anastasia Alevriadou, University of
Western Macedonia, Greece.
©
2014 SPROC LTD. Academic World Education & Research Center. All rights reserved.
Abstract
More children are now being diagnosed with chromosome abnormalities. Some chromosome
disorder syndromes are relatively well known; while others are so rare that there is only limited
evidence about their likely impact on learning and development. For educators, a basic level of
knowledge about chromosome abnormalities is important for understanding the literature and
communicating with families and professionals. This paper describes chromosomes, and the
numerical and structural anomalies that can occur, usually spontaneously during early cell division.
Distinctive features of various chromosome syndromes are summarised before a discussion of the
rare chromosome disorders that are labelled, not with a syndrome name, but simply by a description
of the chromosome number, size and shape. Because of the potential within-group variability that
characterises syndromes, and the scarcity of literature about the rare chromosome disorders,
expectations for learning and development of individual students need to be based on the range of
possible outcomes that may be achievable.
Keywords: chromosomes, chromosome disorders, chromosome abnormalities, rare chromosome
disorders, developmental outcomes.
*ADDRESS FOR CORRESPONDENCE: Linda Gilmore, Faculty of Education, Queensland University of Technology,
Victoria Park Road, Kelvin Grove, QLD 4059 Australia,
E-mail address: [email protected] / Tel.: +61 7 3138 9617, Fax: +61 7 3138 3987
Gilmore, L. (2014). Understanding Chromosome Disorders and their Implications for Special Educators. Cypriot Journal of
Educational Sciences. 9(1), 14-20.
1. Introduction
Many children who attend special education settings are identified as having
chromosome disorders that account for their disabilities. Some chromosome disorders, such
as Down syndrome, are relatively well known while others are much less familiar or virtually
unknown. For the majority of educators who have little or no training in biology,
understanding the complexities of genetics and chromosome disorders can be both daunting
and challenging. Yet a basic level of knowledge about the underlying cause of a child’s
disability is important if teachers are to provide the necessary support for learning and
development. This paper begins by presenting basic information about chromosomes and
chromosome abnormalities. The developmental consequences of the more common
chromosome disorder syndromes are then summarised before the focus turns to the rare,
and at times apparently unique, chromosome disorders that are now being diagnosed with
increasing frequency.
2. Chromosomes Abnormalities
In every cell of the human body (except for the egg or sperm cells and red blood cells)
there are 23 pairs of chromosomes. These are numbered from 1 to 22, largest to smallest,
with the 23rd pair consisting of the sex chromosomes: for females, two X chromosomes; for
males, one X and one Y chromosome (Turnpenny & Ellard, 2005). The descriptors 46, XX or
46, XY are used, respectively, to refer to a female or male with the normal complement of
chromosomes.
Deviations from this normal complement occur in 0.5 to 1% of live births and more than
half of all miscarriages (Carey, 2003; Gardner & Sutherland, 2004; Haydon, 2008). Varying
amounts of chromosome material may be lost, duplicated or rearranged in some way.
These anomalies can be inherited from a parent or, more commonly, occur spontaneously
when chromosomes fail to separate properly during early cell division. The factors that
cause these spontaneous errors are largely unknown, but are presumed to be “accidental”.
Older mothers are more likely to conceive a child with a chromosome disorder. Maternal
age has consistently been associated with Down syndrome and other chromosome disorders
(Hook, 1981; Morris, Wald, & Mutton, 2003) with the risk rising dramatically with advanced
reproductive age.
A number of terms are used when describing chromosome abnormalities. These include
the letters p and q, which refer, respectively, to the shorter and longer arms of the
chromosome. These two arms are joined in the middle by a centromere, and the tips of the
chromosome are known as telomeres. As shown in Figure 1, a numbering system is used to
identify precise regions, bands and sub-bands along the chromosome (Carey, 2003).
Figure 1 Chromosome 16 with the region 16q11.2q13 bracketed
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Gilmore, L. (2014). Understanding Chromosome Disorders and their Implications for Special Educators. Cypriot Journal of
Educational Sciences. 9(1), 14-20.
Chromosome deletions result from breakages and subsequent loss of material, either at the
end of a chromosome (terminal deletion) or at some point within the chromosome (interstitial
deletion). In the example shown in Figure 1, brackets have been inserted to illustrate the region
that has been lost for a person with deletion 16q11.2q13. Breakages may also result in extra
chromosome material (i.e., a third copy of a segment) being present, an anomaly that is referred
to as a duplication or partial trisomy. The most common chromosome disorder, Down syndrome,
usually involves a third complete copy of chromosome 21 (i.e., Trisomy 21).
As well as deletions and duplications, chromosome material can potentially be rearranged in
various ways. Broken segments of two or more chromosomes may exchange places. Known as
translocations, such rearrangements may have little impact on health and development
provided all the chromosome material is still present in each cell and the translocation is thus
balanced. However, there are likely to be problems with reproduction, and in future generations
there is a substantial risk of unbalanced rearrangements that result in missing or additional
chromosome material (i.e., deletions or duplications). Other chromosome anomalies include
inversions, which involve two breakages followed by reconnection of the intervening segment in
reverse order, and rings that are formed when both chromosome tips break off and the two
sticky ends join to form a circle.
3. Chromosome Disorder Syndromes
Some of the more common chromosome abnormalities have been classified as syndromes.
The best known is Down syndrome that is usually caused by an extra chromosome 21 in every
cell of the body. Cases have been reported in which only a proportion of cells have the
additional chromosome (known as mosaic Trisomy 21) and in these cases, developmental
outcomes depend on the type and number of cells that are affected (de A. Moreira, San Juan,
Pereira & de Souza, 2000; Fishler, Koch & Donnell, 1976). A small percentage of individuals with
Down syndrome have a section of chromosome 21 attached to another chromosome, most
commonly chromosome 14.
Other trisomy syndromes include Patau (Trisomy 13) and Edwards’s syndrome (Trisomy 18).
In the relatively small proportion of Trisomies 13 and 18 that result in live births, developmental
consequences are more severe than those associated with Down syndrome, and few children
survive to adulthood (Baty, Jorde, Blackburn & Carey, 1994; Crider, Olney, & Cragan, 2008). The
only other trisomy syndromes that are survivable involve the sex chromosomes, X or Y. An
additional copy of the X chromosome leads to Klinefelter syndrome (47,XXY) in males, or XXX
syndrome in females, while an additional Y chromosome produces XYY syndrome (Leggett,
Jacobs, Nation, Scerif & Bishop, 2010). Further additions of the sex chromosomes are possible –
for instance, XXXX (tetrasomy X) or XXXXX (pentasomy X). There is one survivable condition that
involves a complete missing chromosome. Turner syndrome results from the absence of one X
chromosome, producing the karyotype 45,X in affected females (Powell & Schulte, 2011). In
some cases, only part of the second X chromosome is missing.
Apart from these syndromes, which involve changes in the total number of chromosomes,
there are many others that result from structural changes to one or more chromosomes. These
include Wolf-Hirschhorn, Cri du Chat, Williams, Smith-Magenis, and Velocardiofacial syndromes
that involve deletions on chromosomes 4p, 5p, 7q, 17p and 22q, respectively. Deletions on the
long arm of chromosome 15 inherited from the father produce Prader-Willi syndrome while the
same deletion on the maternally derived chromosome produces Angelman syndrome. Other
less familiar chromosome deletion syndromes include Jacobsen (deletion on 11q) and MillerDieker, which results from a deletion on 17p, but at a different breakpoint from Smith-Magenis
syndrome.
There is considerable variability in the developmental consequences associated with these
chromosome disorder syndromes. Most involve some degree of intellectual disability but the
level of impairment is very variable, both within and across syndromes. More severe levels of
intellectual impairment are associated with Angelman, Smith Magenis, and Cri du Chat
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Gilmore, L. (2014). Understanding Chromosome Disorders and their Implications for Special Educators. Cypriot Journal of
Educational Sciences. 9(1), 14-20.
syndrome. Intelligence is usually within the average range for girls and women with Turner
syndrome, although learning difficulties, especially in mathematics, are common (Mazzocco &
Hanich, 2010; Rovet, 2004). Some individuals with Velocardiofacial syndrome may have
intellectual ability within the average to low average range, while others affected by the
syndrome will have mild to moderate intellectual impairment (Furniss, Biswas, Gumber & Singh,
2011).
Profiles of cognitive strengths and weaknesses are evident too. For instance, children with
Williams’s syndrome tend to have relative strengths in language and auditory memory, but
weaknesses in visuospatial skills (Mervis & John, 2010). Similarly, those with Velocardiofacial
(Furniss et al., 2011) and Turner (Hong, Kent & Scaletta Kesler, 2009) syndromes often perform
less well with visuospatial and nonverbal tasks than on tasks requiring auditory/verbal ability. By
contrast, the cognitive profile for children with Down syndrome shows relative strengths in
visuospatial skills and weaknesses in verbal abilities (Fidler, Hepburn & Rogers, 2006).
In addition to cognitive impairments, there may be problems in areas such as communication,
attention, executive functioning, and behaviour. For some syndromes, there are distinctive
phenotypic features including hyperphgagia (compulsive eating and food obsessions) in Prader
Willi (Whittington & Holland, 2010), and self-injurious behaviour in Smith Magenis (Dykens &
Smith, 1998) and Cri du Chat (Collins & Cornish, 2002) syndromes. Relative strengths are
sometimes displayed, such the desire for social interaction in Angelman (Oliver et al., 2007),
Williams (Jones et al., 2000) and Down (Fidler et al., 2006) syndromes, although overfriendliness and hyper sociability can create social vulnerability.
Mental health problems are also relatively common in chromosome disorder syndromes.
These include attention deficit disorder, phobias and anxiety associated with Williams syndrome
(Stinton, Elison & Howlin, 2010); repetitive, ritualistic behaviours in Prader-Willi syndrome
(Greaves, Prince, Evans & Charman, 2006); and high rates of anxiety and depression, attention
deficit, and psychotic disorders such as schizophrenia in Velocardiofacial syndrome (Gothelf,
Frish, Michaelovsky, Weizman & Shprintzen, 2009).
4. Rare Chromosome Disorders
There is also a wide array of less common or less distinctive chromosome disorders that occur
in live births but which are not identified by a syndrome name. Rather, they are labelled using a
description of the chromosome number, size and shape, called the karyotype. Such labels create
difficulties for families who need to communicate their child’s disorder to others, and potential
confusion for education systems and service providers who are required to categorise the
disability. Diagnoses such as “Deletion 4q33qter with Duplication 10p13pter” are not easily
understood! (In this example, the affected individual has both a deletion of the end segment of
the long arm of chromosome 4, from 4q33 to the end (ter = terminal) as well as a duplication of
the end segment of the short arm (p) of chromosome 10.)
Furthermore, many chromosome disorders are often so rare that there are currently few or
no other cases with the same anomaly reported in the literature or recorded on clinical
databases anywhere in the world, making prognosis difficult and approaches to intervention
uncertain (Gilmore & Campbell, 2006).
Although the overwhelming majority of rare chromosome disorders are known only by their
karyotypes (description of chromosomes), a few have been given syndrome names in recent
years. These include Koolen - DeVries syndrome, which results from a micro deletion of
17q21.31 and Kleefstra syndrome that involves a deletion at 9q34.3. In Pallister-Killian
syndrome, some cells of the body have a small additional chromosome comprising two copies of
the p arm of chromosome 12 (i.e., mosaic tetrasomy 12p). Another unusual anomaly is seen in
individuals with Emanuel syndrome who have an extra chromosome made up of the top (parm)
and middle (centromere) of chromosome 22 plus the bottom (q arm) of chromosome 11. (For
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Gilmore, L. (2014). Understanding Chromosome Disorders and their Implications for Special Educators. Cypriot Journal of
Educational Sciences. 9(1), 14-20.
information about these and a wide range of other rare chromosome disorders see
www.rarechromo.org )
Across all available literature about rare chromosome disorders, characteristics such as
growth retardation, developmental delay, intellectual disability, delayed or impaired speech,
behaviour problems and sensory deficits are frequently documented. For the overwhelming
majority of the vast number of reported chromosome abnormalities that are survivable,
however, research has involved only small samples or individual case reports and, as mentioned
above, for some anomalies there is a total absence of literature. Even when published reports
are available, they are likely to be biased towards individuals with more severe developmental
outcomes, and are thus not a portrayal of the range of outcomes that are potentially achievable
(see Gilmore, 2009).
Indeed, a close scrutiny of existing literature about rare chromosome disorders reveals that,
amidst findings of deficits and adverse developmental outcomes, there are also reports of
individuals who are developing quite typically, at least in some domains. There are, for instance,
numerous cases of individuals with chromosome deletions or duplications who have intellectual
ability within the average range (e.g., Bartels, Pütz, Reintjes, Netzer & Shoukier, 2013; Chen,
Cherry, Hahn & Enns, 2004; Cobb et al., 2010; Gilmore, Cuskelly, Jobling & Smith, 2001; Zarate,
Kogan, Schorry, Smolarek & Hopkin, 2007). Thus, it is clear that adverse developmental
outcomes are not necessarily inevitable.
5. Implications for Special Educators
For educators in both mainstream and special school settings, a basic knowledge of genetics
and chromosomes is important for understanding relevant literature about a student’s disorder.
This knowledge also enables educators to communicate more effectively with families and
professionals. In particular, knowledge about the types of chromosome anomalies that can
occur is valuable for understanding rare disorders whose diagnosis involves a string of letters
and numbers that are otherwise incomprehensible.
Research about the phenotypic cognitive and behavioural profiles of the more common
chromosome disorders enables educators to develop interventions that target a student’s likely
strengths and weaknesses, and to anticipate future potential areas of difficulty. There is,
however, an important caution to keep in mind when reading the literature. Descriptions of
particular disabilities are generally derived from observations and assessments of a group of
individuals with that specific disorder, and conclusions about developmental and learning
characteristics (i.e., the behavioural phenotype) are based on these group data. Unfortunately,
it is not always apparent that some individuals with a particular disorder may do considerably
better than the group, while others may not develop as well. Keeping in mind this caution is
important, because to some extent adult expectations are likely to be influenced by the
literature about a specific disability. Each child will have individual areas of strength and
weakness as well as many personal characteristics that are unrelated to their disorder but which
may be overlooked if a student is defined solely by his or her disability label. An evaluation of
the literature about a child’s disability is valuable to identify particular features that are likely to
be present, so long as there is also a recognition that a specific student may differ in many ways
from others with the same diagnosis.
In relation to the rarer chromosome abnormalities, the extremely limited literature means
that educators have few available resources for guiding their approaches to teaching students
with these disorders. Within education systems that are accustomed to categorising children
according to their learning needs and perceived potential, decisions are inevitably hampered by
the lack of a recognisable disability diagnosis. Even in instances where single or multiple case
reports are available in the published literature, these are unlikely to be representative of others
with the same, or similar, chromosome abnormalities. Thus, for these students, expectations
need to be cautiously optimistic, rather than constrained by negative case reports in the
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Gilmore, L. (2014). Understanding Chromosome Disorders and their Implications for Special Educators. Cypriot Journal of
Educational Sciences. 9(1), 14-20.
literature, and decisions about educational placement and intervention need to be based on a
combination of evidence that includes comprehensive psycho-educational assessment.
One resource that is likely to be of considerable value to educators is the Unique Rare
Chromosome Disorder Support Group (www.rarechromo.org), which has established a rich
database containing the developmental histories of many thousands of individuals with a rare
chromosome disorder. The Unique website offers free information leaflets on a wide range of
rare chromosome disorders, as well as general information about chromosome abnormalities
and their consequences for development and learning. The leaflets, which are derived from a
merging of the scientific literature and unique own database, are rigorously produced, medically
verified, and regularly updated.
In summary, chromosome disorders are relatively common, and an increasing number of
individuals are being diagnosed with rare chromosome disorders that are not identified by a
syndrome name. Educators will benefit from a basic level of understanding of genetics that
maximises their ability to understand relevant literature and to communicate more effectively
with families and professionals. Because of the potential within-group variability that
characterises chromosome disorder syndromes, and the scarcity of literature about the rarer
chromosome disorders, expectations about future learning and development need to be
tentative, taking into consideration the range of possible outcomes that may be achievable for a
student with a chromosome disorder, as well as the unique individual, family and contextual
characteristics that are also likely to influence development and learning.
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