A case of mosaic trisomy 21 with Down's syndrome signs and normal intellectual development.
Test IQ directly in mosaic Down syndrome - normal intelligence is possible even with a large share trisomy cells.
01Research in Context
What this study did
Doctors studied one adult with mosaic Down syndrome. Only a large share of their cells had the extra chromosome.
They ran full IQ tests and checked for Down syndrome facial features. The person looked like they had Down syndrome but scored average on intelligence tests.
What they found
The adult had normal intelligence despite having Down syndrome features. This shows low-level mosaic Down syndrome can have normal cognitive outcomes.
The case suggests we cannot predict IQ from physical appearance in mosaic Down syndrome.
How this fits with other research
Robertson et al. (2017) found dysphagia is common in people with intellectual disability. Their review included Down syndrome cases. This seems different from our case study, but Janet's group studied severe ID cases while our case had normal IQ.
Lin et al. (2005) showed thyroid screening gaps in institutions for people with Down syndrome. Our case suggests some residents might have normal cognition, so blanket assumptions about intellectual ability could be wrong.
Wilde et al. (2017) compared self-injury rates between tuberous sclerosis and Down syndrome. Their Down syndrome group likely included both full and mosaic cases, which our study shows may have very different cognitive profiles.
Why it matters
Always test cognition directly in clients with mosaic Down syndrome. Do not assume intellectual disability based on facial features or diagnosis alone. This changes how you set goals and choose interventions.
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02At a glance
03Original abstract
The present case study describes an adult male with clinical signs of mild Down's syndrome (DS), who performed well at school and reached university level. A karyotype was done on a lymphocyte culture and mosaic trisomy 21 was found in 3% of the 437 cells analysed. Eleven signs from Jackson's checklist were found in the clinical evaluation, which along with the analysis of the subject's dermatoglyphic traits, confirmed the DS diagnosis. Cognitive evaluation was done with several psychological tests and the results were within the average range. This rare phenotypic association shows that normal intellectual development may be possible in DS. This finding could be explained by the low trisomic cell frequency, which may have little effect on the critical tissues for intellectual development, and it might also reflect the wide phenotypic variation in mosaic trisomy 21. Other factors, such as strong family support, early and continued intervention programmes for both physical and speech therapy, and a thorough educational process, also provided opportunities for the development of the cognitive potential of the subject.
Journal of intellectual disability research : JIDR, 2000 · doi:10.1046/j.1365-2788.2000.00246.x