What is Down Syndrome?
Down Syndrome (also known as trisomy 21) is caused by the presence of an extra, full or partial, copy of chromosome 21. The additional genetic material disrupts development, leading to the features seen in Down Syndrome [1]. It is the most common genetic cause of moderate to severe intellectual disability and affects approximately 1 in every 700 live births [2, 3].
The problems that define the syndrome include particular forms of mental retardation, stunted growth and some physical traits and features [4]. People with Down Syndrome are different in their abilities, and the characteristics of Down Syndrome affect people to various degrees [1].
Causes of Down Syndrome
In almost all cases, Down Syndrome is caused by having three copies of chromosome 21 in all the patient's body cells. In rarer cases, the syndrome can be caused by a trisomy 21 chimera or a mosaic of trisomy 21 in some of the body's cells [5].
This form of the condition is not hereditary. It develops purely by chance as an error in a dividing cell while a fetus grows in the first few weeks of gestation [1]. We don’t fully understand why Down Syndrome occurs or the number of risk factors involved in this development. However, the risk of having a baby with Down Syndrome increases with the age of the mother. The likelihood is higher for women aged thirty-five and over [1].
What is IQ?
Based on a method popularised in the early 20th century by the French psychologist Alfred Binet, IQ – described as Intelligence Quotient – represents a numeric measure of an individual’s mental capabilities concerning their peers [6]. Purporting to represent an individual’s innate mental capacity numerically, IQ, therefore, constitutes a score reflecting the general when measured against the specific and figurative when measured against literary merit – all in one blunt assessment. The measurement is arrived at by administering standardised questions to assess human intellectual capacity [7].
IQ tests measure intellectual ability and potential and are designed to reflect general mental competencies, including constraints, such as semantic, visual, verbal and mathematical reasoning, and fluid reasoning. In other words, it is a test of intelligence – pointing to something you are born with – rather than a test of knowledge (of information) representing what you learn via schooling and life experience [8].
IQ tests are made to have an average score of 100. The average IQ differs by country. For instance, the average in the United States is 98 [7]. A score of 116 or more is considered above average—a score of 130 or higher signals a high IQ. Membership in Mensa, the High IQ society, includes people who score in the top 2%, usually 132 or higher [8].
Nevertheless, IQ tests do not capture every dimension of intelligence. They do not, for example, assess creativity well. Nor do they measure either emotional intelligence or critical thinking, which can still contribute enormously to a person’s achievements in life [6].
Down Syndrome and Cognitive Abilities
Down Syndrome, a trisomy of chromosome 21 causing the most common intellectual disability, alters physical and mental development and results in a distinct set of characteristics (a phenotype) that differs from person to person [9].
One of the most robust cognitive deficits in Down Syndrome is verbal short-term memory. They do worse on verbal short-term memory tests than non-verbal short-term memory tasks [10, 11].
Another potential cognitive deficit typical of Down Syndrome is impairment to a complex of skills known as executive function. Executive function includes the skills needed for planning, problem-solving and achieving goals. People with Down Syndrome perform significantly worse on many tests of executive function than people without Down Syndrome [12].
Other abilities affected by Down Syndrome include language skills. It has been shown that linguistic skills in Down Syndrome are, on average, much more impaired than in mental retardation of an unknown aetiology [13].
On top of these cognitive deficits, those with Down Syndrome are also at an ultra-high risk of developing dementia, particularly Alzheimer’s disease, with increasing age being a risk factor, with apparent age-related deficits present only in individuals older than 50 years of age [14].
But equally, although common mental disorders (CMDs) such as anxiety, depression and obsessive-compulsive disorder have been found in the Down Syndrome population, their presence is not strongly associated with long-term detrimental effects on either cognitive or behavioural outcomes [15].
Does Down Syndrome Influence IQ in Affected Individuals?
Down syndrome, which results from trisomy of chromosome 21, is the leading known cause of cognitive impairment. Down Syndrome affects physical and mental development, producing an easily recognisable set of outward features whose expression varies substantially between affected individuals [9].
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Specific factors also seem to moderate the IQ of Down syndrome patients. One study reports that certain genetic polymorphisms play a role. In particular, the MTHFR 677T and transcobalamin 776 G alleles are significantly linked to lower IQ in Down syndrome patients [18].
Further support comes from magnetic resonance spectroscopy, which shows that brain metabolic profiles correlate with IQ in children with Down syndrome and could serve as a valuable predictor of IQ in children with Down syndrome [19].
Similar associations between inflammatory biomarkers and IQ scores have been suggested for individuals with Down syndrome. Indeed, several studies found an inverse relationship between various inflammatory biomarkers, such as C-reactive protein [20].
In conclusion, we address our original question on whether Down syndrome ameliorates or accelerates IQ deficits in affected individuals, and our experiences lead us to conclude that it does. However, it is likely to be influenced by a complex profile consisting of genetic polymorphisms, brain metabolic profiles, and inflammatory biomarkers to a varying degree.