Patients with Down syndrome have decreased buffering of physiologic reactions, resulting in hypersensitivity to pilocarpine and abnormal responses on sensory-evoked electroencephalographic (EEG) tracings.Abnormal physiologic functioning affects thyroid metabolism and intestinal malabsorption patients with trisomy 21 have an increased risk of obesity frequent infections are presumably due to impaired immune responses, and the incidence of autoimmunity, including hypothyroidism and rare Hashimoto thyroiditis, is increased.Molecular analysis reveals that the 21q22.1-q22.3 region, also known as the Down syndrome critical region (DSCR), appears to contain the gene or genes responsible for the congenital heart disease observed in Down syndrome.According to the gene-dosage effect hypothesis, the genes located on chromosome 21 have been overexpressed in cells and tissues of Down syndrome patients, and this contributes to the phenotypic abnormalities.Trisomy 21 (also known by the karyotype 47,XX,+21 for females and 47,XY,+21 for males. Two different hypotheses have been proposed to explain the mechanism of gene action in Down syndrome: developmental instability (ie, loss of chromosomal balance) and the so-called gene-dosage effect. In 1932, it was suggested that a chromosomal anomaly might be the cause, but the anomaly was not demonstrated until 1959.Langdon Down first described the condition in 1866, but its cause was a mystery for many years. It is primarily caused by trisomy of chromosome 21, which gives rise to multiple systemic complications as part of the syndrome.What is Down Syndrome? A drawing of the facial features of a child with Down syndrome.
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