Down syndrome accounts for approximately 10% of all moderate to severe intellectual disabilities. Patients exhibit distinctive facial features—thick epicanthal folds at the inner corners of the eyes causing upward slanting—leading some to refer to it as congenital imbecility, though experts do not fully endorse this term.Other common physical traits include heart defects, tongue protrusion, facial fullness, limb abnormalities, short stature, and progressive obesity. Intellectual disability severity varies widely among individuals, though most experience moderate impairment. Since the 1970s expansion of preschool education programs for children with Down syndrome, however, increasing numbers have achieved mild intellectual disability levels.
There are three primary types of Down syndrome. The first is trisomy 21 (the most common). Children with this type have three copies of chromosome 21 instead of the usual two, resulting in a trisomic condition. The second is mosaic syndrome, caused by developmental abnormalities where some cells have an extra chromosome while others do not.The third is translocation Down syndrome. This occurs when the extra chromosome from the 21st pair attaches itself, either wholly or partially, to another chromosome pair. The likelihood of a child having Down syndrome is significantly influenced by the mother's age. Women under 20 and over 40 are more likely to have children with the condition (the latter being particularly so).For example, the risk of having a child with Down syndrome is 1 in 1,300 for women aged 20–30, 1 in 600 for those aged 30–34, 1 in 300 for those aged 35–39, and 1 in 80 for those aged 40–44.For this reason, doctors sometimes recommend that older women undergo amniocentesis during pregnancy to determine if the fetus has Down syndrome or other chromosomal abnormalities. Recently, some researchers have suggested that factors other than maternal age may also contribute to Down syndrome, such as paternal age, excessive radiation exposure, and certain viral infections.Phenylketonuria occurs when the body cannot convert phenylalanine—a substance found in common foods—into tyrosine. Excessive accumulation of phenylalanine in the body leads to abnormal brain development. Today, newborns can be screened for phenylketonuria within the first few days of life: many states even mandate that infants undergo this screening before hospital discharge.Continuous monitoring and control of phenylalanine levels in the diet from infancy through mid-childhood can prevent mental retardation. Treatment for this condition must begin in infancy; otherwise, severe mental retardation may result. In addition to timely treatment, experts are increasingly focusing on screening parents for the phenylketonuria gene.Although the probability of two phenylketonuria carriers marrying is extremely low, genetic counseling proves invaluable should such a situation arise.
Similar to phenylketonuria, Tay-Sachs disease occurs when both parents are carriers of the gene. This disorder causes progressive brain damage and ultimately leads to death.Diagnostic methods now exist to identify carriers. Additionally, prenatal diagnosis can be performed via amniocentesis during pregnancy. Therefore, intellectual disabilities caused by genetic factors are entirely preventable. Expectant mothers should not overlook these screenings for the sake of their children.