What genetic condition, caused by an extra copy of chromosome 21, is characterized by intellectual disability, distinctive facial features, and increased risk of heart defects?

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Accepted Answer

Down syndrome (Trisomy 21). Down syndrome, also known as Trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra copy of chromosome 21. It is the most common chromosomal cause of intellectual disability, occurring in approximately 1 in 700 live births. Individuals with Down syndrome typically have characteristic facial features (flattened facial profile, upward-slanting eyes, small ears), mild to moderate intellectual disability, and increased risk of certain medical conditions including heart defects, thyroid disorders, and early-onset Alzheimer's disease. The risk of having a child with Down syndrome increases with maternal age, particularly after age 35.

Turner syndrome is a chromosomal condition affecting females in which one of the two X chromosomes is missing or partially missing (45,X karyotype), leading to short stature, ovarian dysfunction, and sometimes heart abnormalities, but it involves a missing sex chromosome rather than an extra copy of chromosome 21. Klinefelter syndrome affects males who have an extra X chromosome (47,XXY karyotype), leading to tall stature, reduced fertility, and sometimes mild learning difficulties, which again involves sex chromosomes rather than chromosome 21. Fragile X syndrome is caused by a mutation on the X chromosome rather than a chromosomal duplication and is the most common inherited cause of intellectual disability, but it has a different genetic mechanism and different characteristics from Down syndrome.

Down syndrome is important on the DSST exam because it illustrates key concepts in genetic and chromosomal influences on development. Students should understand the genetic basis (nondisjunction during cell division), the relationship between maternal age and risk, the range of outcomes for individuals with Down syndrome, and how early intervention and supportive environments can optimize developmental outcomes.

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