Which condition is characterized by mental retardation associated with chromosome abnormalities?

Down syndrome, also known as Trisomy 21, is characterized by mental retardation and is associated with a specific chromosome abnormality involving the presence of an extra copy of chromosome 21. Individuals with Down syndrome typically exhibit various physical features such as a flattened facial profile, slanted eyes, and a single transverse palmar crease, along with developmental delays and intellectual disability due to the genetic anomaly. The extra genetic material disrupts normal development, leading to the cognitive impairments observed in affected individuals.

In other conditions like Turner syndrome, which is caused by the complete or partial absence of one of the X chromosomes in females, cognitive impairment is less pronounced and is not a defining feature. Klinefelter syndrome involves the presence of an extra X chromosome in males, leading to tall stature and potential learning difficulties but not specifically mental retardation as the primary characteristic. Williams syndrome is a microdeletion syndrome associated with a distinctive facial phenotype and cardiovascular issues, but mental retardation is variable and is not universally present as it is in Down syndrome.

Thus, Down syndrome is the condition that primarily represents mental retardation in conjunction with a well-defined chromosomal abnormality.