What is a defining characteristic of Turner's Syndrome?

Turner's Syndrome is a genetic condition that affects females and is characterized by the presence of only one X chromosome instead of the usual two (XX). This chromosomal abnormality, often denoted as monosomy X, leads to a variety of developmental and physiological challenges, such as short stature, reproductive issues, and certain physical features. The defining aspect of this syndrome is indeed the absence of the second X chromosome, which impacts the individual's growth and development.

The other options do not correctly describe Turner's Syndrome. An extra chromosome 21 is associated with Down syndrome, and individuals with Turner's Syndrome have significant differences in their chromosomal makeup. Having two X chromosomes is typical for females and does not relate to Turner's Syndrome. Lastly, while cognitive abilities can vary among individuals with Turner's Syndrome, the syndrome itself is not known for enhancing cognitive functions. Instead, it can be associated with some learning difficulties, although many individuals with this condition are of average intelligence. Thus, the defining characteristic of having a single X chromosome accurately reflects the fundamental genetic characteristic of Turner's Syndrome.