Final answer:
Down syndrome, or trisomy-21, is the most common chromosomal abnormality and is characterized by an individual having an extra chromosome 21. It can be detected through a karyotype, is more likely with advanced maternal age, and features distinct physical and mental traits.
Step-by-step explanation:
Down syndrome, also called trisomy-21, is the most common chromosomal abnormality. It occurs when an individual has three copies of chromosome 21 instead of the usual two, resulting in 47 chromosomes in total. This genetic condition is often a result of chromosomal nondisjunction during meiosis, particularly in the mother's egg cells. As women age, particularly past the age of 36, the likelihood of nondisjunction and therefore the chance of having a child with Down syndrome increases.
Individuals with Down syndrome typically demonstrate both physical and mental characteristics distinct to this condition, including a specific facial appearance, developmental delays, and may require proper assistance throughout their lives. It is notable that Down syndrome is the only autosomal trisomy that allows most affected individuals to survive into adulthood.
A karyotype is a visual representation of an individual's chromosomes and can be used to confirm the presence of an extra chromosome 21, indicating Down syndrome. This condition is recognized as a type of aneuploidy, where there is an abnormal number of chromosomes present.