Final answer:
An individual with trisomy 21 has three copies of chromosome 21, a condition known as Down syndrome, which can result in developmental delays and specific physical features.
Step-by-step explanation:
An individual with trisomy 21 has three copies of chromosome 21. This condition is known as Down syndrome. People with Down syndrome have 47 chromosomes in total instead of the usual 46. This extra genetic material affects their development, leading to physical and intellectual disabilities.
The presence of an extra chromosome 21 is due to nondisjunction, a chromosome separation error during cell division. The likelihood of this nondisjunction increases with the maternal age, especially in mothers over the age of 36.
Down syndrome is one of the few autosomal trisomies that allows individuals to survive into adulthood. It is characterized by specific facial features, including a flat facial profile, upward slanting eyes, and a single deep crease in the palm of the hand; developmental delays and a variety of health issues may also be present.
Diagnosis is often made via karyotyping, where a picture of an individual's chromosomes shows the presence of the extra chromosome 21.