A nurse is educating a group of nursing students about Down syndrome.
A student asks, “What is the primary cause of Down syndrome?” Which response by the nurse is correct?
A “It is primarily caused by the presence of an extra copy of chromosome 22.”..
B “It is mainly caused by an error in cell division, resulting in an extra copy of chromosome 20.”..
C “The primary cause is related to an extra copy of chromosome 21.”..
D “Down syndrome is primarily due to a deletion of chromosome 18.”..
The Correct answer and Explanation is:
The correct answer is: C “The primary cause is related to an extra copy of chromosome 21.”
Explanation:
Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This means individuals with Down syndrome have three copies of chromosome 21 instead of the usual two. This extra genetic material impacts development and causes the physical and intellectual characteristics associated with the condition.
The most common form of Down syndrome is nondisjunction trisomy 21, which occurs in approximately 95% of cases. This is due to an error in cell division called nondisjunction, where chromosome 21 fails to separate properly during the formation of reproductive cells (either the egg or the sperm). As a result, when the egg and sperm combine during fertilization, the embryo ends up with three copies of chromosome 21.
In rare cases, Down syndrome can also result from translocation, where part of chromosome 21 attaches to another chromosome, or mosaicism, where some cells have the extra chromosome while others do not. However, the majority of cases are due to the extra full copy of chromosome 21.
Chromosomal abnormalities like these are not inherited in most cases, but instead happen by chance during the formation of reproductive cells. Certain factors, such as advanced maternal age (women aged 35 and older), can increase the likelihood of nondisjunction occurring.
Down syndrome is not caused by abnormalities in chromosomes 18, 20, or 22. Chromosome 18 abnormalities are linked to conditions like Edwards syndrome, and defects in other chromosomes lead to different syndromes entirely. Understanding the genetic basis of Down syndrome is crucial for diagnosis, care, and education.