Down's syndrome is due to?

Appeared in: KPSC Staff Nurse - 2016

Explanation

Down's syndrome is a chromosomal abnormality, not a single-gene disorder.
It is characterized by the presence of an extra copy of chromosome 21, a condition known as Trisomy 21.
This results from an error in cell division called nondisjunction, leading to a total of 47 chromosomes instead of the usual 46.
The presence of this extra genetic material leads to the characteristic physical and developmental features of the syndrome.

Option A: A missing chromosome describes a condition called monosomy. Down's syndrome is a trisomy, involving an extra chromosome.
Option C: A dominant gene causes a single-gene (Mendelian) disorder, where one copy of a mutated gene is sufficient to cause the condition. Down's syndrome is a chromosomal disorder involving a whole extra chromosome.
Option D: A recessive gene causes a single-gene disorder that requires two copies of the mutated gene. Down's syndrome is caused by an extra chromosome, not a pair of recessive genes.

Clinical Relevance

Nursing practice connection: Knowing Genetic basis of Down's syndrome (Trisomy 21) helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses play a key role in supporting families of individuals with Down's syndrome, providing education on developmental expectations, health screenings (e.g., for cardiac defects, thyroid issues), and available community resources.
The risk of having a child with Down's syndrome increases with maternal age, particularly for women over 35. This is a crucial point for patient education and genetic counseling.
What if the question asked about Turner Syndrome? The answer would change to 'A missing chromosome,' as Turner Syndrome is caused by the absence of one of the two X chromosomes in females (monosomy X).

How to Approach the Question

First, identify the core concept of the question: the cause of Down's syndrome.
Recognize that Down's syndrome is a well-known genetic condition. The question is asking for the specific type of genetic error.
Recall or deduce the difference between chromosomal abnormalities (involving number or structure of whole chromosomes) and single-gene disorders (Mendelian inheritance).
Evaluate the options: 'missing' and 'extra' chromosome refer to chromosomal number, while 'dominant' and 'recessive' gene refer to single-gene inheritance patterns.
Associate Down's syndrome with its common name, Trisomy 21, where 'tri-' implies three copies, meaning one extra chromosome.
Select the option that correctly describes this condition ('An extra chromosome') and eliminate the others based on their definitions.

Concept Tested & Keywords

Q961qiMjjo48O25oZLol5Z

E6 Kaplan Sadock's Synopsis of Psychiatry-2022 (pp 1-3768 of 3768) p. 330-332

E6 Parks TextBook of Preventive & Social Medicine part 2 — Subpart B (pp 233-449 of 464) p. 139-141

Practise the full KPSC Staff Nurse - 2016

Attempt every question from this paper in a timed mock, then review the full solution for each one.