The extra copy of which gene causes Down syndrome?

Appeared in: RIMS Nursing Officer - 2022

Explanation

Down syndrome is caused by the presence of an extra copy of chromosome 21, a condition known as Trisomy 21.
This is the most common nonlethal trisomy and occurs in approximately 1 in 700 live births.
The diagnosis is confirmed by a karyotype analysis, which, as shown in the provided image, reveals three copies of chromosome 21.
The clinical features include intellectual disability, a characteristic facial appearance (flat profile, epicanthal folds), hypotonia, and an increased risk of congenital heart defects and leukemia.

Option A: An extra copy of chromosome 16 (Trisomy 16) is the most common chromosomal abnormality found in first-trimester miscarriages.
Option B: An extra copy of chromosome 18 results in Edwards syndrome (Trisomy 18), a condition with a different set of severe physical abnormalities and a much poorer prognosis than Down syndrome.
Option C: An extra copy of chromosome 20 is not associated with a well-defined, common syndrome like Down syndrome. Mosaic Trisomy 20 can occur, but its clinical presentation is highly variable and often mild.

Clinical Relevance

Nursing practice connection: Knowing Genetic basis of Down syndrome (Trisomy 21) helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Nurses play a vital role in prenatal screening and diagnosis, explaining genetic test results like karyotypes to expectant parents.
Understanding the genetic basis of conditions like Down syndrome is crucial for providing accurate information, offering support, and facilitating appropriate referrals for genetic counseling and support services.
Postnatally, nurses care for infants with Down syndrome, monitoring for associated health issues (e.g., cardiac defects, feeding difficulties) and providing education and resources to the family.

How to Approach the Question

First, analyze the image provided. Identify it as a karyotype, which is an organized profile of a person's chromosomes.
Carefully examine the chromosome pairs. Notice that for chromosome 21, there are three copies instead of the usual two.
Recall or deduce that the presence of three copies of a chromosome is called a 'trisomy'. Therefore, the image shows Trisomy 21.
Connect the specific genetic finding (Trisomy 21) to the clinical syndrome it causes, which is Down syndrome.
Scan the options to find the chromosome number that matches your finding. Option D, 'Chromosome 21', is the correct match.

Concept Tested & Keywords

Concept Tested: Genetic basis of Down syndrome (Trisomy 21)
Stem keywords: extra copy, gene, Down syndrome
Lead-in keywords: which
Clinical cues: The image of the karyotype is a critical cue, showing three copies of chromosome 21.

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

E6 Obstetrics Williams p. 5-15

E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 176-178

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