Q.4

In humans, Down syndrome is caused by

(A) Trisomy 16

(B) Trisomy 18

(C) Trisomy 21

(D) Trisomy 22

Down Syndrome Trisomy 21: Correct Cause and Trisomy Explanations

Down syndrome in humans is caused by trisomy 21, making option (C) the correct answer in this multiple-choice question. This genetic condition results from an extra copy of chromosome 21, leading to characteristic physical and developmental features.

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Correct Answer: Trisomy 21

Trisomy 21 accounts for about 95% of Down syndrome cases, where cells have three copies of chromosome 21 instead of two due to nondisjunction during meiosis. This extra genetic material disrupts normal development, causing intellectual disability, distinct facial features, and increased risk of heart defects. Rare forms include translocation or mosaicism, but full trisomy 21 defines the classic presentation.

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Trisomy 16 Explanation

Trisomy 16 is the most common chromosomal abnormality in miscarriages but rarely results in live births, often as mosaic trisomy 16 with severe outcomes like growth restriction. Mouse models of trisomy 16 mimic some Down syndrome brain defects due to gene synteny with human chromosome 21, but it does not cause human Down syndrome. Affected fetuses typically do not survive to term, distinguishing it from viable trisomy 21.

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Trisomy 18 Explanation

Trisomy 18, or Edwards syndrome, involves an extra chromosome 18 and leads to severe developmental issues, with most infants surviving only days or weeks post-birth. Unlike Down syndrome, it features clenched fists, rocker-bottom feet, and profound intellectual disability, with a high miscarriage rate. It is not associated with the milder cognitive and physical traits of trisomy 21.

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Trisomy 22 Explanation

Full trisomy 22 is extremely rare and usually lethal in utero, while mosaic or partial forms may cause developmental delays but not Down syndrome features. It differs from trisomy 21 by lacking the specific gene dosage effects on chromosome 21 that define Down syndrome. Survival to birth is uncommon, contrasting with the relatively higher viability of trisomy 21 cases.

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