Question

Down syndrome is caused by

• A. Trisomy of Chromosome 21
• B. Tetrasomy of Chromosome 21
• C. Disomy of Chromosome 21
• D. Monosomy of Chromosome 21

Hint:

Key Chromosomal Aneuploidies:
- Down syndrome: Trisomy 21 ($47, +21$).
- Edwards syndrome: Trisomy 18 ($47, +18$).
- Patau syndrome: Trisomy 13 ($47, +13$).
- Turner syndrome: Monosomy X ($45, XO$).
- Klinefelter syndrome: Sex chromosome trisomy ($47, XXY$).

Answer 1

The Correct Option is A

Concept:
Down syndrome is one of the most widely studied chromosomal disorders resulting from autosomal aneuploidy in humans.
First characterized clinically by British physician John Langdon Down in 1866, it is caused by the presence of an additional autosome.
The condition leads to distinct physical characteristics, cognitive disabilities, and developmental delays due to gene dosage imbalance.

Step 1: Cytogenetic Mechanism of the Disorder:
Down syndrome occurs primarily as a consequence of meiotic non-disjunction during oogenesis or spermatogenesis.
During anaphase-I or anaphase-II of meiosis, homologous chromosome pairs or sister chromatids of chromosome 21 fail to separate properly.
This failure produces an abnormal gamete containing an extra copy (2 copies instead of 1) of chromosome 21 ($n+1$).

Step 2: Chromosome Number at Fertilization:
When this abnormal gamete unites with a normal haploid gamete ($n$), the resulting zygote possesses three copies of chromosome 21 instead of the usual homologous pair.
The individual's karyotype contains 47 chromosomes ($2n+1 = 46+1 = 47$) with three copies of chromosome 21.

Step 3: Naming the Chromosomal Condition:
The presence of three copies of a single chromosome instead of the normal pair is defined as trisomy.
Hence, the disorder is classified specifically as Trisomy of Chromosome 21.

Step 4: Final Answer:
Therefore, Down syndrome is caused by the Trisomy of Chromosome 21, corresponding to option (A).