NEET 2026 Zoology Question 138 Solution

Down's syndrome (first described by Langdon Down in 1866) is an autosomal aneuploidy caused by non-disjunction leading to trisomy of chromosome 21 ($2n+1 = 47$). It does not involve sex chromosomes or the loss of an X-chromosome. Therefore, Statement I is definitively false.

• • Absence of an X chromosome causes Turner's syndrome, not Down's.

Turner's syndrome is a sex chromosomal monosomy caused by the absence of one X chromosome in females, resulting in the karyotype $44\text{A} + \text{XO}$ (45 chromosomes). Statement II asserts that it is caused by the "presence of an additional copy of the chromosomes", which describes Klinefelter's ($44\text{A} + \text{XXY}$) or Down's, not Turner's. Hence, Statement II is false.

• • Females are sterile with rudimentary ovaries and lack secondary sexual characters.

Because both statements swap and mischaracterize the underlying genetic lesions, both Statement I and Statement II are incorrect.

• • Matches official NTA final answer key Option B / (2).