Extra chromosome 'X' is present in which of the following cases?

The correct answer is Option (3) → Klinefelter's syndrome

A normal human cell has 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). In Klinefelter's syndrome there is an additional copy of the X chromosome, so the karyotype is 47, XXY. Such a person has a male body build but also shows some female features, such as development of breasts (gynaecomastia), and is sterile.

Option (1) Down's syndrome — Incorrect. Down's syndrome is caused by an extra copy of chromosome 21, which is an autosome (trisomy 21). The sex chromosomes are normal.

Option (2) Turner's syndrome — Incorrect. Turner's syndrome is caused by the absence of one X chromosome (45, X0). An X is missing, not extra.

Option (3) Klinefelter's syndrome — Correct. Klinefelter's syndrome has the karyotype 47, XXY. The second X in a male is the extra X chromosome.

NCERT Extract:
Chromosomal Disorders – “… Down’s Syndrome : The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21). … Klinefelter’s Syndrome : This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY. Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed …. Such individuals are sterile. Turner’s Syndrome : Such a disorder is caused due to the absence of one of the X chromosomes, i.e., 45 with X0, Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters ….”