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 (Figure 4.17 a). 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 (Figure 4.17 b).”