Step 1: Understanding the Concept:
Trisomy means a cell carries three copies of one chromosome instead of the usual two. The question asks about trisomy specifically affecting the sex chromosomes (X and Y), so autosomal trisomies can be set aside right away.
Step 2: Key Approach:
List each named condition by its actual chromosome formula, then pick the one that is both sex-chromosome related and a true trisomy (three chromosomes), not a missing one.
Step 3: Detailed Explanation:
Klinefelter's syndrome has the formula 47, XXY: an extra X chromosome added to a normal male XY set, giving three sex chromosomes total. That is a genuine sex chromosome trisomy. Down's syndrome has the formula 47, +21: an extra copy of chromosome 21, which is an autosome, not a sex chromosome, so it does not answer this question even though it is also a trisomy. Edward's syndrome has the formula 47, +18: again an autosomal trisomy, chromosome 18 this time. Turner's syndrome has the formula 45, X: only one sex chromosome is present, a monosomy, the opposite of trisomy.
Step 4: Final Answer:
Only Klinefelter's syndrome (47, XXY) is a true trisomy of the sex chromosomes, so option 1 is correct.