Final answer:
Klinefelter's syndrome occurs in boys (males) due to the presence of an extra X chromosome (XXY genotype). This condition is not observed in girls (females) who have two X chromosomes.
Step-by-step explanation:
Klinefelter's syndrome occurs in boys (males) because it is related to the presence of a specific sex chromosome. The genotype associated with Klinefelter's syndrome is XXY. In this condition, males have an extra X chromosome, resulting in characteristics such as small testes, enlarged breasts, and reduced body hair. On the other hand, girls (females) have two X chromosomes, and having an extra X chromosome in females does not lead to the development of Klinefelter's syndrome.