Final answer:
Phenotypic differences between normal males (XY) and Klinefelter males (XXY) are due to the presence of an extra X chromosome in Klinefelter males, which, despite inactivation, can still express some genes and impact gene dosage, leading to traits associated with Klinefelter syndrome.
Step-by-step explanation:
The phenotypic differences between normal males (XY) and Klinefelter males (XXY) can be primarily attributed to the presence of the extra X chromosome in Klinefelter males. While one X chromosome is inactivated in Klinefelter males, rendering them with one active X chromosome similar to normal males, the inactivated X chromosome can still express a few genes, which can contribute to the phenotypic differences.
In addition to these gene expressions, imbalance in gene dosage due to the extra X may still impact the individual even after X inactivation, since not every gene on the X chromosome is silenced during the inactivation process. Klinefelter syndrome typically presents with mild intellectual and physical disabilities, and a range of symptoms including small testes, enlarged breasts, reduced body hair, and potential sterility. This compares to the typical phenotype of a normal male who does not exhibit these characteristics.