Final answer:
The gene determining the heaviness of a man's beard is sex-limited, meaning it is present in both sexes but only expressed in males, leading to traits that differ between the sexes.
Step-by-step explanation:
A gene that determines the heaviness of a man's beard would be classified as sex-limited. Sex-limited genes are those that are present in both sexes but are expressed in just one sex and are responsible for certain sexual dimorphism—traits that are different between males and females. In the case of beard heaviness, these genes are turned on only in males and hence are sex-limited. They are not X-linked genes, which are located on the X chromosome and can affect both males and females, although often with different patterns due to the differences in sex chromosome composition (XX for females and XY for males). Y-linked genes are located on the Y chromosome and determine male-specific traits, but beard heaviness is not determined solely by Y chromosome genes. On the other hand, sex-influenced traits are genes that are present in both sexes but are expressed differently in males and females due to hormonal differences, not limited to expression in a single sex.