Which inheritance type would apply to a gene that is expressed in a father and passed to his daughters but is not passed to his sons?

• A. X-linked Recessive
• B. X-linked Dominant
• C. Y-linked
• D. Autosomal Recessive

Answer: (B)

The inheritance type that applies to a gene expressed in a father and passed to his daughters, while not being passed to his sons, is X-linked Dominant. This is due to the fact that males have one X chromosome and one Y chromosome, while females have two X chromosomes. When an X-linked dominant trait is present in a father, he can only pass his X chromosome to his daughters, resulting in them inheriting the trait. Conversely, he passes his Y chromosome to his sons, which means the trait cannot be inherited by them.

In this scenario, any daughter of the affected father will express the trait because she receives his X chromosome. Therefore, if a father has an X-linked dominant trait, all of his daughters will manifest that trait, while none of his sons will due to their receiving his Y chromosome. This is what distinguishes X-linked dominant inheritance from other forms, such as Y-linked, which would pass traits only to sons, or autosomal recessive, which can affect both genders equally and is not dependent on sex chromosomes.