Final answer:
The appositive in the sentence is 'Joe', which gives more specific information about the uncle who bought the tulips.
Step-by-step explanation:
The appositive in the sentence 'My uncle Joe bought a dozen tulips.' is 'Joe.' An appositive is a noun or noun phrase that follows another noun for further identification or to give more information about it. In this case, 'Joe' is placed after 'uncle' to specify which uncle the speaker is talking about. Appositives often provide essential details that clarify which person or thing in a category is being referred to. For example, if someone has more than one uncle, the appositive makes it clear which uncle they mean.