What is the smallest unit of inheritance?

Heredity is the passing of traits to offspring (from its parent or ancestors).

That being said, the smallest unit of Heredity is a gene, because a gene is the smallest unit capable of conveying a genetic message. Technically the smallest unit of heredity is a cistron, but that's essentially the same as a gene.

I would have said a nucleotide, but a single nucleotide can't convey a hereditary message.