Final answer:
The smallest part of a DNA molecule that can be expressed as a trait is a gene. Genes are sequences of DNA that control visible characteristics known as traits, and they can have multiple versions called alleles.
Step-by-step explanation:
The smallest part of a DNA molecule that can be expressed as a trait is a gene. Genes are sequences of DNA that make up chromosomes and control or partially control a number of visible characteristics, known as traits. A single gene may have multiple possible variations called alleles. These alleles determine which version of a particular trait is expressed, such as hair color or eye color.