Final answer:
A stretch of DNA that determines a certain trait is called a gene. Genes are the functional units of chromosomes and code for specific proteins that determine specific characteristics, which are then expressed as traits.
Step-by-step explanation:
A stretch of DNA that determines a certain trait is called a gene.
Genes are the functional units of chromosomes and are made up of specific nucleotide sequences. They code for specific proteins that determine specific characteristics, which are then expressed as traits.
For example, the gene for hair color controls the trait of hair color, with different alleles of the gene determining variations such as blonde, brown, or black hair.