Final answer:
A gene is a segment of a chromosome that encodes the directions for inherited physical and mental characteristics of an organism. Genes are composed of DNA sequences and can have different versions called alleles.
Step-by-step explanation:
The segment of a chromosome that encodes the directions for inherited physical and mental characteristics of an organism is called a gene. Genes are composed of DNA sequences that determine the traits an organism will have. Each gene can have different versions called alleles, which can result in variations in the characteristics of organisms.