A segment of dna on a chromosome that codes for a specific trait is called

Gene

Explanation:

Gene: it is the unit of heredity which is transferred from a parent to offspring, and is usually located on a chromosome and it controls the development of one or more traits. Alternative forms of gene are known as or called alleles.

Some alleles are dominant over other and some are recessive. They may be homozygous or heterozygous. They show law of segregation, is that form each set of alleles, one allele goes in gamete and other goes in other gamete.

Un-linked genes and genes on different chromosomes show law of independent assortment.