A trait may be any single feature or quantifiable measurement of an organism. However, the most useful traits for genetic analysis are present in different forms in different individuals.
A visible trait is the final product of many molecular and biochemical processes. In most cases, information starts with DNA traveling to RNA and finally to protein (ultimately affecting organism structure and function). This is the Central Dogma of molecular biology as stated by Francis Crick.
This information flow may also be followed through the Cell as it travels from the DNA in the Nucleus, to the Cytoplasm, to the Ribosomes and the Endoplasmic Reticulum, and finally to the Golgi Apparatus, which may package the final products for export outside the cell.
The environment plays a large role in the determination of traits. This is natural as all organisms must have input (and output) of energy and matter in order to maintain their overall elevated energy state.
The heritable unit that may produce a trait is called a gene. A gene is a strand of DNA that is part of a very long and compacted string of DNA called a chromosome. An important reference point along this string is the centromere; the distance from a gene to the centromere is referred to as the gene's locus or map location. A chromosomal region known to control a trait while the responsible gene within not being identified is referred to as a quantitative trait locus.
Combinations of different alleles thus go on to generate different traits through the information flow charted above. For example, if the alleles on homologous chromosomes exhibit a "simple dominance " relationship, the trait of the "dominant" allele shows in the phenotype.
Gregor Mendel pioneered modern genetics. His most famous analyses were based on clear-cut traits with simple dominance. He determined that the heritable units, he called "genes", occurred in pairs and could exhibit linkage. His tool was statistics: long before the molecular theory of DNA.