Phenotypic trait

A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or somewhere in between. For example, eye color is a character or abstraction of an attribute, while blue, brown and hazel are traits.

Definition
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.

Cell products are released into the tissue, and organs of an organism, to finally affect the physiology in a way that produces a trait.

Genetic origin of traits in diploid organisms
The heritable unit that may influence 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.

The nucleus of a diploid cell contains two of each chromosome, with homologous (mostly identical) pairs of chromosomes having the same genes at the same loci.

Mendelian expression of genes in diploid organisms
A gene is only a DNA code sequence; the slightly different variations of that sequence are called alleles. Alleles can be significantly different and produce different product RNAs.

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, what he called "genes", occurred in pairs and could exhibit linkage. His tool was statistics: long before the molecular model of DNA was introduced by James D. Watson and Francis Crick.

Some examples of Inherited genes include eye color.

Biochemistry of dominance and extensions to expression of traits
The biochemistry of the intermediate proteins determines how they interact in the cell. Therefore, biochemistry predicts how combinations of different alleles will produce varying traits.

Extended expression patterns seen in diploid organisms include facets of incomplete dominance, codominance, and multiple alleles.