A trait is a distinct variant of a phenotypic character of an organism that may be inherited, be environmentally determined or be a combination of the two. For example, eye color is a character or abstraction of an attribute, while blue, brown and hazel are traits.
A phenotypic trait is an obvious and observable trait; it is the expression of genes in an observable way. An example of a phenotypic trait is hair color, there are underlying genes that control the hair color, which make up the genotype, but the actual hair color, the part we see, is the phenotype. The phenotype is the physical characteristics of the organism. The phenotype is controlled by the genetic make-up of the organism and the environmental pressures the organism is subject to.
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.
Genetic origin of traits in diploid organisms
The inheritable unit that may influence a trait is called a gene. A gene is a portion of a chromosome, which is a very long and compacted string of DNA and proteins. An important reference point along a chromosome is the centromere; the distance from a gene to the centromere is referred to as the gene's locus or map location.
Mendelian expression of genes in diploid organisms
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 we now call genes, occurred in pairs. His tool was statistics: long before the molecular model of DNA was introduced by James D. Watson and Francis Crick.
He is also known to have 'cheated' on his experiments, removing outliers and generally 'tidying' datasets - statistics, and repeating the experiments, shows that his results were just 'too perfect', a point first noted by RA Fisher, in 1936: http://www.cs.brown.edu/courses/csci1950-l/presentations/REVISED--AFCB%202007%20Course%20Fisher%20and%20Mendel.ppt
Biochemistry of dominance and extensions to expression of traits
Extended expression patterns seen in diploid organisms include facets of incomplete dominance, codominance, and multiple alleles. Incomplete dominance is the condition in which neither allele dominates the other in one heterozygote. Instead the phenotype is intermediate in heterozygotes. Thus you can tell that each allele is present in the heterozygote. Codominance refers to the allelic relationship that occurs when two alleles are both expressed in the heterozygote, and both phenotypes are seen simultaneously. Multiple alleles refers to the situation when there are more than 2 common alleles of a particular gene. Blood groups in humans is a classic example. The ABO blood group proteins are important in determining blood type in humans, and this is determined by different alleles of the one locus. 
Schizotypy is an example of a psychological phenotypic trait found in schizophrenia-spectrum disorders. Studies have shown that gender and age influences the expression of schizotypal traits. For instance, certain schizotypal traits may develop further during adolescence, whereas others stay the same during this period.
- Lawrence, Eleanor (2005) Henderson's Dictionary of Biology. Pearson, Prentice Hall. ISBN 0-13-127384-1
- *Campbell, Neil; Reece, Jane (March 2011) , "14", Biology (Sixth ed.), Benjamin Cummings
- Bailey, Regina. "What is incomplete dominance". About.com.
- McClean, Philip. "Variations to Mendel's First Law of Genetics".
- Unknown. "Multiple Alleles".
- Lawrence>Lawrence, Eleanor (2005) Henderson's Dictionary of Biology. Pearson, Prentice Hall. ISBN 0-13-127384-1
- Campbell, Neil; Reece, Jane (March 2011) , "14", Biology (Sixth ed.), Benjamin Cummings