A phenotypic trait, or simply trait, is a distinct variant of a phenotypic characteristic of an organism; it may be either inherited or determined environmentally, but typically occurs as a combination of the two. For example, eye color is a character of an organism, while blue, brown and hazel are traits.
A phenotypic trait is an obvious, observable, and measurable trait; it is the expression of genes in an observable way. An example of a phenotypic trait is hair color; underlying genes, which make up the genotype, "control" the hair color, but the actual hair color, the part we see, is the phenotype. Although all aspects of the phenotype are controlled by the genetic make-up of the organism, at least to some extent, they are also influenced by the environmental conditions the organism is subject to across its ontogenetic development, including various epigenetic processes. Regardless of the relative degree of genetic versus environmental control, the phenotype encompasses all of the characteristics of an organism, including traits at multiple levels of biological organization, ranging from behavior and life history traits (e.g., litter size), through morphology (e.g., body height and composition), physiology (e.g., blood pressure), cellular characteristics (e.g., membrane lipid composition, mitochondrial densities), components of biochemical pathways, and even messenger RNA.
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
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, Biology, 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