Trait de caractère (biologie)

A phenotypic trait, simply trait, or character state 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, having eye color is a character of an organism, while blue, brown and hazel versions of eye colour are traits. The term trait is generally used in genetics, often to describe phenotypic expression of different combinations of alleles in different individual organisms within a single population, such as the famous purple vs. white flower coloration in Gregor Mendel's pea plants. By contrast, in systematics, the term is character state is employed to describe features that represent fixed diagnostic differences among taxa, such as the absence of tails in great apes, relative to other primate groups. A phenotypic trait is an obvious, observable, and measurable characteristic of an organism; it is the expression of genes in an observable way. An example of a phenotypic trait is a specific hair color or eye colour. Underlying genes, which make up the genotype, determine the hair color, but the hair color observed is the phenotype. The phenotype is dependent on the genetic make-up of the organism, and also influenced by the environmental conditions to which the organism is subjected across its ontogenetic development, including various epigenetic processes. Regardless of the degree of influence of genotype versus environment, the phenotype encompasses all of the characteristics of an organism, including traits at multiple levels of biological organization, ranging from behavior and evolutionary history of life 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. Different phenotypic traits are caused by different forms of genes, or alleles, which arise by mutation in a single individual and are passed on to successive generations.