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A phenotypic trait, simply trait, or character state is a distinct variant of a phenotype characteristic of an organism; it may be either heredity or determined environmentally, but typically occurs as a combination of the two.Lawrence, Eleanor (2005) Henderson's Dictionary of Biology. Pearson, Prentice Hall. For example, having eye color is a character of an organism, while blue, brown and hazel versions of eye color are traits. The term trait is generally used in genetics, often to describe the phenotypic expression of different combinations of 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 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. (2025). 9781501752773, Cornell University Press. ISBN 9781501752773
The phenotype is dependent on the genetic make-up of the organism, but is also influenced by the environmental conditions that the organism was subjected to during its ontogenesis 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.
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.
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