In genetics, a trait refers to any genetically determined characteristic. In technical terms, a genetic trait is amenable to segregation analysis rather than quantitative analysis. A dominant lethal trait is a trait that is expressed if present in the genome and therefore precludes having descendants. All such cases must necessarily be sporadic and must represent new mutations, not inherited ones (because a potential parent with the trait would die before being able to pass it along). A mendelian trait is one that segregates in accordance with the laws of genetics set forth by Gregor Mendel. Sickle cell trait is a mendelian trait. It refers to the situation in which a person has one copy (and one copy only) of the gene for sickle cell but does not have sickle cell disease (which requires two copies of the sickle cell gene). If two people with sickle cell trait have children together, each of their children has a one in four chance (25%) of having sickle cell disease. A nonpenetrant trait is a genetic trait that is in the genome but does not manifest itself in the individual. By contrast, a penetrant trait is one that manifests itself. If, for example, if 100 people have a particular genetic trait but only 80 of them express it, the penetrance of that trait is 80%.
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A qualitative characteristic; a discrete attribute as contrasted with metric character. A t. is amenable to segregation rather than quantitative analysis; it is an attribute of phenotype, not of genotype. [Fr. from L. tractus, a drawing out, extension]
- Bombay t. Bombay phenomenon.
- categorical t. in genetics, a feature that can conveniently and effectively be analyzed by sorting into classes either because there is no satisfactory way of measuring it (as with blood group s) or because it falls into natural classes so that the variation among classes far exceeds that within classes ( e.g., the phenotypic effects of many enzyme polymorphisms); existence of categories suggests but does not prove the operation of a major, simple, underlying cause. SYN: qualitative t..
- chromosomal t. a t. dependent on a recurrent chromosomal aberration.
- dominant t. an outstanding mental or physical characteristic. See dominance of traits.
- dominant lethal t. t., expressed in the phenotype if present in the genotype, that precludes having descendants. All such cases are necessarily sporadic and must represent new mutations as the usual methods of classical genetics provide no means of demonstrating any genetic component whatsoever, except for tenuous arguments such as advanced paternal age. Molecular biology may help although the methods may be tedious; if there is an epistatic gene that may mask the t., the logic is more tractable, though complex.
- galtonian t. a quantitative genetic t. due to contributions from many more of less equally important loci that resembles a continuous t..
- intermediate t. a measurable t. in which there is some evidence of the operation of a simple major cause, but in which the variation within the putative categories is such as to cause overlap and hence ambiguity in classification of any particular reading.
- liminal t. SYN: threshold t..
- marker t. a t. that may be of little importance in itself but which by association, linkage, or other means facilitates the detection, anticipation, or understanding of a disease or (for genetic diseases) the localization of the causative gene on the karyotype.
- mendelian t. a categorical t. that segregates in accordance with a single-locus genetic system.
- nonpenetrant t. a genetic t. that is not phenotypically manifest because of nongenetic factors; it therefore does not include recessivity, epistasis, hypostasis, or parastasis but does include environmental factors and pure random effects such as lyonization.
- penetrant t. a t. that in the appropriate genotypes is phenotypically manifest; strictly, it is the t. that is penetrant, not the gene. See penetrance.
- qualitative t. SYN: categorical t..
- recessive t. dominance of traits.
- sickle cell t. the heterozygous state of the gene for hemoglobin S in sickle cell anemia.
- threshold t. a t. that falls into natural groups that originate not in categorically distinct causes but in whether or not the outcome attains critical values; e.g., gallstones may result from a categorical cause or from unusual levels of causal factors that themselves show no evidence of grouping. SYN: liminal t..

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trait 'trāt, Brit usu 'trā n an inherited characteristic

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(trāt) 1. any genetically determined characteristic; see also entries under character, gene, and inheritance. 2. sometimes, more specifically, the condition prevailing in the heterozygous state of a recessive disorder, as in sickle cell anemia. 3. a distinctive behavior pattern.

Medical dictionary. 2011.

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