9.2 Review of Genetic Nomenclature and Symbols

A gene is a hereditary unit that occupies a specific position (locus) within the genome or chromosome and has one or more specific effects upon the phenotype of the organism and can mutate into various forms (alleles). A genotype is the specific allelic composition of a cell or organism. Normally, only the genes under consideration are listed in a genotype, while the alleles at the remaining gene loci are considered to be wild type. A phenotype is the detectable outward manifestation of a specific genotype. In describing a phenotype, usually only the characteristics under consideration are listed while the remaining characters are assumed to be wild type (normal).

Gene Names and Symbols

Usually, gene names are unique and their corresponding symbols are unique letters or combinations of letters. So, for example, the “vermillion” gene in Drosophila is represented by the letter “v “, while “vg ” is the symbol for the “vestigial” gene and “vvl ” is the symbol for the “ventral veins lacking” gene locus. Note, however, that the same letter symbols may represent a different gene in another organism. Gene symbols and gene names are typically shown italicized  text, but not always.

The normal, or wild type, form of a gene is usually symbolized by superscript plus sign, “+”. E.g., ” a+ “, ” b+ “, etc. or it is sometimes abbreviated to just “+”. A forward slash is occasionally used to indicate that the two symbols are alleles of the same gene, but on homologous chromosomes.

A typical mutant form of the gene, of which there can be many, can be symbolized by a superscript minus sign, “-“. E.g.,  ” a“, ” b “, etc., or sometimes abbreviated to just “a“, “b“, etc. (no superscript). Therefore, if the genotype of a diploid organism is given as a+/a, it means there is a wild type allele and mutant allele of the “a” gene at the “a” locus. This may also be abbreviated to +/a.

In some species of diploids, the dominant allele is typically designated with the uppercase letter(s), while the recessive allele is given the lowercase letter(s). For example, in Mendel’s peas the dominant Rough allele is “R”, while the recessive smooth alleles is “r”.

Simple graphic illustrating how chromosomes, loci and alleles in a cell, and how we depict them as text.
Figure 9.2.1 A Diagram Illustrating Chromosomes, Loci, and Alleles in a Cell, and How We Depict Them as Text [Long description]

Media Attribution


Locke, J. (2017). Figure 2. A diagram illustrating how chromosomes, loci and alleles…[digital image]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 18, p. 2). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf

Long Description

  • Figure 9.2.1 Chromosomes, loci, and alleles in a cell are usually depicted by solid black lines, small black lines perpendicular to these solid lines, and letters (capital or lowercase) respectively. [Back to Figure 9.2.1]]


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Introduction to Genetics Copyright © 2023 by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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