Terms and Nomenclature

• Terms

Allele: One of two or more alternative forms of a gene occupying the same locus on a chromosome and concerned with the same biochemical and/or developmental process.

Independent assortment: Random transmission of alleles at two or more unlinked loci to gametes via meiosis. Also known as Mendel's second law.

Linkage: The inherited association of alleles at loci due to their physical proximity on a chromosome. The absence of linkage is independent assortment. The degree
of linkage is estimated as the percentage of recombination between loci. This value is often converted to centimorgan units. For example, the r and s loci are 16.4
cM apart on chromosome 7.

Linkage map: An ordered representation of genes on a chromosome derived from observed percentages of recombination. In a well-described genome, the
number of maps = the number of linkage groups. For example, in barley there are 7 linkage groups and 7 chromosome maps.

Locus: The site or position of a particular gene on a chromosome. For example, the vrs1 locus is on chromosome 2. In any given diploid barley plant, there can be two alleles at the locus.The term marker locus has a more general definition: see marker locus.

Marker lcous: A locus with two or more forms (alleles) that is used for inheritance studies and linkage map construction. The marker may be defined by alleles at
locus, where the locus is a gene. The marker locus may be also defined as a DNA landmark, where the "alleles" are differences in DNA product size (i.e. molecular
markers such as AFLPs, RFLPs, SSRs, etc.). In this case, the alleles may also be referred to as polymorphisms.

Segregation: Random transmission of alleles at a locus from parents to progeny via meiosis. Also known as Mendel's first law.

• Nomenclature

Symbols of hereditary factors, derived from their original names, should be written in Roman letters of distinctive type, preferably italics, and be as short as possible.
Example (recessive male sterility):
ms: a msms individual will be male sterile

Whenever unambiguous, the name and symbol of a dominant should begin with a capital letter and those of a recessive with a small letter.
Example (dominant male sterility):
Ms: Msms and MsMs individuals will be male sterile

Molecular markers are introducing a new wrinkle in the nomenclature issue, due to their abundance. In the North American Barley Genome Mapping Project, we've adopted the convention of naming the locus by the probe (i.e. ABG123, where ABG stands for American Barley Genomic). If the probe identifies multiple loci, we use a letter suffix: i.e. ABG123A, ABG123B, etc.). A British group mapping in wheat, on the other hand, puts an "X" prefix on a RFLP locus (i.e. XABG123).

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