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Appendix 1 – Simple Nomenclature

You have probably encountered symbols in high school genetics and perhaps in previous chapters assigned to you in this course. Chapter 1 points out that a position on a chromosome is a gene locus, which simply means the location at which a gene is found. At that position, the gene could be one of several variants that we call alleles. The set of alleles comprise an organism’s genotype, which might affect the organism’s phenotype through the action of the genotype.

When you are solving genetics problems you have to keep track of different alleles of one or more genes. This can become confusing, so understanding how a gene works, how alleles vary, how they are expressed, and the influence they have on phenotype is important. A consistent nomenclature system – a set of rules dictating how you name alleles – becomes immensely helpful.


Figure A1.1: Two chromosomes with the same gene but different alleles, A and a.  (Image used with permission of provided it includes the link