• On paired chromosomes, every gene has a corresponding opposite gene on the other chromosome.  Therefore, at the location of an given gene, there are only two possible alternative genes that can occupy that location (one on each chromosome).   There may be several alternative mutant genes that can occupy that location; but only two can occupy at any given instance.  Each alternative gene is called an allele.

• The site of each gene's location on a chromosome is called a "locus"; and each locus can be occupied by only one gene at a time.   However, chromosomes are generally, paired (side by side duplicates - with matching locations) which allows for two genes to occupy any given location  (one on each of the paired chromosomes).

 
• Alternative gene variations (alleles) can occupy a given location in various paired combinations.  The examples below show alternate gene combinations at the pattern locus.  The alternative mutants in the chart below occur only at the pattern locus.  All normal genes are represented with the symbol for wildtype, "+".