Homozygous and heterozygous

Here I take the classical example used to explain genetics in budgerigars: the blue gene bl. There exist at least four different alleles (forms) of this gene. In this example we only use the two most frequent ones:

• the not mutated wild allele: bl⁺ (the superscript '+' always indicates the wild allele)
• the mutated 'blue' allele: bl

Since genes come in pairs there are 3 possible combinations of these two alleles:

bl+ --- bl+

bl+   bl  --- = --- bl    bl+

bl  --- bl

When both allels are equal the organisme is said to be homozygous for the gene. When the allels are different the organisme is said to be heterozygous for the gene.