Genetics of Opaline

Summary:
Opaline - sex-linked recessive
Normal - dominant

Opaline is a sex-linked mutation. This means that the gene is located on the x-chromosome. Male budgies have two x-chromosomes (XX) and female budgies have an x-chromosome and a y-chromosome (XY). Since the gene is recessive to normal, male budgies must have two opaline genes (one on each x-chromosome) to be an opaline variety. However, since female budgies have only one x-chromosome, if their x-chromosome has the opaline gene, they will be the opaline variety. It is because females need only one gene to express the trait that sex-linked mutations such as opaline are more common in female budgies. Here, we will represent the opaline gene on the x-chromosome as "X^o" and the x-chromosome with the normal gene as "X". There are three genotype possibilities for a male budgie:

• XX - Two normal genes resulting in a normal male budgie.
• XX^o - One normal gene and one opaline gene resulting in a normal male budgie that is split for opaline.
• X^oX^o - Two opaline genes resulting in an opaline male budgie.

And there are two genotype possibilities for a female budgie:

• XY - A normal gene resulting in a normal female budgie.
• X^oY - An opaline gene resulting in an opaline female budgie.

Below are some punnet square examples of pairings. As you will see, if your goal is to breed a sex-linked mutation like opaline, besides breeding two visually opaline budgies, the best results will be from the pairing of a male who is split for opaline to an opaline female.