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 Z-chromosome. Male budgies have two Z-chromosomes (ZZ) and female budgies have a Z-chromosome and a W-chromosome (ZW). 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 Z-chromosome, if their Z-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 Z-chromosome as "Z^o" and the Z-chromosome with the normal gene as "Z". There are three genotype possibilities for a male budgie:

• ZZ - Two normal genes resulting in a normal male budgie.
• ZZ^o - One normal gene and one opaline gene resulting in a normal male budgie that is split for opaline.
• Z^oZ^o - Two opaline genes resulting in an opaline male budgie.

And there are two genotype possibilities for a female budgie:

• ZW - A normal gene resulting in a normal female budgie.
• Z^oW - 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.