The half-sider is actually not a true variety. The trait is not genetically inherited. Rather, it is a congenital condition. Visually, this budgie is split vertically, with the appearance and color of two distinct varieties appearing in splotches or sections divided by the vertical center line. I believe that this is a condition called tetragametic chimerism in which fraternal twin zygotes fuse at a very early stage in the womb, forming one individual with the tissues and DNA of both twins.

Basic Genetics:
NONE - This is a congenital condition
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	photo courtesty of Cagdas at Kushane.com Visit his site to see more pics of his halfsider and other budgies!
The halfsider budgie has two different sets of genetic material.