Mutation Effects
Inheritance: Recessive

Male and Female: The color of the beak is changed from the normal orange/red to a light orange to yellow. Males still have more red than the females. The legs and feet are also of a lighter color approaching yellow. This change is usually not as dramatic as the change in beak color.

Fledgling: Young Yellow Beaks look like regular beak finches, whether the normal color of the fledglings beak is black or horn or pink. As the beak begins to change color, you can see the lighter color showing through. Because the color of the legs and feet can vary with fledglings, this is not a reliable indicator of YB fledglings.

Identifying Splits
Male and Female: No way to detect Yellow Beaks that I have found.

Combinations
The Yellow Beak mutation can be combined with any other mutation or combination of mutations. There seems to be some debate as to whether the yellow beak is more attractive on a dark Zebra like the Black Face that offers the beak more contrast or if a diluted beak should be on a dilute bird like the Chestnut Flanked White (CFW). I find it attractive on just about any Zebra finch. Like all White Zebras though, they offer an interesting contrast only in the presence of normal Zebras. Someone once told me that they are like spices. A little is good, but a meal they do not make.

Notes
This is an old mutation that shows up in many lines. Being a recessive gene it can remain hidden for some time before showing up. They were probably imported with regular wild-caught stock long ago and were not isolated or focused on until the 1960's.

It seems that the lightest yellow can be achieved with YB x YB, or YB x split YB matings rather than split YB x split YB matings.

The hens are usually more attractive since their beaks are lighter in color and can truly be yellow. The male's beak will always look a little orange.