A double yolk occurs when a hen's body releases more than one egg during her daily ovulation cycle. And, just like humans, it's possible for two — or more — eggs to make their way from the ovary and through the reproductive tract. The overall odds of a hen laying a double yolk are one in 1,000. So I was very surprised when I cracked this egg into the pan and saw a double yolk. The next egg I cracked in was exactly the same and by the end of the week, after I had cracked open all of the eggs in the box I was astonished to see that every single egg in the entire box of 1 dozen was a "double yolker".
The eggs came from a small local farm so I guess that certain hens have a genetic disposition to develop double yolk eggs.
After doing some online research I discovered that:
Heredity can cause some hens or breeds to have a higher propensity for double yolks, but it most often occurs in pullets that are just beginning to lay.
Each time we’ve raised young pullets, we’ve seen a few double-yolked eggs. But as each group got older, they quit producing the double-yolked eggs.
Double-yolk eggs are not really that rare (about 1 in every 1000) but aren’t often seen in grocery store eggs today because commercial operations candle the eggs, separate out the double-yolks, and sell them to make egg-containing products.
If you are planning to incubate eggs for hatching, it’s generally recommended that double-yolk eggs NOT be included in the incubating eggs. Double-yolk eggs rarely hatch successfully.