"Various combinations of pure breeds were originally used to create egg laying hybrids, the most popular being a Rhode Island Red cockerel and a Light Sussex hen. These breeds have of course been modified over the years to create better parent stock that will lay even more eggs. It is thought the White Leghorn has also been used and this is where the battery hen has got her large floppy comb from".
Poultry Keeper - This is where I found the info
They have been bred as the most prolific layers and lay more eggs in their lifetime than any other breed. This is when they are kept in an artificial environment with artificial light constantly. These ladies will not produce as many eggs as a battery hen would and will stop laying when daylight drops to less than fourteen hours a day. Fantastic I might get three eggs out of each of them this year! An egg still seems a very long way off. We will probably fight over the first in this house or share it! For now I will have to console myself with eggs of the Chocolate variety. I hope you are all enjoying a Happy Easter Weekend.