The maternal Z is not a common denominator of female chickens. Female chickens have a Z chromosome from the sperm and males have one each from the sperm and the egg. For a laying hen to have a female offspring, it has to pass on a W. Therefore, if you make the mother hen's ovary produce eggs with a lethal z, but a non-lethal W, normal sperm will produce viable ZW females and Zz males with a lethal mutation.