Sorry Chooks man.... the Cuckoos don't even lay a 4 but I am going to fix that!
every thing could be fixed , just time and some work that all.
you know before the marans hen was bred ,the dark egg did not exist in any breed used to create it . the magic happen in crossing .some mutation happen when the genes cross over and congregate in new way ( new sequence ).
when the dark egg appeared they selected for that trait . this is a reason why the marans breed is so rich on varieties .because too many different breeds used to create it .
controlled crossing with a good plan will restore the lost trait ( like a dark egg ) .
chooks man