Heredity can play a big part in whether they hatch early or late. There are lots of other things too that can cause early or late but heredity is real. The hatcheries operate on a schedule. The one that mail chicks to us have to ship at a certain time, any that hatch late aren't going to make it. The big hatcheries that hatch for commercial meat or laying operations are also on a tight schedule. They are not going to keep any that hatch late. If you set up a commercial hatchery that's one of the recommendations, do not keep any in your breeding flock that hatched late. Of course, those hatcheries use incubators, not under broody hens.
My eggs from my mixed flock consistently hatch a full day and sometimes even more early. That's the same whether it is under a broody hen or in my incubator. It is pretty rare, broody or incubator, for me to get many chicks to hatch on the 21st day. They are almost all early. That's been pretty consistent for several years. Heredity is the only thing I can think of to explain that consistency with my eggs hatching, especially the broody hens.
Over the years I have butchered a lot of pullets and hens. Some were laying, some were not. I even butchered a broody hen once. There is a big difference in what a laying hen and one not laying looks like inside. If a hen is laying there are many ova of varying sizes growing to make yolks for eggs. I haven't counted them but I'd guess maybe 10 or 12 in some advanced stage of growing. The plumbing that comprises her internal egg making factory is very visible, soft, and moist. If a hen is not laying her ova are tiny, close to pinhead size, and clustered very tightly. Nothing growing at all. And her internal plumbing is dried up, you almost have top look for it to find it. Very definite physical differences.
That's why I don't believe the stories where someone started their hens laying by feeding them hot pepper of something else and get results in a couple of days. If that happens it is coincidence, those hens had been getting their bodies ready to lay anyway.
My eggs from my mixed flock consistently hatch a full day and sometimes even more early. That's the same whether it is under a broody hen or in my incubator. It is pretty rare, broody or incubator, for me to get many chicks to hatch on the 21st day. They are almost all early. That's been pretty consistent for several years. Heredity is the only thing I can think of to explain that consistency with my eggs hatching, especially the broody hens.
Over the years I have butchered a lot of pullets and hens. Some were laying, some were not. I even butchered a broody hen once. There is a big difference in what a laying hen and one not laying looks like inside. If a hen is laying there are many ova of varying sizes growing to make yolks for eggs. I haven't counted them but I'd guess maybe 10 or 12 in some advanced stage of growing. The plumbing that comprises her internal egg making factory is very visible, soft, and moist. If a hen is not laying her ova are tiny, close to pinhead size, and clustered very tightly. Nothing growing at all. And her internal plumbing is dried up, you almost have top look for it to find it. Very definite physical differences.
That's why I don't believe the stories where someone started their hens laying by feeding them hot pepper of something else and get results in a couple of days. If that happens it is coincidence, those hens had been getting their bodies ready to lay anyway.
. Oh little question about that - is yesterday when I put the eggs under her day one or is today day 1?
Maybe tufting is dominant and she can still produce tufted chicks without having both parents be tufted. I'm sure it's a challenge and a genetic adventure! The tufting lethal ness would make me charge more for my chicks but don't think I would breed for chicks without them as to me they are a key feature of the breed that gives it's unique quality. But that's ME. 
). 
