The way I read that they used the Vantress male line and the Cobb female line to make the Cobb 500. The company was Cobb Vantress Inc. So yes, separate male and female lines combine to make that broiler, even today. I'll copy a line from that article to show part of why I think this.
In the search for a productive female that would fit this new direction
I don't know if the Cobb 500 takes advantage of dwarfism, some broiler lines do. Dwarfism is a recessive sex linked trait that reduces the size of the bird, reducing feed costs and some of the problems with overgrowth. Not sure how familiar you are with sex linked breeding, but this would enable the entire female side to be dwarf as long as the male side is pure for not-dwarf. I'll offer this as an example of the genetic tweaking that has gone on with the broilers over the years.
Selective breeding to develop that fast growth takes generations of chickens. You are not going to get the growth you see in today's Cornish X with a simple cross of a true Cornish with a White Rock. Generations of chickens means years. By all means try that cross. I trust what I see more than what I read on the internet so see for yourself. You should get a bird with more breast meat than most other crosses since you have that Cornish in there. Try to start with the best stock you can. The more chicks you hatch the better your chances of getting good breeders. I'm not prepared to hatch and raise more than I can eat to get really good choices for breeders in my backyard.
I'm basing the following on my own thoughts, not anything I've read specifically about the broilers. What is the real purpose of the grandparent flocks? Why don't you just have two parent flocks? The way hybrids work it would seem inconsistent to have separate important traits in the flocks that combine to make the father of the broiler though you do want different traits in the flocks that make the father versus the mother of the broiler. I think the grandparent flocks for the purpose of genetic diversity, instead of just having two parent flocks.
When you create a new breed or in this case a type, you want to remove all traits that don't give you what you want. A simple example, if you want a white bird get rid if any genetic diversity would give you any other color. You want to eliminate certain genetic traits that give you diversity in critical traits.
But if you don't maintain genetic diversity in other traits over generations they become less thrifty. They can lose fertility or become more susceptible to diseases. Inbreeding is how these birds were originally developed, but once you get to a certain point you have to manage inbreeding. Breeders have different techniques for that, I don't know which techniques are used on the broiler grandparent flocks. That's part of how they manage them. They have maintained these flocks for decades without them becoming too inbred.
But with the parent flocks I suspect they want to take advantage of hybrid vigor. When you cross two different genetic lines you often get a boost due to what we call hybrid vigor. You don't have to cross two different breeds to take advantage of this. Even if you cross two strains of Delaware from different flocks that have been genetically separated for several generations you can get a boost in hybrid vigor.
Developing and managing the Cornish X is a lot more than just genetics. But please, try it yourself and come back on here to report your results. It's how you and we learn.
