[[[[[........With genetics, first generation offspring is very controlled if the parents are pure. You know exactly what you are going to get. But with the second generation, where you are mating crosses, you generally don't know what you are going to get.......]]]]
The stock has been carefully bred for decades now. The genes are very concentrated, so that the grandparent birds all of them homozygous for the desired traits. They aren't going to pass on anything else. The company that breeds them knows exactly how the genes are going to nick. They know what is dominant and what is recessive and they know what flock contains what gene and how it will cross with the second flock. Perhaps some of the traits are color linked so they can cull the wrongly colored ones. They are not breeding willy-nilly.
These are not backyard chickens with unknowns back a few generations in the pedigrees.
Simplified. Maybe flock A and Flock B are both homozygous for dominant white. All of their offspring in Flock C will be homozygous for white (and whatever traits they are bred for, perhaps fast growth) and Flock C can only pass on 2 Dominant white genes.
Then flock D is red but has genes for huge breast meat, and flock E is black and has the genes for huge breast meat (and whatever else, perhaps large leg bones). Flock F will have chicks that are either black or red and have huge breasts, but when they are bred to Flock C with Homozygous white genes, all the offspring (that you are buying for your home) will be white and also have the genes for huge breast meat which are concentrated in flock F. (but they will carry a hidden gene for either black or red)
You do the same at home on a very small scale if you are breeding purebreds that are all a specific color. The color genes are concentrated and only that color can be passed down to the next generation. If you have purebred RIR's all your chicks will hatch out red.
Now, because the Cornish Cross get some genes from one side of their family and other genes from the other side, you are the one who could get all sorts of different things if you breed them together.
It's very common in plant breeding. One rose has concentrated genes for red and another has concentrated genes for fragrance and you breed them together and you get red fragrant roses every time. You can't get those concentrated genes in the parents of Cornish Cross unless you are controlling the genes of their parents, ie: the grandparents of your Cornish Cross. There are decades of controlled breeding and the concentrating of genes behind the Cornish Cross that you purchase.
