Simmental is a good example of a breed that is no longer purebred, as like you say, they are "up breeding to turn the cattle black. Simangus are in fact, crossbreds. I assume you are talking about Balancers when you said Gelbvieh, These can be 75% to 25 % Gelbvieh and the rest red or black Angus, This is not a stablized breed yet. Black Hereford and Brangus are established breeds. BH's are 5/8th Herford and 3/8th Angus., Br are 5/8 angus and 3/8 Brahma. Breed a Brangus to a Brangus, and you get a Brangus. Cattle have 60 chromosomes.. 29 pairs that make a Hereford a Hereford, or a Black angus a Black angus, and 1 pair of sex chromosones. So if you breed a herford to angus, the calf.. black baldy..has 1 set of angus Chromosomes and 1 set of Hereford chromosomes. Half a black baldy's eggs will have a full set of Hereford chromosomes, and half will have Angus. Same with a black baldy bull. If you breed a black baldy cow to a black baldy bull, 25% of the caves will be full blooded angus, 50% will be black baldies, and 25% will be a red hereford. Breed a Black hereford to a Black hereford, and 100% of the calves will be Black Hereford.
For a new breed to be developed from a 2 way cross, it is 3 generations. For example, when they developed Brangus. The first cross was a full blooded Brahma to a full blooded Angus, This is called F1. Then they took the 12/Br 1/2 Ang, and bred them back to Angus. This produced a 3/4 angus and 1/4 Brahma. ..these are F2. These were then bred back to 1/2 Angus and 1/2 Brahma, and those calves were 5/8ths Angus and 3/8ths Brahma, and they now have their own genetic material in their 29 pairs of BRANGUS chromosomes. Breeding a Brangus to a Brangus can never result in a Brahma calf or an Angus calf.