All chickens are of the species Gallus gallus domesticus. Varieties of a species are often called breeds. There are hundreds of breeds of chickens in existence. Domesticated chickens all descend from the ancestor the Red Junglefowl (Gallus gallus). Chickens have been bred for thousands of years, and apart from the recognized breeds, there are an infinite amount of variations, caused by crossbreeding. For example, if you cross a barred rock with a Lakenvelder, they will have offspring that will have traits of both parents, but won't be a true genetic breed.
A breed is considered a true genetic breed if, when mated with another induvidual of that breed, it reproduces offspring that are of the same breed type. So, if you wanted to create a new breed by crossing barred rocks and Lakenvelders, it would take many generations of breeding to create a true genetic breed.
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