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Sex linked chicks are ones that hatch and you can tell from the coloring which are boys and which are girls. Breeding certain breeds together will give you these results and they often produce reds, blacks, buffs and some funny mixes of those colors when grown depending on parents. They do not breed true but will usually lay around 250-300 eggs a year the first 2 years and then slow down after that a good bit.
Nadine is right. Certain genes are "sex linked", being contributed by the sire or dam. Half of the genetic make up is contributed by either. The advantage of taking advantage of these "sex linked" traits is that the cockerels can be seperated from the pullets at hatch based on feather color etc. A rhode based (red) cock on a white (silver) hen produce red pullets and mostly white cockerels making them easy to differentiate from each other at hatch. The same cock on a barred bird (like a barred rock) makes for barred cockerels and black hens. They can be seperated at hatch by a white spot on the cockerels head and an all black chick for the pullet. Cockerels in commercial laying operations are considered a waste product, and they want to dispose of them before they invest any money or labor into them.