Here is a great explaination from a site out of the U.K.
Sex Linked or Auto Sexing ?
There appears to be much confusion over the difference between auto sexing and sex linked birds. Although both are sexed by the colour of their down, for a sex linked combination you need to keep both the parent breeds for example; if you wanted a gold/silver cross of Rhode Island Red (being gold) and Light Sussex (being silver) then you will need to keep both of those breeds to produce a chick that can be sexed on hatch because the sex linkage will not work for a second generation, only on the first generation crosses. If you put the adult chicks back to the father you will not be able to tell the difference between the boys and girls on hatch as you will simply have a hybrid
With an auto sexing breed because it is a pure breed ie. it breeds true, then whichever way you do the cross, father to daughter, mother to brother, aunts, uncles, whichever way you cross you will always be able to distinguish the boys from the girls on hatch because they are a pure breed.
There is a bonus to keeping more than one auto sexing bred and that is crosses of auto sexing birds. Virtually any cross of an auto sexing breed, in either way, will give you birds that can be distinguished on hatch by the colours of their downs and give a very good hybrid with excellent laying ability. There is only one cross that isnt recommended and that is the Buff to Silver cross.
Simple explanation - sex link is cross bred 'mutt' or 'barnyard' chicken. If you breed them back to one another you will not get more of the same chickens.
The easiest explanation comes from feathersite
Sex-Links are chickens which can be sexed at hatching, usually by color but sometimes by feather development. They are planned crosses, which are hardier and more productive than their parents' respective breeds. (Ed. note: Be aware that these are breed hybrids and will not breed true.)
(Ed. note: If you came to this page by some name that is not mentioned herein, it's because I can't keep up with all the commercial names that sex-linked chickens are marketed under, but I try to link the names to this page when I come across them.)
Both Red and Black Sex-Links use a red male for the father. Either a Rhode Island Red or a New Hampshire may be used.
Black Sex-Links are produced using a Barred Rock as the mother. Both sexes hatch out black, but the males have a white dot on their heads. Pullets feather out black with some red in neck feathers. Males feather out with the Barred Rock pattern along with a few red feathers. Black Sex-Links are often referred to as Rock Reds.
Red Sex-Links are the result of various crosses. White Rocks with the silver factor (the dominant white gene would produce all white offspring) are crossed with a New Hampshire male to produce the Golden Comet. Silver Laced Wyandotte crossed with New Hampshire gives the Cinnamon Queen. Two other crosses are obtained with Rhode Island White x Rhode Island Red, and Delaware x Production Red. These two crosses are simply called Red Sex-Links. Males hatch out white and, depending on the cross, feather out to pure white or with some black feathering. Females hatch out buff or red also depending on cross, and they feather out in one of three ways.
Buff with white or tinted undercolor (such as Golden Comet, Rhode Island Red x Rhode Island White)
Red with White or tinted undercolor (Cinnamon Queen)
Red with Red undercolor (Delaware x Production Red) (In this color pattern it is almost impossible to distinguish daughters' color from father's color.)
B/R hen + RIR roo = Black Sex Link
Delaware hen + RIR roo = Red Sex Link
White Leghorn hen + RIR = Red Star
White Rock hen + RIR roo = Golden Comet
Silver Laced Wyandotte hen + RIR roo =
There are other ways to come up with the sex links, like using New Hampshire roos instead of RIR's, but these seem to be the most widely used crosses.
And if you reverse the roo and hen breeds you come up with something completely different.
Normal white leghorns will not work in a sex linked cross. They do not contain the genetics that would produce a sex linked cross. Special breeds of white leghorn will work. They would have to be bred for the task. The same would be true for white rock- they have to be bred for doing sex linked crosses. Females used to do sex linked crosses must contain specific genes or the down color of the chicks can not be used to sex the chicks.
All black sex links are genetically similar; Non barred male on barred female.
This thread has some great info. I am building a "breeding" pen and coop for my RIR roo, some RIR hens, Barred Rock hens and White Rock hens to produce some additional laying stock. Does anyone know of a way I can determine if my White Rocks have the silver factor or can I breed them with a specific type of roo to get offspring with the silver factor? I would really like tp produce some Golden Comets as I already have a few and they are great birds. Thanks in advance for any info you can offer as I am novice to poultry breeding and genetics.