Naked neck is dominant.. but also has a dose effect, which means there can be a visible difference between birds carrying one or two copies(AKA pure) for the NN gene.
You can breed showgirl to showgirl, they will be fine.. a common reason breeders don't do that often is because they feel it makes "too naked" chicks(they're just getting some chicks pure for the naked neck gene).. but this is a personal taste and if the stock also has pea comb*, that can make them even more naked than the usual turken. Yet this same effect can be seen as a bonus: showgirl and silkie crosses can and do throw chicks with small bowties.. normally(in turkens) a cross between naked neck and not naked neck results in birds with huge bowties that cover the entire lower neck. So this makes it extremely easy to do showgirl-silkie crosses.
If your showgirls have good feather and body type then you can breed however you wish. If they could use some improving, then it's really best to breed with a pure silkie with good or better feather and body type.. this will help a lot in making 'better looking' showgirls. It will be easy as showgirl x pure silkie will throw at least half showgirls.. 100% showgirls if the showgirl is pure for the naked neck gene.
*Silkie standard calls for walnut, which is genetically both rose and pea comb genes combined, however a lot of silkies simply have a modified rose comb without the pea comb gene present.
