This comes from Sigrid Van Dort's latest Silkie book. Its an explanation for why a single comb can crop up in a flock where its not been seen in the past. "Single combs can occur in R/R when the rose comb fraction in the semen is dead because the rooster didn't mate often enough. In pure rose combs its advised to let the rooster mate the hen at least every three days. And a single comb can occur when r+ and p+ come together when hen and rooster are R/r+, P/p+, R/R, P/p+." She goes on to further describe combinations that could cause the occurrence of a single comb. Most of it has to do with which genes the breeder birds possess and how it comes together in breeding.