Here how to breed a pure chooks strain just from Roosters.Example ; Marans. its called " Absorption crossing"
1) Marans Rooster BCM X your hens = F1 50% marans gene.
2) Cross F1 Cockerel ( the one look like a maran phenotype) cross him to his sister/half sister F1 ( maran phenotype ) = F2 in this generation they will be a lot variation in the phenotype.
3) Pick the F2 pullets ( the one that are more marans looking and lay darker eggs ) CROSS them to a pure BCM Rooster = F3 ,they are 75% marans.
4) cross this F3 cockerel to his F3 sisters / half sisters . always go with the marans looking chooks = F4 a litle variation in this generation but not as much as the F2.they still 75% marans genes
5) Pick the F4 best pullets ( marans type) and cross them to a BCM Rooster = F5 they will be 87.5% marans genes.
6) Cross the F5 cockerel to his sisters/half sisters . pick the marans looking chooks = F6 no much variation in this generation by now most of the chooks looks like marans and you will see the one s currying any recessive genes they are a cull .
7) Pick the best F6 pullets and Cross them to a BCM Rooster = F7 this generation will breed true marans
ONE thing is very important is to CROSS the sibling together each type you use a new Rooster. WHY . is to breed out any recessive genes IF you don t do that you will never breed true marans BECAUSE the recessive genes of the original strain crossed to the marans will be passed down.
HOPE that Help