Most pedigree breeders will line breed in some form , while I am not pedigree now , 9 of my 10 hill rams all have a certain ram in their genes as do most of the females , this is to try for uniformity of type . I hope you have deep pockets if you want to line breed pedigrees . First forget about your existing sheep it will take ages to breed up , join the DORSET DOWN association and read every pedigree book and sale catalogue you can find, speak to every breeder you can and go to sales/flock visits then eventually decide which male or female line you really like . TRY then to buy as many females with the particular ram or ewe in their pedigree and a ram with the same as you can , hopefully the weak genes will have been bred out