Katchit

Hello; i am new to the forum. I would like to ask you about selective breeding and the chances of producing a Taurus (bull) that displayed favourable traits and phenotypes of 2 selectively-picked cattle; thus to produce a better offspring as a result. Taurus have 30 chromosome pairs; so what are the potential outcomes of the breeding? How do favourable phenotypes become dominant? is that just a result of the phenotype being heterozygous dominant? Am i correct in thinking that it is important to keep as many different phenotype combinations in genes as possible; as this allows for diversity within a species?!? if the potential phenotypic combinations were reduced; would this mean that the species would begin to look more and more alike with continued breeding? Thank you for your time and help; much appreciated.