Traditional breeding and selection
A slightly more advanced application is whereby the breeder selects one or a few individuals as above but then crosses these with other individuals in order to incorporate a new trait. In the traditional setting, farmers prefer to mate their cows with large strong bulls in an effort to combine desirable traits into their own herd. Plant and animal breeders often combine traits from other varieties, wild relatives in the same or closely related species using this method. In addition to the gene(s) for the desired trait, other undesirable genes are also dragged along. It then becomes necessary to invest time in weeding these out through repeated backcrossing.