Alternative explanation: linkage maps
What?
A linkage map is a graphical representation of all the genetic positions of markers (and sometimes genes of monogenic traits) relative to each other.
How?
First, a set of genetic markers must be developed. Then, a population is scored for polymorphic markers. Statistics will show which markers are linked on the same linkage group. In addition, the recombination frequencies between markers can be used to estimate their order and genetic distances. This way, per linkage group a linear map can be drawn showing the estimated order and approximate distances between marker loci.
Why?
Markers are easily made visible. Scoring for traits is often laborious. Selection for favorable genes may be substituted by selection for a linked marker. This can also be done in other populations (e.g. breeding populations), if the marker also
segregates in those populations. So, ideally, creating the map is a one-time effort after which the map can be used many times.