Genetics of Mapping Molecular Loci

Genetics of Mapping
Molecular Loci

Genetics of Mapping Molecular Loci

Each of the mapping populations will give a specific segregation ratio at each locus. The knowledge of these ratios is important to determine if the population is expressing a skewed segregation ratio at any locus. The following are the ratios that you would expect at each locus for codominant and dominant makers segregating in the three types of populations.

Population Codominant loci Dominant loci

F₂ population 1:2:1 3:1

Backcross population 1:1 1:1*

RI population 1:1 1:1

*To score a dominant maker in a backcross population, you must cross the recessive parent with the F₁ plant. Therefore to score RAPD loci you would need to create two populations, each one developed by backcrossing to one of the two parents. For this reason, backcross populations have not been used for mapping RAPD loci.

Once you have analyzed your segregating population by RFLP, RAPD or isozyme makers and have determined that the segregation ratio of each locus does not deviate from the expected ratio, you are ready to begin developing the map. (It should be noted here that scientists which develop molecular maps normally include those loci with skewed segregation ratios in their mapping analysis.) All of the segregation data is then compiled and used to derive the linkage relationship among the markers. This analysis is performed using computers and one program widely used is called MAPMAKER. This procedure is based on the maximum likelihood method. The output from this program is a linear relationship among the markers and the distance between the markers is measured in centimorgans. (You will be learning to use this software package, and the specifics of its use will be given at that point.)