Step 1. Development of a mapping population
To make a linkage map, first we need a segregating plant population. This is a population resulting from sexual reproduction. The production involves:
- Choosing two parents that differ for one or more traits of interest, and that have DNA markers that reveal differences (polymorphic markers).
In autogamous species, the parents need to be homozygous (inbred lines). In allogamous species, where many species do not tolerate inbreeding, parents are typically heterozygous at many loci. It is possible to use the direct F1 population (segregating in the case of heterozygous parents!). In some crops a generation of forced selfing of a single heterozygous plant may be used (but how to be sure, a priori, that it is heterozygous for a particular trait in that case?). Care should be taken that the population segregates for traits of interest (avoid crosses AA x AA, AA x aa, aa x aa etc). - Crossing the parents, and creation of a segregating population, consisting of at least 150 to 250 individuals. High-resolution mapping requires even larger populations.
The mapping procedure depends on the population type that is used: - F2, RIL or BC of inbred parents
- haploids or doubled haploids
- the F1 of heterozygous parents
- the progeny from selfing of one heterozygous (allogamous) parent
Summary
→ To create a linkage map, a segregating plant population must be "genotyped" for the marker alleles occurring in each member of the population
→ The parents of a mapping population are usually chosen to contrast in traits of interest, but the same population may be also used for other traits
→ For a mapping population in an autogamous crop the F1 of a cross should be heterozygous for many markers and for the traits of interest. For a mapping population of an allogamous crop, at least one parent must be heterozygous for markers and the traits of interest.
I want an alternative explanation | Move on to: Development of a mapping population in autogamous species