Mapping and Mapping Populations

The Classes
of Molecular Markers

Detecting DNA
Polymorphisms

Mapping and
Mapping Populations

Genetics of Mapping
Molecular Loci

Specialized Mapping
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Mapping and Mapping Populations

F₂, backcross, and recombinant inbred are the three primary types of mapping populations used for molecular mapping. An F₂ population is developed by selfing (or intermating for cross pollinated species) among F₁ individuals. These F₁ individuals are developed by crossing two parents that show significant polymorphism for whichever type of loci you are going to score. Backcross populations are developed by crossing the F₁ with one of the two parents used in the initial cross. The major drawback to using F₂ or backcross populations is that the populations are not eternal. Therefore, your source of tissue to isolate DNA or protein will be exhausted at some point in time. You then would have to begin mapping again in another population.

Populations of recombinant inbred lines can be a powerful solution to this problem. Recombinant inbred lines are developed by single-seed selections from individual plants of an F₂ population. (Because of this procedure, these lines are also called F₂-derived lines.) Single-seed descent is repeated for several generations. At this point, all of the seed from an individual plant is bulked. For example, a F_3:4 RI population underwent single-seed descent through the F₃ generation, and was bulked to develop the F₄. This population of seed can then be grown to obtain a large quantity of seed of each individual line. Importantly, each of the lines is fixed for many recombination events.

RI population level of inbreeding % within-line homozygosity at each locus

F_3:4 75.0

F_4:5 87.5

F_5:6 92.25

F_6:7 96.875

F_7:8 98.4375

F_8*9 99.21875

These lines have several uses. First, they can be used be used to derive a map because it essentially is an eternal F₂ population with unlimited mapping possibilities. Additionally, these lines can be scored for morphological traits (such as disease resistance or flower color) or quantitative traits (such as yield or maturity). This morphological trait data can then be compiled and those traits can be placed on the developing molecular map. These lines are especially powerful for analyzing quantitative traits because replicated trials can be analyzed using identical genetic material. The quantitative trait data can then be used to determine if any molecular markers are closely associated with those traits.