Then select the Heterozygotes and repeat this again and again until the final F n+1: Heterozygote is closer in all other traits/ phenotypes to the original S cultured plant. Most R: plant disease-resistant traits originate from wild un-bred/ not yet cultured types. These wild types bearing the Resistant trait of interest, when bred into the S disease-susceptible cultured-bred type, the cross between the 2 always has the R trait-gene come together with a big piece of wild unwanted DNA.
This unwanted piece of DNA is only eliminated by repeating the cross again to the original cultured-bred type and selecting each generation only the best H plants. Best meaning the ones with the “smaller” piece of unwanted DNA.
To summarize, what the breeder does is a loop of:
- Make a Cross between an H: R/S plant and an S: S/S cultured plant.
Output: F1 50% H: R/S and 50% S: S/S - Select only the F1: H: R/S plants and Cross again to the original S: S/S plant.
Output: F2 50% H: R/S and 50% S: S/S - Select only the F1: H: R/S plants and Cross again to the original S: S/S plant.
Output: F3 50% H: R/S and 50% S: S/S - Repeat from 2-3 as many times as required.
How can using DNA markers ease or rationalize or even speed up this process?
- Select the 50% H plants using a DNA marker as close or even inside the R or any other gene of interest.
- Out of the 50% H plants, select the plants that are genetically as close to the original S: S/S plant.
Practically, how would you do it?
How can using DNA markers for Back-Cross selections actually assist you?
Here is the workflow we suggest:
1. DNA extraction + Real-time PCR H plants selection.
