What is a practical reason to use cross populations to locate genomic regions associated with a trait?

• A. It increases power to detect QTL by recombination in mixed backgrounds.
• B. It reduces genetic variation and makes mapping harder.
• C. It eliminates the need for genotyping by using only phenotypes.
• D. It makes the experiment easier by avoiding recombination.

Answer: (C)

Cross populations are valuable because they bring together diverse genetic backgrounds and create many recombination events across the genome. This abundance of recombination helps break up long linked blocks of DNA, so the association between a trait and nearby genetic markers can be narrowed down to smaller genomic regions. By comparing marker genotypes with the observed phenotypes across individuals from different parental backgrounds, you can detect which marker alleles track with the trait, giving you power and precision to locate the genomic regions involved.

Using only phenotypes would not reveal where in the genome the trait is located, since phenotype data alone tells you nothing about the underlying DNA variants. Likewise, relying on less recombination (or reducing genetic variation) would make it harder to separate the effects of closely linked loci and would reduce mapping resolution. Recombination is a crucial aspect of mapping, and genetic variation across cross populations provides the necessary signal to link traits to specific genomic regions.