To map Eda's effects on plate number, which cross design is appropriate?

• A. Cross a high-plated marine with a low-plated freshwater, then analyze offspring genetics and plate phenotypes to map the Eda region.
• B. Cross two high-plated individuals from different populations.
• C. Cross marine with freshwater and just observe plate numbers.
• D. Do a backcross to an outgroup species.

Answer: (A)

To map Eda’s effect on plate number, you need a cross that creates segregating offspring for the allele of interest so you can link phenotype to genetic markers. Crossing a high-plated marine individual with a low-plated freshwater individual brings together the contrasting Eda alleles, producing offspring that differ in plate number. By genotyping these offspring across many markers and recording their plate phenotypes, you can see which genomic regions co-segregate with the plate trait, pinpointing the Eda region. The other options don’t provide this mix of contrasting alleles and segregating phenotypes (or rely on an outgroup), so they don’t enable effective mapping.