Which cross design would you use to map the Eda effect on plate number?

Which cross design would you use to map the Eda effect on plate number?

• A. Cross high-plated marine with low-plated freshwater and analyze offspring to map Eda.
• B. Cross high-plated marine with low-plated freshwater and analyze offspring to map Eda.
• C. Cross two marine individuals and compare to freshwater.
• D. Cross freshwater with an outgroup species.

Answer: (B)

To map a gene that controls plate number, you need a cross that makes the two relevant alleles segregate in the offspring. The Eda gene has a strong effect on plate number in sticklebacks: marine fish tend to be fully plated, while freshwater fish carry the allele that reduces plating. Crossing a high-plated marine individual with a low-plated freshwater individual creates progeny that inherit different Eda alleles, producing variation in plate number among the offspring. By linking those plate-number differences to genetic markers across the genome, you can locate the Eda region. Other crosses don’t provide informative segregation for Eda: crossing two marine individuals would mostly carry the same Eda allele and show little variation, and crossing with an outgroup introduces many confounding genetic differences that make it hard to map a single gene's effect.

To map a gene that controls plate number, you need a cross that makes the two relevant alleles segregate in the offspring. The Eda gene has a strong effect on plate number in sticklebacks: marine fish tend to be fully plated, while freshwater fish carry the allele that reduces plating. Crossing a high-plated marine individual with a low-plated freshwater individual creates progeny that inherit different Eda alleles, producing variation in plate number among the offspring. By linking those plate-number differences to genetic markers across the genome, you can locate the Eda region.

Other crosses don’t provide informative segregation for Eda: crossing two marine individuals would mostly carry the same Eda allele and show little variation, and crossing with an outgroup introduces many confounding genetic differences that make it hard to map a single gene's effect.