Stickleback Practice Test

This question is about proving causality by directly manipulating the genetic variant and watching the effect on the trait. The strongest way to test if a variation in Eda causes changes in plate number is to swap the Eda allele between individuals using genome editing and then compare plate numbers in a shared genetic background. By editing only the Eda allele and keeping all other genes and the environment the same, any difference in plate number can be attributed to the specific Eda variant. If the swapped allele leads to a different plate count, that provides direct evidence that Eda variation drives the trait. Observing natural populations or comparing knockouts alone introduces confounds. A cross between species or transferring Eda across backgrounds brings many uncontrolled genetic differences and environmental factors, muddying the link between Eda variant and plate number. A knockout shows whether Eda is necessary for the trait, but it doesn’t test how natural allelic differences influence plate number in a controlled background.