(A) Rock pigeons can have one of four patterns on their wings: t-check (which is the most melanistic), checker, bar and barless (which is the least melanistic). (B) The genomic region that controls wing pattern in pigeons, dubbed the C locus, harbors a copy number variable (CNV) region that contains between one and four repeated units per chromosome. Increased copy number was associated with higher levels of melanism (shown by the grayscale): this suggests that the CNV region contains an element that regulates the expression of a gene called NDP. In addition, a mutation (purple star) in the start codon for this gene is likely responsible for the barless phenotype. Note: the unlabeled pigeon is an intermediate between the checker and t-check phenotypes. Images courtesy of the Genetic Science Learning Center, University of Utah; www.learn.genetics.utah.edu.