"Elaine Ostrander and Gordon Lark collaborated on a project to analyze the genetic basis of body size in dogs. They used both genotype and phenotype information to identify the gene IGF1 that controls growth. The IGF1 gene has two alleles, I and B. Large dogs have the genotype II, and small dogs have the genotype BB. If heterozygous dogs are medium-sized dogs, what does this indicate about the pattern of inheritance?"