FIGURE 6-4 Segregation patterns of dominant and recessive mutations in crosses between true-breeding strains of diploid organisms. All the offspring in the first (F₁) generation are heterozygous. If the mutant allele is dominant, the F₁ offspring will exhibit the mutant phenotype, as in part (a). If the mutant allele is recessive, the F₁ offspring will exhibit the wild-type phenotype, as in part (b). Crossing of the F₁ heterozygotes among themselves also produces different segregation ratios for dominant and recessive mutant alleles in the F₂ generation.