Figure 1. Mendel's experiment demonstrating the properties of dominance and recessivity. Cross-pollination between pure-bred lines of peas grown from round and wrinkled seeds gave rise only to round seeds (F₁ generation). However, these F₁ plants produced wrinkled seeds as well as round seeds when intercrossed, in a ratio of about three round to one wrinkled (F₂ generation). Mendel explained this pattern by postulating that the phenotype was determined by the combination of factors R and r. The round is dominant over the wrinkled trait because the round trait is manifested in the heterozygote Rr. Conversely, wrinkled is recessive to round.

Figure 2. Dominance relationships between a pair of alleles A and B. (a, b) Phenotypes corresponding to the different genotypes AA, AB and BB are indicated by filled rectangles of different tones. (c) In many dominantly inherited diseases, the phenotype associated with the homozygous mutant BB has not been observed; hence it is not known whether allele B is a true dominant or semidominant, with respect to A.

Figure 3. Typical pedigrees showing autosomal dominant and autosomal recessive inheritance. Affected and unaffected individuals are denoted by filled and open symbols (square, male; circle, female) respectively. (a) Autosomal dominant inheritance of mutant allele B. Transmission of the phenotype occurs vertically between generations. On average, 50% of the offspring of an affected individual are themselves affected, irrespective of sex. (b) Autosomal recessive inheritance of mutant allele B. Consanguinity is frequent, as shown here (closely spaced parallel lines). Usually only a single sibship is affected, with previous and succeeding generations free of the disease. (c) If there is extensive inbreeding or the recessive mutant allele B is very common, pseudodominant inheritance may occur.

Figure 4. Relationship between protein level and metabolic activity. Most proteins act at the asymptotic end of the activity curve. A 50% reduction in protein compared with the wild-type level, caused by a heterozygous loss-of-function mutation, results in a reduction in activity of less than 10% (assumed to reflect the phenotype); complete loss of the protein abolishes activity. Hence the phenotype of the heterozygote resembles wild type and the mutation is recessive.

Figure 5. Common mechanism of dominant negative mutation. (a) Dimerization mediated by the left half of the normal monomeric protein activates the function of the right half (shown as a change to shaded fill). (b) Heterozygous mutation that abolishes the activation domain but does not affect dimerization will cause half of the normal protein to become sequestered into nonproductive signaling complexes.