Figure 1. Autosomal and X-linked recessive inheritance. Squares and circles represent males and females, respectively. The open symbols represent normal individuals. For autosomal recessive inheritance, completely black symbols represent affected individuals who are homozygotes, i.e. they have two copies of the disease allele (a). Half-filled symbols represent heterozygote carriers, i.e. they have one copy of the disease allele (a), one copy of the normal allele (A), and have a normal phenotype. For X-linked inheritance, circles with black dots are heterozygote carriers and filled squares are affected males with one disease allele on their single X-chromosome, i.e. hemizygous. X^N and X^M represent the normal (N) and mutated (M) allele, respectively, on the X-chromosome. Y is the Y-chromosome.

Figure 2. A metabolic pathway. A, B, E, F represent intermediate metabolites in the normal (a) and disease-involved (b) pathways. The size of the letters indicates the relative amount of each intermediate that is present in the steady state.

Figure 3. (a) Idealized retrovirus (RNA virus) used for gene therapy. LTR, GAG, POL and ENV represent the long terminal repeats, the glycoprotein coat protein, reverse transcriptase and envelope protein, respectively, that are present in the infective or wild-type virus. (b) The GAG, POL and ENV are removed from a potential therapeutic virus and replaced by the gene of interest. The sequence is needed to produce viral particles that contain all the elements for infection, i.e. it is a packaging signal.