Diseases with a Mendelian inheritance pattern

Many inherited diseases occur through a mutation within a single gene and with a pattern of inheritance that fits Mendelian genetics. This means that mutations within one or both copies of a gene result in clinical effects with a dominant, codominant or recessive expression pattern (Table 3.32.1 and Fig. 3.32.1). Dominant inheritance means that a mutation within one copy of a gene produces clinical effects when the second gene copy is normal. Recessive inheritance means that mutations are required within both copies of a gene in order that a clinical effect becomes evident. Individuals possessing one recessively inherited mutated gene and one normal gene are termed carriers: that is they may pass the mutation to offspring but do not show clinical evidence of the disease. X-linked disorders are those in which a mutation is present within an X-chromosome, usually in a recessive manner; therefore, the effect of the mutation is masked in females whose second gene copy is normal (these individuals are carriers) but is present in males carrying the mutation since there is no second copy of the gene present within the Y-chromosome.

Table 3.32.1 Examples of genetic diseases

a Approximately 20% of males inheriting the fragile X mutation are phenotypically normal but can act as carriers.

Fig. 3.32.1 Mendelian inheritance of diseases. C, normal gene; c, abnormal gene.