Patterns of inheritance - Genetic syndromes

Patterns of inheritance

Autosomal dominant: Mendelian pattern of inheritance where the presence of a single abnormal allele is able to produce the disease. There may be reduced expression of the condition if the condition does not have full penetrance (see Fig. 14.1 and Table 14.1).

Autosomal recessive: Mendelian pattern in which both genes must be defective to produce the clinical phenotype (see Fig. 14.2 and Table 14.2).

X linked conditions are those that appear on the X chromosome, i.e. are linked to the sex of an individual. There is no male-to-male transmission, daughters of an affected male will be obligate carriers. In X linked dominant conditions, females may also demonstrate the clinical phenotype, and may also demonstrate mosaicism as a result of random X inactivation (see Fig. 14.3 and Table 14.3).

Mitochondrial: A number of conditions that do not follow normal Mendelian patterns of inheritance. In this set of conditions males and females may be affected, but only the offspring of female sufferers may be affected. These conditions result from the maternally inherited mitochondrial DNA, which code for at least 37 mitochondrial enzymes (see Table 14.4).