Almost all diseases have a genetic component. In children
and young adults in particular, many of the disorders
causing long-term morbidity and mortality are
genetically determined. The molecular basis of most
Mendelian (or ‘single-gene’) diseases has now been
determined, and our understanding of the abnormalities
in cell function responsible for the clinical presentation
is improving. It has also become clear that variants in
many genes contribute to the pathogenesis of several
common diseases such as asthma, rheumatoid arthritis
and osteoporosis. In this chapter, we review key principles
of cell biology, cellular signalling and molecular
genetics, with emphasis on the diagnosis and assessment
of patients with genetic diseases.