How important is the study of one patient? In the analysis of genetic diseases, studies of individual patients have been extraordinarily useful for determining chromosomal localization of disease and the ordering of loci on an individual chromosome. Studies of X-linked ichthyosis illustrate this very well.
X-linked ichthyosis (recently reviewed by Shapiro1 is very common, 1/2000 to 1/6000 males, and is life-long. The complexity of the inherited disorders of keratinization and their treatments requires the special skills of the dermatologist for diagnosis, treatment, prognosis, and genetic counseling. Formal genetic analysis of affected patients shows the disease was caused by a defect on the X chromosome. It is just over a decade since the molecular basis of the disorder, a deficiency of the microsomal enzyme steroid sulfatase, was first determined; testing the cultured fibroblasts of patients for this enzyme is a standard way of verifying the diagnosis. Finding increased blood cholesterol
Goldsmith LA. Look at the Genes, See What's in the Jeans. Arch Dermatol. 1990;126(5):585-586. doi:10.1001/archderm.1990.01670290027003