It is thought that sometimes in meiosis these two copies, on the same chromatid, align and recombine leading to excision of the DNA between them as in the figure below. Sometimes deletions can cover more than one gene. When this occurs we have an example of a contiguous gene syndrome . For instance, some patients with Tuberous sclerosis also suffer from polycystic kidney disease. The two loci TSC2 and PKD are adjacent on chromosome 16 and can be simultaneously deleted.
Some deletions become so large that they are visible on well stretched metaphase chromosomes.
Contiguous gene syndromes will become likely. More on this later. Trinucleotide expansion The commonest inherited cause of mental retardation is a syndrome originally known as Martin-Bell syndrome. Patients are most usually male, have a characteristic elongated face and numerous other abnormalities including greatly enlarged testes. The pattern of inheritance of this disease was, at first, puzzling.