The technical advances in molecular biology have endowed us with a wealth of knowledge, which has allowed us to identify the cause of diseases not only at a single gene level but at a greater magnitude, where a substitution or deletion of a single base pair can be identified. Our present task is to establish a clear link between phenotype and nucleotide sequence. Obviously, a gene is no longer an imaginary entity. Recent discoveries in a number of bewildering traits, whose inheritance do not follow simple mendelian rules, have caused much amazement. For example, fragile X-syndrome, spine and...