(I) Although in the great majority of cases a person's genetic make-up results in normal development, genes can occasionally be a source of problems. (II) If a defective gene inherited from one parent is recessive, the dominant - and usually normal - allele from the other parent can prevent this particular problem.(III) Indeed, some human disorders are entirely hereditary and are passed along according to the same principles of inheritance that determine eye colour and nose shape. (IV) Other genetic disorders are not inherited but may result from errors during cell division in meiosis. (V) Chromosomes and the genes they carry can also be made abnormal by radiation, drugs, viruses, chemicals and perhaps even the aging process.