This lecture covers the molecular and chemical basis of genetic diseases, focusing on abnormalities in genes and chromosomes. Specific examples such as Phenylketonuria, Hereditary Angioedema, and Haemophilia A are discussed in detail, including their molecular mechanisms, mutations, and metabolic implications. The discovery and development of small molecule drugs for genetic disorders, like Platensimycin, are also explored, highlighting the need for new antibiotics and the challenges faced by pharmaceutical companies in this field.