Q.The genetic defect adenosine deaminase deficiency may be cured permanently by a) Enzyme replacement therapy b) periodic infusion of genetically engineered lymphocytes having ADA cDNA c) administering adenosine deaminase activators d) introducing bone marrow cells producing ADA into embryo at an early stage of development

Gene therapy corrects a hereditary disease by delivering a normal, functional copy of a gene, via a vector, into the cells of a patient carrying a mutant allele, so that the expressed normal gene product restores a normal phenotype; it has mainly targeted single-gene disorders like cystic fibrosis and haemophilia, for which no other effective treatment exists. Two strategies exist — gene augmentation therapy (adding a working gene copy) and gene inhibition therapy (silencing a harmful dominant gene with an antisense gene) — and two levels of application: somatic cell gene therapy (targeting non-reproductive cells like bone marrow, blood or skin; corrects the patient but is not inherited) and germline gene therapy (targeting eggs or sperm; heritable, and far more ethically contentious). The landmark case is ADA-deficiency SCID, first treated clinically by French Anderson in 1990 in a four-year-old girl: lymphocytes are drawn from the patient's blood, infected in culture with a disabled retrovirus carrying a working ADA gene, and reinfused; because these engineered cells are not immortal the patient needs periodic repeat infusions, and only introducing the corrected gene at the early embryonic stage would offer a truly permanent cure.

"Gene therapy definition ADA deficiency SCID example" and "gene therapy class 12 biology important questions" are frequently searched terms in the Biotechnology and its Applications chapter of the NCERT-aligned CBSE Class 12 Biology curriculum, a favourite NEET application-based topic. The 1990 ADA-SCID case discussed here, including why repeat infusions are needed, is a commonly asked case-study question in competitive biology exams.