In the context of hereditary diseases, consider the following statements: 1. Passing on mitochondrial diseases from parent to child can be prevented by mitochondrial replacement therapy either before or after in vitro fertilization of egg. 2. A child inherits mitochondrial diseases entirely from mother and not from father. Which of the statements given above is/are correct?

Answer: C. Both statements about mitochondrial diseases are correct. MITOCHONDRIAL DISEASES are a group of disorders caused by mutations in MITOCHONDRIAL DNA (mtDNA) — the small circular genome inside mitochondria (the 'powerhouses' of the cell). These disorders manifest as muscle weakness, neurological symptoms, organ failure (Leigh syndrome, MELAS, MERRF, Leber's hereditary optic neuropathy etc.). Statement 1 is CORRECT. MITOCHONDRIAL REPLACEMENT THERAPY (MRT) is a clinical IVF-related technique pioneered to prevent transmission of mitochondrial diseases. Two main approaches: - MATERNAL SPINDLE TRANSFER (MST): the mother's nuclear DNA is transferred from her egg (with diseased mitochondria) into a healthy donor egg from which the nucleus has been removed, BEFORE in vitro fertilisation. - PRONUCLEAR TRANSFER (PNT): performed AFTER fertilisation — the pronuclei (nuclear DNA from sperm and egg) of the patient's fertilised egg are transferred into an enucleated donor zygote with healthy mitochondria. The resulting child has nuclear DNA from both parents and mitochondrial DNA from the donor. The UK approved MRT in 2015; first MRT babies have been born. So MRT can prevent transmission either BEFORE or AFTER IVF, depending on the technique. Statement 2 is CORRECT. MITOCHONDRIAL DNA IS INHERITED EXCLUSIVELY FROM THE MOTHER (maternal inheritance). This is because: - The egg cell contributes the entire cytoplasm (with hundreds of thousands of mitochondria) to the zygote. - The sperm's few mitochondria (in the tail) are actively destroyed during fertilisation or are diluted to negligible levels. - Hence MITOCHONDRIAL DISEASES caused by mtDNA mutations are MATRILINEALLY INHERITED — a child gets them entirely from the mother, never from the father. This explains why MRT focuses on replacing maternal mitochondria. Source: NCERT Class 12 Biology Chapter 'Principles of Inheritance and Variation' / The Lancet on Mitochondrial Replacement Therapy.