Complete UPSC Notes — What is mitochondrial disease, how MRT creates a "three-parent baby," Maternal Spindle Transfer (MST) vs Pronuclear Transfer (PNT) with step-by-step diagrams and comparison table. UK's 8 healthy babies (NEJM, July 2025). PYQs from UPSC 2020 & 2021.
🔋 Mitochondria = Cell's Power PlantmtDNA = Inherited Only from MotherPYQ: Pronuclear Transfer (UPSC 2020)PYQ: MRT & Inheritance (UPSC 2021)8 Healthy Babies Born in UK (NEJM 2025)37 Genes from Donor · 99.9% Parents' DNA
Think of your cells as phones. Each phone has a battery — that's the mitochondria. Mitochondria generate over 90% of the energy your cells need to function. Now imagine some phones have faulty batteries — they keep dying, overheating, or shutting down. That's mitochondrial disease. When too many "phone batteries" fail, the organs they power (brain, heart, muscles, eyes) start failing too — which can be fatal. The catch? You inherit your batteries only from your mother. If mom has faulty batteries, all her children will have faulty batteries too.
📌 Key Facts:Mitochondria = "power plants" inside almost every cell. Have their own DNA (mtDNA) — separate from nuclear DNA. mtDNA is inherited exclusively from the mother (sperm mitochondria are destroyed during fertilisation). Mitochondrial diseases affect energy-dependent organs: brain, heart, muscles, eyes, liver. About 1 in 200 children in the UK is born with a mitochondrial disorder. These diseases include muscular dystrophy, epilepsy, heart problems, and intellectual disabilities.
📌 Why Only From Mother? During fertilisation, the sperm contributes only nuclear DNA — its mitochondria are destroyed. The egg contributes both nuclear DNA and all mitochondria. So a child's mitochondrial DNA comes 100% from the mother. If the mother's mtDNA has mutations → all children inherit the disease. There is no treatment to "fix" faulty mtDNA once you're born.
Section 02
👶 What is MRT? — The Three-Parent Baby
💡 The "Egg Repair" Analogy
Imagine the mother's egg is a phone with a perfect screen and processor (her nuclear DNA — determines appearance, intelligence, etc.) but a faulty battery (her mitochondria). A donor provides a phone with a good battery (healthy mitochondria) but we don't want the donor's screen. So we take the mother's screen (nuclear DNA) and put it into the donor's phone body with the good battery (healthy mitochondria). The result? A phone with mom's screen + donor's battery. The baby gets: nuclear DNA from both parents (99.9%) + mitochondrial DNA from the donor (0.1%, ~37 genes) = three genetic contributors = "three-parent baby".
📌 What the Baby Gets: ~20,000 genes from mother & father (nuclear DNA = appearance, personality, health traits) + ~37 genes from mitochondrial donor (mtDNA = only energy production, NOT physical characteristics). The baby is 99.9% genetically the parents' child. The donor contributes only the "battery," not the "blueprint."
Section 03 — Technique 1
🔬 Maternal Spindle Transfer (MST) — Step by Step
📌 Key Point: MST is done BEFORE fertilisation — on unfertilised eggs.
🔬 Maternal Spindle Transfer (MST) — Before Fertilisation
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1
Mother's Egg — Remove Nuclear DNA
Mother's egg has healthy nuclear DNA but faulty mitochondria. The spindle (structure holding the chromosomes) is carefully removed from the mother's egg, leaving behind the faulty mitochondria.
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2
Donor's Egg — Remove Her Nuclear DNA
Donor's egg has healthy mitochondria but we don't want her nuclear DNA. The donor's spindle (her nuclear DNA) is removed and discarded. What remains: an egg with healthy mitochondria but no nuclear DNA — an "empty shell" with a good battery.
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3
Insert Mother's Spindle into Donor Egg
Mother's spindle (her nuclear DNA) is inserted into the enucleated donor egg. Now we have: mother's nuclear DNA + donor's healthy mitochondria in one egg.
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4
Fertilise with Father's Sperm → Embryo
The reconstructed egg is fertilised with the father's sperm via IVF. The resulting embryo has: nuclear DNA from mother + father, mitochondria from donor = three genetic parents.
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5
Implant → Healthy Baby Born
Embryo is implanted in mother's uterus. Baby is born with 99.9% parents' DNA and healthy mitochondria from donor. No mitochondrial disease.
Section 04 — Technique 2
🔬 Pronuclear Transfer (PNT) — Step by Step
📌 Key Point: PNT is done AFTER fertilisation — on fertilised eggs (zygotes). This is what UPSC asked about in 2020!
🔬 Pronuclear Transfer (PNT) — After Fertilisation
🥚+🔬
1
Fertilise Both Eggs
Both the mother's egg and the donor's egg are fertilised with sperm via IVF. This creates two zygotes (fertilised eggs) — each containing pronuclei (the combined nuclear DNA from egg + sperm).
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2
Remove Pronuclei from Both Zygotes
The pronuclei (containing nuclear DNA) are removed from both zygotes. Mother's zygote: pronuclei removed → left with faulty mitochondria (discarded). Donor's zygote: pronuclei removed and discarded → left with healthy mitochondria.
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3
Transfer Mother's Pronuclei → Donor Zygote
Mother's pronuclei (her + father's nuclear DNA) are transferred into the enucleated donor zygote (which has healthy mitochondria). Result: parents' nuclear DNA + donor's healthy mitochondria.
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4
Implant → Healthy Baby Born
Reconstructed embryo implanted in mother. Baby has 99.9% parents' DNA + healthy donor mitochondria. Three genetic parents. No mitochondrial disease.
Section 05 — Comparison
📊 MST vs PNT — Comparison Table
Parameter
Maternal Spindle Transfer (MST)
Pronuclear Transfer (PNT)
Timing
Before fertilisation (on unfertilised eggs)
After fertilisation (on zygotes)
What is transferred
Mother's spindle (chromosomes in meiotic structure)
Mother's pronuclei (combined nuclear DNA from egg + sperm)
Fertilisation step
Reconstructed egg is fertilised after transfer
Both eggs are fertilised before transfer
Donor egg requirement
One donor egg (unfertilised)
One donor egg + additional sperm to fertilise the donor egg
Embryo creation
Only one embryo created
Two embryos created (mother's + donor's), then one is discarded
Ethical concern
Lower — no embryo destroyed
Higher — donor embryo is destroyed after pronuclei removal
mtDNA carryover risk
Very low (~1–2%)
Slightly higher (~2–5%)
First baby born
2016 (USA/Mexico — to avoid Leigh syndrome)
Technique pioneered by Newcastle Fertility Centre, UK
UK regulation
Both approved by UK Parliament (2015). Only Newcastle Fertility Centre has a licence. HFEA approves each case individually.
UPSC relevance
Understand the process
Directly asked in UPSC 2020!
Section 06 — Current Affairs
🆕 Latest Developments
🇬🇧 8 Healthy Babies Born in UK (July 2025)
The New England Journal of Medicine published results from Newcastle Fertility Centre showing 8 healthy babies born via MRT since 2023 — the largest clinical test so far. The study confirmed early signs of safety. 32 patients have been authorised for MRT treatment in the UK to date. UK remains the only country to have formally regulated MRT (since 2015).
🌍 First Three-Parent Baby (2016)
The world's first three-parent baby was born in 2016 in Mexico (procedure done by US doctor John Zhang). The mother carried genes for Leigh syndrome (fatal neurometabolic disorder). She had previously experienced 4 miscarriages and lost 2 children to the disease. The baby was born healthy using MST technique.
🇮🇳 India's Position
India does not currently have specific legislation governing MRT. The Assisted Reproductive Technology (ART) Act, 2021, and the Surrogacy (Regulation) Act, 2021, regulate IVF but do not specifically address mitochondrial donation. Any future MRT in India would require new regulatory frameworks.
⚖️ Ethical Debate
Critics raise concerns: creating embryos with 3 genetic parents, destroying donor embryos (in PNT), germline modification (changes pass to future generations), "slippery slope" to designer babies. Supporters argue: prevents fatal diseases, child is 99.9% parents' DNA, only 37 mitochondrial genes affected.
Section 07 — Previous Year Questions
🧾 UPSC PYQs
UPSC 2020Prelims — GS Paper I
In the context of recent advances in human reproductive technology, "Pronuclear Transfer" is used for:
AFertilization of egg in vitro by the donor sperm
BGenetic modification of sperm-producing cells
CDevelopment of stem cells into functional embryos
DPrevention of mitochondrial diseases in offspring
📌 Explanation Answer: (d) Prevention of mitochondrial diseases in offspring. Pronuclear Transfer (PNT) is one of two MRT techniques (the other being MST). In PNT, the pronuclei from a fertilised mother's egg are transferred into a donor's enucleated zygote with healthy mitochondria — creating an embryo free of mitochondrial disease. This creates a "three-parent baby." It is NOT about fertilisation (a), sperm modification (b), or stem cell development (c).
UPSC 2021Prelims — GS Paper I
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?
A1 only
B2 only
CBoth 1 and 2
DNeither 1 nor 2
📌 Explanation Answer: (c) Both 1 and 2. Statement 1 ✓ — MRT can be done before fertilisation (MST — maternal spindle transfer) OR after fertilisation (PNT — pronuclear transfer). Both prevent mitochondrial disease transmission. Statement 2 ✓ — Mitochondrial DNA is inherited exclusively from the mother. Sperm mitochondria are destroyed during fertilisation. Therefore, mitochondrial diseases come only from the mother.
Section 08 — Practice
📝 UPSC-Style MCQs
Q1In a "three-parent baby" born via MRT, the child inherits:
a) Equal DNA from all three parents (33% each)
b) ~99.9% nuclear DNA from mother and father, and ~0.1% mitochondrial DNA from a donor
c) Nuclear DNA from the donor and mitochondrial DNA from the mother
d) 50% DNA from the mother and 50% from the donor
The child gets nuclear DNA from both parents (99.9%) and mitochondrial DNA from the donor (~0.1%, about 37 genes). The donor contributes only the "battery" (mitochondria), not the "blueprint" (nuclear DNA). Answer: (b).
Q2The key difference between MST and PNT is:
a) MST uses nuclear DNA; PNT uses mitochondrial DNA
b) MST is performed before fertilisation; PNT is performed after fertilisation
c) MST requires three parents; PNT requires only two
d) MST is used for cancer treatment; PNT is for genetic diseases
MST (Maternal Spindle Transfer) is performed on unfertilised eggs (before fertilisation). PNT (Pronuclear Transfer) is performed on zygotes (after fertilisation). Both involve transferring nuclear DNA from the mother's egg/zygote into a donor's egg/zygote with healthy mitochondria. Both create three-parent babies. Answer: (b).
Q3Which country was the first to formally regulate Mitochondrial Replacement Therapy?
a) United States
b) United Kingdom
c) Japan
d) India
The UK Parliament approved MRT regulations in 2015, making the UK the first country to formally regulate the technique. Newcastle Fertility Centre received the first licence in 2017. As of July 2025, 8 healthy babies have been born via MRT in the UK. The first three-parent baby was born in Mexico (2016), but Mexico didn't have formal regulations — the US doctor went to Mexico because MRT wasn't approved in the US. Answer: (b).
Section 09
🧠 Memory Aid
🔑 Lock These In for Prelims Day
MITO
Mitochondria = cell's "power plant." Own DNA (mtDNA). Generates 90%+ of cell energy. Inherited ONLY from mother.
Baby gets ~99.9% DNA from parents (nuclear). Only ~0.1% (~37 genes) from mitochondrial donor. Donor = "battery," not "blueprint."
MST
Maternal Spindle Transfer = BEFORE fertilisation. Transfer spindle (chromosomes) from mother's egg to donor's enucleated egg. Then fertilise.
PNT
Pronuclear Transfer = AFTER fertilisation. Both eggs fertilised first → pronuclei from mother's zygote transferred to donor's enucleated zygote. UPSC 2020!
UK FIRST
UK = first country to regulate MRT (2015). Newcastle Fertility Centre = only licensed clinic. 8 healthy babies born (NEJM, July 2025).
2016
World's first three-parent baby: Mexico, 2016. Mother had Leigh syndrome genes. US doctor John Zhang used MST technique.
PYQ 2021
MRT can be done before (MST) OR after (PNT) IVF ✓. mtDNA inherited only from mother ✓. Answer: (c) Both correct.
PYQ 2020
Pronuclear Transfer = prevention of mitochondrial diseases in offspring. Answer: (d).
Section 10
❓ FAQs
Does the baby look like the mitochondrial donor?
No. The mitochondrial donor contributes only 37 genes — all of which are related to energy production, not physical appearance. All traits like eye colour, hair, height, facial features, intelligence, and personality are determined by nuclear DNA, which comes entirely from the mother and father. The baby will look like its parents, not the donor. Think of the donor as providing the "battery" for the phone — the battery doesn't change the phone's screen, colour, or brand.
Is this "designer baby" technology?
No. MRT is not the same as designer babies. Designer baby technology would involve selecting or modifying traits like intelligence, appearance, or athletic ability — which involves editing nuclear DNA. MRT only replaces mitochondrial DNA (the "battery") and does not modify any traits. Its sole purpose is to prevent the transmission of mitochondrial diseases that can be fatal. However, critics argue it could be a "slippery slope" — if we allow modifying one type of DNA, it may normalise modifying other types.
Why is this controversial?
The controversy centres on several issues: (1) Germline modification — changes made to mtDNA will be passed to all future generations through the female line, making this a permanent genetic change. (2) Embryo destruction — PNT requires creating and destroying a donor embryo. (3) Three genetic parents — raises questions about identity, legal parentage, and the child's right to know their genetic origins. (4) Consent — the future child cannot consent to having their germline modified. (5) Uncertainty — long-term effects on children born via MRT are still unknown (the oldest are only ~10 years old).
What is Leigh syndrome?
Leigh syndrome is a severe neurological disorder caused by mutations in mitochondrial DNA. It typically appears in the first year of life and progressively affects the brain, muscles, and nerves. Symptoms include loss of motor skills, seizures, breathing difficulties, and cognitive decline. It is usually fatal within 2–3 years of onset. The world's first three-parent baby (2016) was born to a mother who carried Leigh syndrome genes — she had previously lost two children to the disease and experienced four miscarriages.
Section 11 — Mains
📜 Probable Mains Questions
Probable Question 1
"What is Mitochondrial Replacement Therapy? Explain the techniques involved and discuss the ethical concerns surrounding 'three-parent babies.'"
Probable Question 2
"Discuss the significance of mitochondrial inheritance in understanding hereditary diseases. How does Pronuclear Transfer help prevent transmission of mitochondrial diseases?"
Section 12
🏁 Conclusion
👶 Three Parents, One Purpose — Preventing Suffering
In 2016, a boy was born in Mexico — the world's first three-parent baby. His mother carried genes for Leigh syndrome, a devastating mitochondrial disease that had already killed two of her children. The technique that saved her next child — Maternal Spindle Transfer — replaced her faulty cellular "batteries" with healthy ones from a donor, while keeping 99.9% of the child's DNA from his parents.
By July 2025, eight healthy babies had been born via MRT at Newcastle Fertility Centre in the UK — the only clinic in the world with a formal licence — and their early results, published in the New England Journal of Medicine, showed promising signs of safety. The technique remains controversial, touching on fundamental questions about germline modification, genetic identity, and the boundaries of reproductive technology.
For UPSC: Know the two techniques (MST = before fertilisation, PNT = after fertilisation), the 99.9% vs 0.1% DNA split, that mtDNA is inherited only from the mother, and that PNT was directly asked in UPSC 2020 (prevention of mitochondrial diseases). Also know: UK was first to regulate (2015), first baby born in Mexico (2016), 8 babies in UK (2025 NEJM).
