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?
Contents13
- A1 only
- B2 only
- CBoth 1 and 2
- DNeither 1 nor 2
Show answer
Answer: (C) Both 1 and 2
Statement 1 is correct:
Mitochondrial Replacement Therapy (MRT) is a form of IVF that replaces a mother's defective mitochondrial DNA with healthy donor mitochondrial DNA.
This can be done before fertilization (spindle transfer) or shortly after (pronuclear transfer), resulting in a baby with nuclear DNA from both parents but healthy mitochondrial DNA from the donor.
Statement 2 is correct:
In nearly all mammals including humans, mitochondrial DNA is inherited exclusively from the mother.
Fathers do not pass on their mitochondria.
Since mitochondrial diseases come only from the mother, MRT targets the maternal line.
Both statements are correct.
Answer: (c).
Mitochondrial replacement therapy became the first gene therapy technique approved in multiple countries to prevent hereditary diseases from passing to children.
UPSC is testing whether students understand that inheritance works differently for mitochondrial DNA versus nuclear DNA - mitochondria come only from the mother's egg cell.
The question checks if students know both the medical technique and the biological principle behind why only maternal mitochondria are inherited.
Mitochondrial Inheritance Pattern
Science And Technology mitochondrial diseases parent to child mother father
Mitochondrial Inheritance: Why Only from Mother
Mitochondrial DNA is inherited exclusively from mother in humans
Fathers contribute zero mitochondria to offspring
Mitochondrial diseases affect maternal lineage only
Each cell has hundreds of mitochondria with their own DNA
Why Maternal Only
Mitochondria are cellular powerhouses with their own DNA separate from nuclear DNA. During fertilization, only the egg contributes mitochondria to the embryo — sperm mitochondria are actively destroyed after fertilization.
• Egg cells: contain thousands of mitochondria in cytoplasm
• Sperm cells: mitochondria stay in tail, don't enter egg during fertilization
Nuclear vs Mitochondrial DNA
Aspect | Nuclear DNA | Mitochondrial DNA |
|---|---|---|
Location | Cell nucleus | Mitochondria (cytoplasm) |
Inheritance | Both parents (50-50) | Mother only (100%) |
Copy number | 2 copies per cell | Hundreds-thousands per cell |
Size | ~3 billion base pairs | 16,569 base pairs |
Genes | ~20,000-25,000 | 37 genes |
Clinical Significance
Mitochondrial diseases affect energy production in cells — heart, brain, muscles most vulnerable
Maternal family history is key diagnostic clue for mitochondrial disorders
Variable expression — same mutation can cause different severity in family members
Heteroplasmy — cells can have mix of normal and mutated mitochondrial DNA
Trap: Assuming both parents contribute to mitochondrial inheritance like nuclear DNA
Trap: Confusing mitochondrial DNA with Y-chromosome (paternal) inheritance patterns
Confusion: Mitochondrial diseases can affect males even though inheritance is maternal
Mitochondrial Replacement Therapy
Science And Technology mitochondrial replacement therapy in vitro fertilization before or after
Mitochondrial Replacement Therapy: Preventing Inherited Diseases
MRT replaces defective maternal mitochondria with healthy donor mitochondria
Can be done before (spindle transfer) or after fertilization (pronuclear transfer)
Results in three-parent baby — nuclear DNA from both parents, mitochondrial DNA from donor
UK was first country to legalize MRT in 2015
The Problem MRT Solves
Women with mitochondrial diseases face passing defective mitochondria to all their children. MRT allows them to have genetically related children with healthy mitochondria from a donor woman.
Two MRT Techniques
Technique | When Performed | Process | Nuclear DNA Source |
|---|---|---|---|
Spindle Transfer | Before fertilization | Remove nucleus from patient's egg, insert into donor's enucleated egg, then fertilize | Patient's egg nucleus |
Pronuclear Transfer | After fertilization | Remove both pronuclei from fertilized patient embryo, insert into donor's enucleated fertilized egg | Patient's fertilized embryo |
Spindle Transfer Process
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Donor Preparation**
Remove **nucleus** from healthy donor egg, keeping cytoplasm with good mitochondria`"]
s2["`**Patient Preparation**
Extract **nucleus** from patient's egg (contains patient's nuclear DNA)`"]
s3["`**Nuclear Transfer**
Insert patient's nucleus into donor's **enucleated egg**`"]
s4["`**Fertilization**
Fertilize reconstructed egg with partner's **sperm** via IVF`"]
s5["`**Result**
Embryo with patient's nuclear DNA + partner's nuclear DNA + **donor's mitochondrial DNA**`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5MRT Visualization
Source: ScienceDirect.com — Mitochondrial transfer: Implications for assisted ... · www.sciencedirect.com
Ethical & Legal Status
UK legalized MRT in 2015 — first country to allow clinical use
Three-parent baby term is misleading — 99.9% DNA still from biological parents
Donor contributes less than 0.1% of total DNA (only mitochondrial)
Regulatory approval required in most countries — still experimental in many places
Trap: Thinking MRT can only be done before fertilization — both before and after are possible
Confusion: MRT does not alter nuclear DNA — only replaces mitochondrial DNA
Trap: Assuming MRT creates genetically modified babies — nuclear genes remain unchanged
Hereditary Diseases Classification
Science And Technology hereditary diseases
Types of Hereditary Diseases by Inheritance Pattern
Autosomal dominant — one mutated gene copy causes disease
Autosomal recessive — both gene copies must be mutated
X-linked — genes on X chromosome, mainly affects males
Mitochondrial — inherited only from mother
Inheritance Patterns Compared
Pattern | Inheritance Source | Risk Pattern | Example Diseases |
|---|---|---|---|
Autosomal Dominant | Either parent | 50% chance if one parent affected | Huntington's disease, Marfan syndrome |
Autosomal Recessive | Both parents (carriers) | 25% chance if both parents carriers | Sickle cell anemia, Thalassemia |
X-linked Recessive | Mother to son mainly | 50% chance for sons of carrier mothers | Hemophilia, Color blindness |
Mitochondrial | Mother only | All children of affected mother at risk | Leber's optic neuropathy, MELAS |
Genetic Disease Categories
# Hereditary Diseases
## Single Gene Disorders
- Autosomal Dominant
- Autosomal Recessive
- X-linked
- Y-linked
## Chromosomal Disorders
- Down syndrome
- Turner syndrome
- Klinefelter syndrome
## Mitochondrial Disorders
- Maternal inheritance
- Energy metabolism defects
- Multi-organ involvement
## Multifactorial
- Diabetes
- Heart disease
- Cancer predispositionTrap: Confusing X-linked (affects mainly males) with mitochondrial (affects both sexes equally)
Trap: Thinking autosomal recessive diseases skip generations — carriers are unaffected
Confusion: Mitochondrial diseases can affect males despite maternal inheritance