Extra-Chromosomal Inheritance
1. Introduction to Extra-Chromosomal Inheritance
Classical Mendelian genetics mainly explains inheritance through genes located on chromosomes present in the nucleus. However, not all hereditary information is restricted to nuclear chromosomes. Cells contain cytoplasmic organelles such as mitochondria and, in plants and algae, chloroplasts. These organelles contain their own genetic material and can transmit certain traits from one generation to another.
Inheritance involving genetic information outside the nuclear chromosomes is broadly described as extra-chromosomal inheritance, also called cytoplasmic inheritance in many classical genetics contexts.
Extra-chromosomal inheritance is particularly important because it does not always follow the classical Mendelian ratios of 3:1 or 9:3:3:1. Instead, inheritance patterns may depend strongly on which parent contributes the cytoplasm, how organelles are distributed during cell division, the number of organelle genomes present, and interactions between organellar and nuclear genes.
2. Index / Table of Contents
- Introduction to Extra-Chromosomal Inheritance
- General Features of Extra-Chromosomal Inheritance
- Cytoplasmic Inheritance
- Mechanism of Cytoplasmic Inheritance
- Mitochondrial Inheritance
- Mitochondrial DNA and Its Characteristics
- Maternal Transmission of Mitochondria
- Heteroplasmy and Homoplasmy
- Threshold Effect and Mitochondrial Phenotypes
- Mitochondrial Pedigree Pattern
- Chloroplast Inheritance
- Mirabilis jalapa and Leaf-Color Inheritance
- Infectious Heredity
- Kappa Particles in Paramecium
- Maternal Effect
- Shell Coiling in Snails
- Maternal Effect vs Maternal Inheritance
- Comparison of Extra-Chromosomal Inheritance
- How to Identify Cytoplasmic Inheritance
- CSIR-NET / GATE Important Points
- Common Mistakes
- Memory Tricks
- 10 Practice MCQs
- Quick Revision
3. General Features of Extra-Chromosomal Inheritance
Extra-chromosomal inheritance differs from ordinary Mendelian inheritance in several important ways. These differences arise because cytoplasmic components are distributed differently from nuclear chromosomes during reproduction and cell division.
Important characteristics
- The hereditary determinant is located outside the conventional nuclear chromosomes.
- Mitochondria can carry their own DNA.
- Chloroplasts can carry their own DNA.
- Many cytoplasmic organelles are transmitted through the cytoplasm of the gamete.
- In many organisms, the egg contributes much more cytoplasm to the zygote than the sperm.
- Consequently, mitochondrial inheritance is frequently maternal in animals.
- Chloroplast inheritance can be maternal, paternal, or biparental depending on the organism.
- Organelle genomes may exist in multiple copies within a single cell.
- Different copies of organelle genomes can sometimes carry different genotypes, a condition called heteroplasmy.
- Cytoplasmic inheritance may not produce classical Mendelian ratios.
- The phenotype may depend on the distribution and proportion of organelles rather than simply on a pair of nuclear alleles.
- Cytoplasmic inheritance can interact strongly with nuclear genes.
4. Cytoplasmic Inheritance
Cytoplasmic inheritance refers to inheritance determined by genetic material or hereditary factors located in the cytoplasm rather than solely in the nuclear chromosomes.
The cytoplasm contains many structures and molecules that can influence phenotype. Among these, mitochondria and chloroplasts are particularly important because they contain their own genomes.
In addition to organellar DNA, classical genetics has identified unusual cytoplasmic hereditary particles that can influence phenotype. The classical example of infectious heredity involving kappa particles in Paramecium demonstrates how cytoplasmic factors can be inherited and can also behave like infectious elements.
Major examples
- Mitochondrial inheritance.
- Chloroplast inheritance.
- Kappa particles in Paramecium.
- Certain cytoplasmic plasmids and other organelle-associated hereditary elements.
Why does cytoplasmic inheritance occur?
During fertilization, the egg usually contributes a large volume of cytoplasm, while the sperm contributes relatively little cytoplasm. Therefore, organelles present in the egg can be transmitted to the offspring in large numbers.
This is one of the major biological reasons why mitochondrial inheritance in many animals follows a maternal pattern.
5. Mechanism of Cytoplasmic Inheritance
Cytoplasmic inheritance begins with the presence of hereditary information in cytoplasmic components. During cell division, these components are distributed into daughter cells.
↓
Replication of organelle genome
↓
Distribution of organelles
↓
Transmission to daughter cells
↓
Phenotypic expression
Unlike nuclear chromosomes, organelles do not necessarily segregate according to a simple Mendelian mechanism. Multiple organelles may be distributed randomly or semi-randomly among daughter cells.
This can result in different proportions of normal and mutant organelles in different cells.
Such variation becomes especially important in mitochondrial disorders, because tissues can tolerate different proportions of mitochondrial dysfunction depending on their energy requirements.
6. Mitochondrial Inheritance
Mitochondria are semi-autonomous organelles that contain their own genetic material. They are involved in oxidative phosphorylation and ATP production, as well as several other cellular processes.
Human mitochondrial DNA, or mtDNA, is a small circular DNA molecule. It encodes a limited number of proteins, ribosomal RNAs, and transfer RNAs important for mitochondrial gene expression and oxidative phosphorylation.
However, mitochondria are not genetically independent. Most proteins required for mitochondrial structure and function are encoded by nuclear genes and imported into mitochondria.
They contain their own genome but depend extensively on nuclear genes for their structure, replication, gene expression, metabolism, and maintenance.
7. Mitochondrial DNA – Important Characteristics
- Human mtDNA is generally circular.
- It is located within mitochondria rather than the nucleus.
- There are many copies of mtDNA per cell.
- Mitochondria can vary in number among different cell types.
- The mitochondrial genome contains genes involved in oxidative phosphorylation and mitochondrial protein synthesis.
- Mitochondrial ribosomal RNAs and transfer RNAs are encoded by mtDNA.
- Most mitochondrial proteins are actually encoded by nuclear genes.
- Mitochondria are transmitted through the cytoplasm.
- In humans and many animals, mtDNA is predominantly maternally inherited.
- Mitochondrial genomes can undergo mutations and may contribute to human disease.
8. Maternal Transmission of Mitochondria
In many animals, including humans, mitochondrial inheritance is predominantly maternal. This means that mitochondrial DNA in the offspring is usually derived from the mother.
The reason is primarily related to the relative contribution of the egg and sperm during fertilization.
↓
Egg containing mitochondria
↓
Zygote
↓
Offspring mitochondria
The sperm contributes its nuclear genome to the zygote, but paternal mitochondria are usually eliminated or otherwise prevented from making a substantial contribution to the long-term mitochondrial population of the embryo in typical mammalian inheritance.
9. Mitochondrial Pedigree Pattern
Mitochondrial inheritance produces a distinctive pedigree pattern.
- Affected mothers can transmit mitochondrial variants to both sons and daughters.
- Affected fathers generally do not transmit mitochondrial variants to their children in the classical maternal inheritance model.
- All children of an affected mother may inherit the mtDNA variant, but the severity or phenotype may differ among offspring.
- The phenotype can vary because the amount and distribution of mutant mtDNA can differ among cells.
- The pedigree can show maternal-line transmission across generations.
Mother → Son + Daughter
but
Father → usually no mtDNA transmission
10. Heteroplasmy and Homoplasmy
One of the most important concepts in mitochondrial genetics is heteroplasmy.
A cell can contain many copies of mitochondrial DNA. If all mitochondrial DNA copies are genetically identical, the condition is called homoplasmy.
If a mixture of different mitochondrial genomes exists within a cell or organism, the condition is called heteroplasmy.
| Term | Meaning |
|---|---|
| Homoplasmy | All mitochondrial genome copies are essentially the same genotype. |
| Heteroplasmy | Different mitochondrial genome types coexist within the same cell or organism. |
Heteroplasmy is important because the proportion of mutant mtDNA can influence whether a phenotype is observed and how severe it becomes.
11. Threshold Effect in Mitochondrial Inheritance
A mitochondrial phenotype may not appear when the proportion of mutant mitochondria is low. A certain critical level may need to be exceeded before cellular dysfunction becomes clinically or phenotypically apparent.
This concept is known as the threshold effect.
↓
Normal cellular function may be maintained
↓
Mutant mtDNA increases
↓
Threshold reached
↓
Phenotypic dysfunction
Different tissues can have different energetic demands. Therefore, tissues with high ATP requirements, such as skeletal muscle, heart, and nervous system, may be particularly sensitive to mitochondrial dysfunction.
This helps explain why individuals carrying mitochondrial DNA variants can show different clinical manifestations and why affected members of the same family may show variable severity.
12. Replicative Segregation
During cell division, mitochondria are distributed to daughter cells. Because many mitochondria and mtDNA molecules may be present, the distribution of different mtDNA variants can vary between daughter cells.
This phenomenon is often referred to as replicative segregation.
- Different daughter cells can receive different proportions of mutant mitochondria.
- Different tissues can therefore contain different mutant mtDNA loads.
- The proportion of mutant mtDNA can change across generations of cells.
- This contributes to variable expression of mitochondrial phenotypes.
13. Chloroplast Inheritance
Chloroplasts are photosynthetic organelles found in plants and algae. Like mitochondria, chloroplasts contain their own DNA and are capable of replicating within cells.
Inheritance of chloroplast genes is called chloroplast inheritance or plastid inheritance.
The inheritance pattern can vary among species. Chloroplasts may be inherited maternally, paternally, or from both parents depending on the organism.
Important points
- Chloroplasts contain their own genome.
- Chloroplast DNA encodes some proteins and RNAs involved in chloroplast function.
- Many chloroplast proteins are encoded by nuclear genes.
- Chloroplast inheritance is not universally maternal.
- Different plant species can show different patterns of plastid inheritance.
- Classical experiments with Mirabilis jalapa demonstrate cytoplasmic inheritance of chloroplast-related leaf coloration.
14. Mirabilis jalapa and Leaf-Color Inheritance
One of the classic examples of cytoplasmic inheritance is the inheritance of leaf color in the four-o'clock plant, Mirabilis jalapa.
This plant can produce branches or tissues with green, white, or variegated leaves.
The phenotype is associated with chloroplasts. Therefore, the phenotype of the offspring is strongly influenced by the chloroplasts contributed through the maternal tissue or ovule.
Three important plant types
- Green branches: contain functional chloroplasts capable of normal photosynthesis.
- White branches: contain defective or non-functional chloroplasts.
- Variegated branches: contain a mixture of normal and defective chloroplasts.
The important lesson from this experiment is that the phenotype of the offspring depends on the cytoplasmic organelles contributed by the maternal parent rather than following a simple nuclear Mendelian ratio.
15. Cytoplasmic Variegation
Variegation refers to the presence of different colored regions in the same plant, commonly green and white areas.
At the cellular level, variegation can arise because different cells contain different proportions or types of chloroplasts.
Cells containing functional chloroplasts can produce green tissues, whereas cells lacking functional chloroplasts can produce white tissues.
When reproductive cells are produced from different tissues, the chloroplast population transmitted to offspring can differ.
↓
Different cellular chloroplast composition
↓
Different leaf coloration
↓
Cytoplasmic inheritance
16. Infectious Heredity
Infectious heredity is a special type of cytoplasmic inheritance in which a cytoplasmic hereditary element can be transmitted through cell division and can also spread from one cell to another.
The classical example comes from the protozoan Paramecium.
Some Paramecium strains contain cytoplasmic particles known as kappa particles. These particles are associated with the ability of the host cell to produce a substance that can kill susceptible Paramecium cells.
The kappa particle system is historically important because it demonstrates that hereditary information can involve cytoplasmic entities and can be maintained through cell generations.
17. Kappa Particles in Paramecium
In classical genetic studies of Paramecium, certain strains were described as killer strains because they could produce a toxic factor that killed sensitive strains.
The killer phenotype depends on the presence of cytoplasmic kappa particles, which are associated with particular nuclear genetic backgrounds that allow their maintenance.
Important components
- Killer strain: Paramecium containing kappa particles and the appropriate genetic background.
- Kappa particles: Cytoplasmic hereditary particles associated with the killer phenotype.
- Non-killer strain: A strain lacking the required combination for the killer phenotype.
The kappa particles are often described in classical genetics as cytoplasmic infectious elements because they can be transmitted cytoplasmically and can contribute to the killer phenotype.
18. Features of Infectious Heredity
- The hereditary determinant is associated with a cytoplasmic element.
- The element can be transmitted from parent cell to daughter cell.
- Certain cytoplasmic elements can spread between cells under appropriate conditions.
- The phenotype can depend on both cytoplasmic and nuclear genetic factors.
- Kappa particles in Paramecium are the classical textbook example.
19. Maternal Effect
Maternal effect is fundamentally different from mitochondrial inheritance.
In maternal-effect inheritance, the phenotype of the offspring is determined by the genotype of the mother, particularly because the mother deposits gene products such as mRNAs or proteins into the egg before fertilization.
The gene responsible for the phenotype is usually a nuclear gene. Therefore, maternal effect should not automatically be classified as cytoplasmic inheritance.
→ genetic material itself is in the cytoplasm.
Maternal effect:
→ nuclear gene in mother determines products deposited into egg, which influence offspring development.
20. Mechanism of Maternal Effect
During oogenesis, the mother produces specific RNAs and proteins that are stored in the developing egg. After fertilization, these maternal products can control early embryonic development before the embryo begins substantial expression of its own genome.
↓
Maternal gene expression during oogenesis
↓
mRNA / protein deposited in egg
↓
Embryonic development
↓
Offspring phenotype
This means that the phenotype observed in the offspring can reflect the mother's genotype rather than the offspring's genotype at the relevant maternal-effect locus.
21. Shell Coiling in Snails – Maternal Effect
A classic example of maternal-effect inheritance is the direction of shell coiling in certain freshwater snails, traditionally represented by Limnaea in genetics textbooks.
Shell coiling can be dextral or sinistral.
- Dextral: right-handed coiling.
- Sinistral: left-handed coiling.
The direction of shell coiling is influenced by maternal gene products that affect early embryonic cleavage orientation.
The important conceptual point is that the genotype of the mother determines the developmental environment of the egg, which then affects the phenotype of the offspring.
Mother's genotype → maternal RNA/protein → early embryo → shell coiling phenotype
22. Maternal-Effect Genetic Crosses
Maternal-effect problems can initially appear confusing because the genotype of the offspring does not necessarily predict its own phenotype immediately. Instead, the phenotype is determined by the mother's genotype.
Consider a simplified maternal-effect gene with:
d = recessive allele
Suppose the maternal genotype determines the phenotype of the offspring. Then an offspring that genetically carries a particular allele may show a phenotype determined by the mother's genotype during its own embryonic development.
This can create a characteristic one-generation delay in phenotype relative to genotype.
23. Maternal Effect vs Maternal Mitochondrial Inheritance
This is one of the most important distinctions in extra-chromosomal inheritance.
| Feature | Mitochondrial Inheritance | Maternal Effect |
|---|---|---|
| Location of responsible genetic information | Mitochondrial genome | Usually nuclear gene in mother |
| What determines phenotype? | Inherited mitochondrial genotype | Mother's genotype through deposited products |
| Transmission pattern | Often maternal in animals | Maternal genotype determines offspring phenotype |
| Classical example | Mitochondrial disorders | Snail shell coiling |
| Major concept | Cytoplasmic genetic material | Maternal gene products |
24. Maternal Inheritance Does Not Always Mean Maternal Effect
The word "maternal" can cause confusion. Maternal inheritance and maternal effect are not synonymous.
In maternal inheritance, a hereditary determinant is transmitted through the mother, often because it is located in the cytoplasm, such as mitochondrial DNA.
In maternal effect, the phenotype of an offspring is determined by the mother's nuclear genotype through products deposited into the egg.
Maternal effect: Mother's nuclear gene → maternal mRNA/protein → embryo → phenotype
25. Comparison of Major Extra-Chromosomal Inheritance Types
| Type | Genetic Basis | Classical Example | Key Feature |
|---|---|---|---|
| Cytoplasmic inheritance | Cytoplasmic genetic determinants | Organelle inheritance | Non-Mendelian transmission |
| Mitochondrial inheritance | mtDNA | Human mitochondrial traits | Usually maternal in animals |
| Chloroplast inheritance | Plastid DNA | Mirabilis jalapa | Leaf-color variegation |
| Infectious heredity | Cytoplasmic infectious particles | Paramecium kappa particles | Hereditary factor can spread |
| Maternal effect | Usually maternal nuclear genotype | Snail shell coiling | Maternal products determine early phenotype |
26. Mendelian vs Extra-Chromosomal Inheritance
| Feature | Mendelian Nuclear Inheritance | Extra-Chromosomal Inheritance |
|---|---|---|
| Primary genetic location | Nuclear chromosomes | Cytoplasm / organelles or cytoplasmic factors |
| Typical segregation | Chromosomal segregation | Organelle/cytoplasmic segregation |
| Classical ratios | Often predictable Mendelian ratios | May not follow simple ratios |
| Parent contribution | Usually both parents contribute nuclear alleles | May show strong maternal or cytoplasmic bias |
| Examples | Autosomal dominant/recessive | Mitochondrial, chloroplast, infectious heredity |
27. How to Identify Extra-Chromosomal Inheritance
When a pedigree or genetic cross is given in an examination, use a systematic approach.
Step 1: Look for maternal transmission
If affected mothers transmit a trait to both sons and daughters while affected fathers do not transmit it, mitochondrial inheritance should be considered.
Step 2: Check whether the phenotype depends on maternal genotype
If the mother's genotype determines the phenotype of her offspring, consider maternal effect.
Step 3: Look for organelle-specific phenotypes
Leaf variegation and chloroplast-associated phenotypes can indicate plastid inheritance.
Step 4: Look for unusual cytoplasmic particles
If a problem mentions kappa particles or killer strains in Paramecium, recognize infectious heredity.
Step 5: Consider heteroplasmy
Variable expression among offspring from the same mother may occur because the proportion of mutant mitochondria differs among eggs or tissues.
28. Nuclear-Cytoplasmic Interaction
Although mitochondria and chloroplasts have their own genomes, their function depends heavily on nuclear genes.
Therefore, many phenotypes are produced by interaction between nuclear and cytoplasmic genomes.
Organelle genes +
Environmental conditions
↓
Phenotype
This is particularly important because an organelle may contain only a small fraction of the genes required for its complete function. Most organelle proteins are encoded by nuclear DNA and transported into the organelle.
29. Why Extra-Chromosomal Inheritance Is Often Non-Mendelian
Mendelian inheritance depends heavily on the segregation and independent assortment of homologous nuclear chromosomes during meiosis.
Cytoplasmic organelles do not generally follow exactly the same segregation rules.
A cell can contain many mitochondria or chloroplasts, and each organelle may contain multiple copies of its genome. During cell division, these organelles are distributed among daughter cells.
Consequently, offspring can receive different numbers or proportions of different organelle genomes.
This produces inheritance patterns that may not fit simple Mendelian ratios.
30. CSIR-NET / GATE / Life Science High-Yield Points
- Extra-chromosomal inheritance involves hereditary determinants outside the conventional nuclear chromosomes.
- Mitochondria possess their own genome.
- Chloroplasts possess their own genome.
- Mitochondrial inheritance is commonly maternal in animals.
- Mitochondrial DNA can be heteroplasmic.
- Homoplasmy means essentially identical mitochondrial genomes within the population being considered.
- Heteroplasmy means coexistence of different mitochondrial genomes.
- The threshold effect is important in mitochondrial phenotypes.
- Replicative segregation contributes to variation in mitochondrial mutant load.
- Mirabilis jalapa is a classic example of chloroplast inheritance.
- Leaf variegation in Mirabilis is associated with chloroplasts.
- Kappa particles in Paramecium are the classical example of infectious heredity.
- Maternal effect is generally caused by maternal gene products deposited in the egg.
- The classic maternal-effect example is shell coiling in snails such as Limnaea.
- Maternal effect does not necessarily mean that the gene is mitochondrial.
- Mitochondrial inheritance and maternal effect must be distinguished.
- Chloroplast inheritance can be maternal, paternal, or biparental depending on the species.
- Mitochondrial inheritance generally does not show classical autosomal Mendelian ratios.
- The father contributes nuclear DNA to sons and daughters but generally does not transmit mitochondrial DNA in the standard maternal inheritance model.
31. Common Mistakes Students Make
Mistake 1: Maternal inheritance = maternal effect
This is incorrect. Mitochondrial inheritance can be maternal because mitochondria are transmitted through the egg. Maternal effect usually involves a nuclear gene in the mother whose products influence offspring development.
Mistake 2: All organelle inheritance is maternal
This is not universally true. Chloroplast inheritance varies among organisms and can be maternal, paternal, or biparental.
Mistake 3: Mitochondria have no DNA
Mitochondria contain their own DNA.
Mistake 4: Mitochondria are completely independent
Mitochondria are semi-autonomous. They depend heavily on nuclear genes.
Mistake 5: Heteroplasmy means two alleles of a nuclear gene
No. Heteroplasmy refers to coexistence of different mitochondrial genome types within a cell or organism.
Mistake 6: Kappa particles are nuclear genes
Kappa particles are cytoplasmic hereditary factors associated with the classical infectious heredity example in Paramecium.
Mistake 7: Maternal effect means mitochondria
Maternal-effect traits can be caused by nuclear genes expressed in the mother and deposited as products into the egg.
32. Easy Memory Tricks
Mitochondria
"Mito = Mother"
Remember the classical maternal inheritance pattern in animals.
Chloroplast
"Chloro = Chlorophyll = Green"
Think of Mirabilis jalapa → green/white/variegated leaves.
Infectious heredity
"Kappa = Killer"
Kappa particles → killer phenotype in Paramecium.
Maternal effect
"Mother makes the message"
Mother's nuclear genotype → maternal mRNA/protein → offspring phenotype.
Heteroplasmy
"Hetero = different"
Different mitochondrial genomes coexist.
Homoplasmy
"Homo = same"
Essentially the same mitochondrial genome type predominates.
33. One-Line Memory for Examination
Chloroplast → Mirabilis jalapa
Kappa particles → Paramecium → Infectious heredity
Snail shell coiling → Maternal effect
Heteroplasmy → Mixed mitochondrial genomes
Threshold effect → Mutant mtDNA must reach critical level
34. Practice MCQs – 10 Important Questions
Test your understanding with the following 10 multiple-choice questions. Click Show Answer after attempting each question.
Mitochondria contain their own DNA outside the nuclear chromosomes and can transmit hereditary information. Therefore mitochondrial inheritance is a major example of extra-chromosomal inheritance.
Human mitochondrial inheritance is predominantly maternal because the egg provides most of the cytoplasm and paternal mitochondria are normally not maintained as the major mitochondrial population in the embryo.
Heteroplasmy refers to the coexistence of different mitochondrial genome types within a cell or organism.
Mirabilis jalapa is a classical example of cytoplasmic inheritance involving chloroplasts and leaf-color variegation.
Kappa particles are the classical cytoplasmic hereditary factors associated with the killer phenotype in Paramecium and are an important example of infectious heredity.
In maternal-effect inheritance, the mother's nuclear genotype determines maternal products such as mRNAs or proteins deposited into the egg. These products influence early development of the offspring.
Shell coiling in classical Limnaea snail genetics is a well-known maternal effect in which maternal gene products influence early embryonic development.
In the classical maternal inheritance pattern, an affected mother can transmit mitochondrial variants to both sons and daughters.
Different tissues may contain different proportions of mutant and normal mitochondrial genomes. This heteroplasmy, together with threshold effects, can contribute to variable phenotype.
Kappa particles in Paramecium are a classical example of infectious cytoplasmic heredity.
35. Quick Revision Chart
| Topic | Key Point | Classic Example |
|---|---|---|
| Extra-chromosomal inheritance | Inheritance outside conventional nuclear chromosomes | Organelle inheritance |
| Cytoplasmic inheritance | Cytoplasmic hereditary determinants | Mitochondria / chloroplasts |
| Mitochondrial inheritance | Usually maternal in animals | Human mtDNA |
| Heteroplasmy | Different mtDNA types coexist | Mitochondrial genetics |
| Chloroplast inheritance | Plastid DNA transmission | Mirabilis jalapa |
| Infectious heredity | Cytoplasmic element can contribute to a heritable phenotype and spread | Kappa particles in Paramecium |
| Maternal effect | Mother's nuclear genotype determines offspring phenotype through maternal products | Snail shell coiling |
CHLOROPLAST → MIRABILIS
KAPPA → PARAMECIUM → KILLER
SNAIL COILING → MATERNAL EFFECT
HETEROPLASMY → MIXED mtDNA
36. Final Exam Strategy
Extra-chromosomal inheritance questions are often designed to test whether you can distinguish between similar-sounding concepts. The safest strategy is to identify the location and source of the hereditary information.
- If the question mentions mitochondria or mtDNA, think mitochondrial inheritance.
- If the question describes green, white, or variegated leaves in Mirabilis, think chloroplast inheritance.
- If the question mentions kappa particles or killer Paramecium, think infectious heredity.
- If the phenotype depends on the mother's genotype through products deposited into the egg, think maternal effect.
- If different mitochondrial genomes coexist, the term is heteroplasmy.
- If the same mitochondrial genome type predominates, the term is homoplasmy.
- If the phenotype appears only after mutant mtDNA exceeds a critical level, think threshold effect.
- If mitochondrial distribution changes among daughter cells, think replicative segregation.
ULTIMATE EXAM MEMORY
Extra-chromosomal inheritance
↓
Cytoplasmic hereditary factors
Mitochondria
→ maternal inheritance
Chloroplast
→ Mirabilis jalapa → leaf variegation
Kappa particles
→ Paramecium → infectious heredity
Maternal effect
→ mother's nuclear genotype → maternal products → offspring phenotype
Heteroplasmy
→ different mitochondrial genomes
Threshold effect
→ mutant mtDNA reaches critical level before phenotype becomes apparent
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