Model Answer
0 min readIntroduction
Cytoplasmic inheritance, also known as non-Mendelian inheritance, refers to the transmission of genes located outside the nucleus – specifically within organelles like mitochondria and chloroplasts. Unlike nuclear DNA which undergoes recombination and segregation during meiosis, cytoplasmic DNA is typically inherited maternally, meaning it’s passed down only from the mother. This inheritance pattern deviates from the principles established by Gregor Mendel, where genes are inherited equally from both parents. Understanding cytoplasmic inheritance is crucial for comprehending the genetic basis of certain plant traits and human diseases.
Mechanism of Cytoplasmic Inheritance
Cytoplasmic inheritance occurs due to the presence of DNA within organelles like mitochondria and chloroplasts. These organelles possess their own circular DNA, ribosomes, and can synthesize some of their own proteins. The key features of cytoplasmic inheritance are:
- Maternal Inheritance: Most cytoplasmic genes are inherited solely from the mother because the egg cell contributes the majority of the cytoplasm to the zygote. Sperm cells contribute very little cytoplasm.
- Non-Mendelian Segregation: The segregation patterns of cytoplasmic genes do not follow Mendel’s laws of independent assortment or dominance.
- High Mutation Rate: Mitochondrial and chloroplast DNA have higher mutation rates compared to nuclear DNA, potentially leading to genetic diversity.
- Heteroplasmy: Cells can contain a mixture of different mitochondrial DNA molecules (wild-type and mutated), leading to variable expression of traits.
Organelles Involved and Genes
Mitochondrial Inheritance
Mitochondria are responsible for cellular respiration and contain a relatively small genome (around 16,569 base pairs in humans). Mitochondrial genes encode for components of the electron transport chain, tRNA, and rRNA. Mutations in mitochondrial genes can cause a variety of human diseases, often affecting tissues with high energy demands like muscles and nerves.
- Example: Human Mitochondrial Myopathies: Mutations in genes like MT-TL1 (tRNA leucine gene) can cause Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS).
- Genes Involved: MT-ATP6, MT-ATP8, MT-CO1, MT-CO2, MT-CO3, MT-ND1-6, MT-CYB (These genes encode for proteins involved in oxidative phosphorylation).
Chloroplast Inheritance
Chloroplasts are the sites of photosynthesis and are found in plant cells and algae. They contain a larger genome than mitochondria (around 120-160 kb). Chloroplast genes encode for proteins involved in photosynthesis, such as those in photosystems I and II.
- Example: Four O’Clock Plant (Mirabilis jalapa): Karl Correns observed that leaf variegation (patches of white and green) in four o’clock plants was inherited maternally. If a variegated plant was crossed with a green plant, all the progeny inherited the variegated phenotype from the variegated parent. This is because the variegation is caused by mutations in chloroplast genes.
- Genes Involved: Genes encoding for ribosomal proteins, photosystem proteins (psbA, psbC, rbcL), and other proteins involved in chloroplast function.
Specific Examples Illustrating Cytoplasmic Inheritance
| Organism | Trait | Organelle Involved | Gene(s) Involved | Inheritance Pattern |
|---|---|---|---|---|
| Four O’Clock Plant (Mirabilis jalapa) | Leaf Variegation | Chloroplast | psbA (D1 protein of photosystem II) | Maternal |
| Yeast (Saccharomyces cerevisiae) | Growth on specific media | Mitochondria | pet5 (cytochrome b gene) | Maternal |
| Humans | Leber’s Hereditary Optic Neuropathy (LHON) | Mitochondria | MT-ND4 | Maternal |
Male Sterility in Plants: Cytoplasmic male sterility (CMS) is a common phenomenon in many plant species, where plants are unable to produce functional pollen. This is often due to mutations in mitochondrial or chloroplast genes that disrupt pollen development. CMS is exploited in hybrid seed production.
Conclusion
Cytoplasmic inheritance represents a significant deviation from Mendelian genetics, highlighting the importance of extranuclear DNA in determining phenotypic traits. The maternal inheritance pattern and unique characteristics of organellar genomes contribute to its distinct segregation patterns. Understanding cytoplasmic inheritance is crucial not only for plant breeding and understanding plant traits but also for comprehending the genetic basis of human mitochondrial diseases. Further research into the complexities of heteroplasmy and the interplay between nuclear and cytoplasmic genes will continue to refine our understanding of this fascinating aspect of genetics.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.