Table of Contents
- Introduction to Extrachromosomal Inheritance
- Characteristics of Extrachromosomal Inheritance
- Types of Extrachromosomal Inheritance
- Modes of Extrachromosomal Inheritance
- Differences between Extrachromosomal and Chromosomal Inheritance
- Significance of Extrachromosomal Inheritance
- Challenges in studying Extrachromosomal Inheritance
Introduction to Extrachromosomal Inheritance
- Inheritance is the transfer of genetic information or traits from one cell or individual to another.
- Most inherited traits follow patterns of chromosomal inheritance, where genes on the chromosomes control the traits.
- Some traits do not follow this conventional pattern and are caused by extrachromosomal inheritance.
- Extrachromosomal inheritance, also known as cytoplasmic or extranuclear inheritance, refers to the inheritance of traits that are not controlled by chromosome genes.
- These traits are determined by genetic materials located outside the chromosomes.
- This form of inheritance occurs in the cytoplasm of cells and involves genes present in cytoplasmic organelles like mitochondria and plastids.
- The extrachromosomal hereditary factors have the ability to self-replicate and can be transmitted sexually or asexually.
- It is important to study these non-chromosomal factors to gain a comprehensive understanding of heredity.
- The early recognition of extrachromosomal inheritance started with the demonstrations by Carl Correns.
- Correns observed that heredity is not solely governed by the nucleus.
- Correns demonstrated that hereditary factors can also be present in the cytoplasm, not just the nucleus.
- Over time, extrachromosomal inheritance was observed in many cases in plants and animals.
Characteristics of Extrachromosomal Inheritance
Several characteristics associated with extrachromosomal inheritance:
- Extrachromosomal inheritance does not conform to traditional Mendelian inheritance patterns.
- The inheritance of extrachromosomal factors occurs independently of nuclear genes.
- In certain instances, extrachromosomal traits are maternally inherited because the egg provides more cytoplasm to the zygote than the male parent does.
- Extrachromosomal inheritance can result in unique phenotypic changes that do not follow Mendelian patterns.
- Extrachromosomal genes can show vegetative (somatic) segregation, a phenomenon rarely seen in nuclear genes.
Types of Extrachromosomal Inheritance
There are two main types of extrachromosomal inheritance:
1. Chloroplast inheritance
- Chloroplasts are organelles in plant cells essential for photosynthesis, containing their own DNA (chloroplast DNA or cpDNA) distinct from nuclear DNA.
- Chloroplast gene inheritance was first identified by Carl Correns and Erwin Baur in 1909.
- Correns' study on Mirabilis jalapa (the four o’clock plant) revealed that leaf color transmission was maternally determined by the ovule’s source.
- Baur's research on geranium (Pelargonium zonale) showed that chloroplast genes could be inherited from both parents or solely from the male parent, resulting in variegated plants.
- Recent research at the Max Planck Institute of Molecular Plant Physiology with tobacco plants challenges the belief that chloroplasts are only maternally inherited. Under specific environmental conditions, chloroplasts from the father can also be passed to offspring.
2. Mitochondrial inheritance
- Mitochondria, found in eukaryotic cells, generate cellular energy and contain their own DNA (mitochondrial DNA or mtDNA).
- mtDNA, circular in structure, encodes 37 genes on 16.5 kb of DNA and is the primary form of extrachromosomal inheritance in animals.
- Margit and Sylvan Nass discovered mitochondrial DNA in 1963.
- Mitochondria are predominantly inherited uniparentally, mainly from the mother, as the zygote receives mitochondria exclusively from the mother, with minimal or negligible paternal contribution.
- In 2018, a controversial claim suggested that mtDNA could be inherited from fathers. Later research showed that biparental inheritance might involve mitochondrial DNA fragments migrating into the nucleus and integrating with chromosomes, but primary mitochondrial DNA inheritance remains maternal. This supports the concept of maternal inheritance.
- mtDNA has a higher mutation rate than nuclear DNA. Mutations in mtDNA can significantly impact and are associated with various diseases.
Modes of Extrachromosomal Inheritance
1. Uniparental inheritance
Uniparental inheritance involves genetic material or traits being inherited from a single parent, either the mother or the father. In most eukaryotes, the genomes of extrachromosomal organelles are maternally inherited. For instance, in humans, mitochondrial DNA is inherited exclusively from the mother due to the significant cytoplasmic contribution of the egg compared to the minimal contribution from the sperm. Carl Correns’ experiments with four o’clock plants also demonstrated uniparental inheritance of chloroplast DNA, specifically from the maternal parent.
2. Biparental inheritance
Biparental inheritance is a less common form of extrachromosomal inheritance where genetic material from both parents contributes to the traits encoded by extrachromosomal organelles. This occurs when both maternal and paternal parents transfer extrachromosomal genetic material to the offspring. In 1909, Baur observed the inheritance of leaf phenotypes in Pelargonium cultivars, describing the transmission of chloroplasts through biparental inheritance.
3. Vegetative segregation
Vegetative segregation is a form of extrachromosomal inheritance characterized by the random distribution of cytoplasmic elements during cell division in asexual reproduction. During this process, extrachromosomal DNA in the cytoplasm is randomly allocated to daughter cells, leading to an unequal distribution of cytoplasmic content.
Differences between Extrachromosomal and Chromosomal Inheritance
Significance of Extrachromosomal Inheritance
Extrachromosomal inheritance is crucial for understanding evolutionary processes, inheritance patterns, and relationships between different species or groups.- Maternal inheritance of extrachromosomal elements, such as mitochondrial DNA, aids in tracing maternal lineages and studying human population history, providing valuable insights into ancestral relationships.
- Mutations or changes in extrachromosomal elements can lead to genetic disorders or diseases. Studying extrachromosomal inheritance mechanisms is essential for understanding these inherited disorders.
- Mitochondrial DNA has unique characteristics that make it useful for forensic identification. Its circular shape and multiple copies provide greater resilience compared to nuclear DNA. The presence of specific genes and hypervariable regions allows mtDNA to serve as a fingerprint for identification purposes.
- Extrachromosomal inheritance has also been instrumental in mapping the chloroplast and mitochondrial genomes of many species.
Challenges in studying Extrachromosomal Inheritance
- The lack of precise segregation in extrachromosomal inheritance complicates the study of inheritance patterns for extrachromosomal genes.
- The complexity of extrachromosomal inheritance has led researchers to primarily focus on chromosomal factors, which are relatively easier to understand.
- The exact characteristics and components involved in extrachromosomal inheritance remain unclear, hindering a comprehensive understanding of this process.
- Extrachromosomal hereditary factors can be lost through selection, while chromosomal inheritance tends to be more precise and consistent.
