Inheritance is the transfer of genetic information or traits from one cell or individual to another. While most inherited traits follow patterns of chromosomal inheritance, where genes on the chromosomes control the traits, there are some traits that do not follow this conventional pattern. These traits 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. Instead, they 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, who 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
There are several characteristics associated with extrachromosomal inheritance:
- Extrachromosomal inheritance does not follow the typical Mendelian inheritance patterns.
- The inheritance of extrachromosomal factors is independent of genes located within the cell nucleus.
- In some cases, extrachromosomal traits are inherited exclusively from the mother. This is because the egg contributes more cytoplasm to the zygote compared to the male parent.
- Extrachromosomal inheritance can lead to characteristic phenotypic changes that are not inherited in a Mendelian pattern.
- Extrachromosomal genes can exhibit vegetative (somatic) segregation which is rare in nuclear genes.
Types of Extrachromosomal Inheritance
There are two main types of extrachromosomal inheritance that are briefly discussed below:
1. Chloroplast inheritance
- Chloroplasts are organelles located in plant cells that play a vital role in photosynthesis. They possess their own DNA, known as chloroplast DNA (cpDNA), which is distinct from nuclear DNA.
- The inheritance of chloroplast genes was first discovered by Carl Correns and Erwin Baur in 1909.
- Correns conducted a study on Mirabilis jalapa, commonly known as the four o’clock plant, where he observed that the transmission of leaf color was strictly maternal, determined by the color of the ovule’s source.
- Baur found in his experiment in geranium (Pelargonium zonale) that chloroplast genes can also be inherited from both parents or from the male parent only, resulting in variegated plants.
- Recent research conducted at the Max Planck Institute of Molecular Plant Physiology with tobacco plants presents new evidence that challenges the commonly held belief that chloroplasts are solely inherited from the mother plant. The researchers discovered that under specific environmental conditions, chloroplasts from the father can also be passed on to the offspring.
2. Mitochondrial inheritance
- Mitochondria are cellular structures present in eukaryotic cells that are responsible for generating energy. They also contain their own unique DNA, known as mitochondrial DNA (mtDNA).
- mtDNA is the main form of extrachromosomal inheritance in animals. mtDNA is circular and encodes 37 genes on 16.5 kb of DNA.
- Margit and Sylvan Nass discovered the DNA in mitochondria in 1963.
- Mitochondria are primarily inherited uniparentally, mostly maternally. The zygote receives mitochondria exclusively from the mother, while the paternal contribution of mitochondria is minimal or negligible.
- In 2018, a controversial claim suggested that children can inherit mtDNA from their fathers. However, subsequent research found that in cases of biparental inheritance, mitochondrial DNA fragments can migrate into the nucleus and integrate with the chromosomes. These mitochondrial DNA fragments are inherited alongside the nuclear chromosomes but the primary inheritance of mitochondrial DNA still occurs from the mother. This research confirms that the concept of maternal inheritance is still true.
- MtDNA exhibits a higher rate of mutational change compared to nuclear DNA. Mutations in mtDNA can have significant effects and are associated with various diseases.
Modes of Extrachromosomal Inheritance
1. Uniparental inheritance
Uniparental inheritance refers to a mode of inheritance where genetic material or traits are inherited from a single parent, either the mother or the father. The genomes of extrachromosomal organelles are maternally inherited in most eukaryotes. For example, in humans, mitochondrial DNA is inherited solely from the mother. This is due to the significant contribution of cytoplasm from the egg to the zygote compared to the relatively minimal contribution from the sperm. Carl Correns’ experiments with four o’clock plants also demonstrated uniparental inheritance of chloroplast DNA, specifically through 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 the extrachromosomal organelles. This can occur when there is a transfer of extrachromosomal genetic material from both the maternal and paternal parents to the offspring. Baur (1909) observed the inheritance of leaf phenotypes in Pelargonium cultivars, describing the transmission of chloroplasts through biparental inheritance.
3. Vegetative segregation
Vegetative segregation is a mode of extrachromosomal inheritance that involves the random distribution of cytoplasmic elements during cell division in asexual reproduction. In this process, extrachromosomal DNA within the cytoplasm is randomly segregated into daughter cells, resulting in unequal distribution of cytoplasmic content.
Differences between Extrachromosomal and Chromosomal Inheritance
|Characteristics||Extrachromosomal inheritance||Chromosomal inheritance|
|Location||Genetic material is located in the cytoplasm or cytoplasmic organelles such as mitochondria or chloroplast.||Genetic material is located within the nucleus, specifically on chromosomes.|
|Inheritance pattern||The inheritance patterns do not follow classic Mendelian principles.||Genetic traits on nuclear chromosomes generally follow Mendelian patterns of inheritance.|
|Segregation pattern||Segregation is different from Mendelian segregation. Alleles may or may not segregate.||Chromosomes undergo precise and controlled segregation.|
|Size||Extrachromosomal genetic elements are often smaller compared to nuclear chromosomes.||Nuclear chromosomes are larger and more complex structures.|
|Parental inheritance||Extrachromosomal genes are often inherited from only one parent, known as uniparental inheritance.||Nuclear genes are inherited from both parents, following a biparental inheritance pattern.|
Significance of Extrachromosomal Inheritance
- Extrachromosomal inheritance is important for understanding evolutionary processes. It helps to study inheritance patterns and explore relationships between different species or groups.
- Maternal inheritance in extrachromosomal elements like mitochondrial DNA helps trace maternal lineages and study human population history. It is valuable for understanding ancestral relationships.
- Mutations or changes in extrachromosomal elements can cause genetic disorders or diseases. Studying the mechanisms of extrachromosomal inheritance is important for understanding these inherited disorders.
- Mitochondrial DNA has unique characteristics that make it useful for forensic identification. Its circular shape and multiple copies make it more resilient than nuclear DNA. The presence of specific genes and hypervariable regions enables mtDNA to act as a fingerprint for identification purposes.
- Extrachromosomal inheritance has also been useful in mapping the chloroplast and mitochondrial genomes in many species.
Challenges in studying Extrachromosomal Inheritance
- The lack of precise segregation in extrachromosomal inheritance makes it challenging to study the inheritance patterns of extrachromosomal genes.
- The complexity of extrachromosomal inheritance has led researchers to focus mainly on studying chromosomal factors, which are comparatively easier to understand.
- The exact characteristics and components involved in extrachromosomal inheritance are unclear, which makes it difficult to have a complete understanding of this process.
- Extrachromosomal hereditary factors could be lost through selection, whereas chromosomal inheritance tends to be more precise and regular.
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