Extrachromosomal Inheritance: Introduction, Types, Examples

Introduction:

• The genes of nuclear chromosomes have different roles in inheritance, cellular, metabolism, development and mutation of the organism, in which they occur. Though, the genes of nuclear chromosomes have a significant and play key role in the inheritance and transmission of almost all characters from one generation to other generation, they cannot be considered as the sole means of inheritance.
• Because, certain experimental evidence suggests transmission of some genetic information from one generation to the next generation through the cytoplasm and is known as extracellular or cytoplasmic inheritance.
• The role of cytoplasm in hereditary is determined from result of reciprocal crosses in which the sources of male and female gametes are reversed.
• In chromosomal inheritance, it makes no difference in the transmission of a gene whether it comes from which parent and identical phenotypes can be obtained through reciprocal crossing.
• But, if there is extranuclear inheritance, the resulting phenotypes in reciprocal crosses are nonidentical and indicate uniparental transmission, in most cases from the maternal parent.
• Extrachromosomal inheritance refers to the DNA that found in the cytoplasm that is also known as extranuclear inheritance, non-mendelial pattern of heredity, uniparental inheritance, maternal inheritance or cytoplasmic inheritance.
• In 1949, Boris Ephrussi discovered the inheritance of extrachromosomal in yeast.

Extrachromosomal Inheritance in eukaryotes:

Various scientists have investigated extrachromosomal inheritance in different eukaryotes. Some of the most significant examples of extranuclear inheritance in eukaryotes are as follows:

Plastid inheritance

• Plastids (chloroplasts) possess their own genetic information and consequently, express its own pattern of inheritance due to plasma genes which is located in plastid is known as plastid inheritance.
• In 1908, Correns and Baur discovered inheritance of plastid which show non-Mendelian pattern of inheritance.
•  All the knowledge about plastid inheritance in higher plants based on studies on chlorophyll variegation in leaves. Variegation is the occurrence of white or yellow dots of varying sizes on a green background in leaves.

Correns and Baur found that the four o’ clock plant, Mirabilis Jalapa, in contrast to other higher plants, contain three types of leaves and parts as follows:

• Full green leaves or branches having chloroplasts
• White leaves or branches having no chloroplasts
• Variegated or mixed leaves or branches with green and white patches.

The white part of these leaves contains leucoplasts, while the green area contains chloroplasts. Because the leucoplasts are unable to produce food through photosynthesis, the white branches of this plant survive by receiving nourishment from green parts.

Correns discovered that the flowers on green branches produced only green offspring, regardless of the genotype and phenotype of pollen parent. Likewise, flower from the white branches produced only white offspring regardless of genotype and phenotype of pollen parents. However, these offspring will die soon because they lack chlorophyll and cannot carry on photosynthesis in order to survive. The flowers from the variegated branches yielded mixed progeny of green, white and variegated plants in various ratios.

The   above experiment clearly reveals that the phenotype of the progeny is identical to that of the female parent. Except when the female parent is variegated, all the three combinations occur.

 Mitochondrial inheritance

In fungi Neurospora crassa, a variety of mitochondrial mutation are inherited via the female parents. But, the poky strain of Neurospora crass was the best studied mutant, which was first isolated by Mitchell and Mitchell in 1952.

Poky strain grows very slow, forms small colony and exhibits the poorly differentiated mitochondria. The mitochondria are deficient in the membrane bound cytochrome a and b, which is essential for respiratory electron transport chain. It also has a significantly reduced amount of small ribosomal units.

 This trait seems to be inherited in certain strains in non-mendelial fashion, indication its extra chromosomal nature.

In a cross between normal female and poly male the resultant ascospores all produce normal colonies. And in reciprocal cross between poky female and normal male parents, all the resultant colonies are poky. This suggests that the poky feature is inherited through maternal heredity. Mitochondrial mutants are also known in paramecium, Trypanosoma and yeast.

Fig: Mitochondrial inheritance

Maternal effect

• Maternal inheritance is the transmission of the mitochondrial genome from a mother to all of her children, with no paternal mtDNA contribution. During fertilization, a human sperm cell with few mitochondria makes no substantial contribution to the zygote.
• Where the development of some characters in several organism is either governed or influenced by the genotype of the female parent, it is known as maternal effect. Such phenotype expression of maternal genes in offspring may be transient or may last the entire life of the individual.
• The substance which produces the maternal effects in the progeny in the life are found to be transcriptional products of maternal nuclear genes like mRNA, tRNA, rRNA, which have been produced during oogenesis and which exist in the cytoplasm of fertilized eggs in the form of inactive protein coated mRNA molecules or inactive rRNA and tRNA.
• These maternal genes transcriptional products produce their phenotypic effects during early cleavage and blastulation when there is very little or no transcription because the maternal and paternal genes of the zygote remain engaged in mitotic replication or DNA duplication.
• The classic trait exhibiting maternal impact is the coiling direction of snail shells. The offspring’s coiling phenotype is determined by the mother’s genotype. Likewise, the popular example of maternal inheritance is cytoplasmic male sterility in maize which reveals that nuclear genes don’t play any role in sterility; instead, sterility is passed down through the egg cytoplasm to children.

Differences between Extrachromosomal and Chromosomal Inheritance:

Feature	Extrachromosomal Inheritance	Chromosomal Inheritance
Definition	Inheritance of genetic material not found on chromosomes.	Inheritance of genetic material located on chromosomes.
Location	Found in organelles like mitochondria and chloroplasts.	Found in the nucleus within chromosomes.
Mode of Transmission	Typically maternal (e.g., mitochondrial DNA)	Genetic traits on nuclear chromosomes generally follow Mendelian patterns of inheritance.
Examples	Mitochondrial DNA, chloroplast DNA.	Nuclear DNA in chromosomes.
Size	Extrachromosomal genetic elements are often smaller compared to nuclear chromosomes.	Nuclear chromosomes are larger and more complex structures.
Inheritance Pattern	Non-Mendelian, often maternal lineage.	Mendelian inheritance (autosomal dominant, recessive, sex-linked).
Implications in Disease	Mitochondrial diseases, often maternally inherited.	Wide range of genetic disorders (e.g., cystic fibrosis, hemophilia)