Extrachromosomal Inheritance: Do you know that our genetic material, i.e., DNA, is arranged on chromosomes that store, transfer, and express the genetic information? Nuclear DNA that is present in the nucleus of a cell controls all the phenotypes that are transferred from the parents to offspring, and this process is called Mendelian Inheritance.

DNA is also present in mitochondria and plastids. Before fertilization, a sperm cell loses most of its cytoplasm and cytoplasmic organelles, and only the sperm nucleus enters the egg. So, the zygote receives the genome of plastids and mitochondria only from the maternal parent, and thus this phenomenon is known as extra chromosomal inheritance or cytoplasmic inheritance. Read this article to know more about extrachromosomal inheritance, how it occurs, and some non-mendelian inheritance examples.

What is Extrachromosomal Inheritance?

Extrachromosomal Inheritance is defined as a form of a non-mendelian inheritance that is governed by the DNA present in the cytoplasm. It refers to the transmission of genes that occurs outside the nucleus, also known as extranuclear inheritance, found in most eukaryotes.

It commonly occurs in the cytoplasmic organelles such as mitochondria and plastids or in cellular parasites like viruses or bacteria. The traits present in the mitochondria or plastids do not follow the  Mendelian principles of inheritance. It was first reported by Carl Correns in 1908 in Mirabilis jalapa (Four o’clock plant) plastid DNA for leaf colour. The extrachromosomal genome consists of a few genes of several thousand base pairs, mostly encoding rRNA, tRNA, and other proteins for their DNA metabolism.

No cytoplasm is contributed by the male gamete, which means whatever cytoplasm is present in the zygote is contributed by the female gamete only. Since this cytoplasmic inheritance is coming from the mother or maternal side, this inheritance is also referred to as maternal or uniparental inheritance.

Fig: Diagrammatic Representation of Cytoplasmic Inheritance

Criteria for Extrachromosomal Inheritance

The criteria for extrachromosomal inheritance are as follows:
1. The extrachromosomal DNA does not follow the Mendelian pattern of inheritance, unlike the common nuclear DNA.
2. The extrachromosomal DNA doesn’t have its own protein synthetic machinery for the process of replication, transcription, and translation. So, it synthesizes its own DNA and makes its own protein.
3. The extrachromosomal DNA is inherited from the maternal side because the female gamete contains more cytoplasm than the male gamete.
4. All the progenies obtained by this inheritance have the phenotype of only one parent, i.e., the mother.
5. The extranuclear genes present in the mitochondria and plastids cannot be mapped to the chromosomes in the nucleus.
6. This inheritance is not affected by substituting the nucleus with a different genotype.

Extrachromosomal Inheritance Examples

There is no scientific evidence for the occurrence of extrachromosomal inheritance. However, according to one theory, the female gamete (egg/ovum) is bigger than the male gamete (sperm), which contains more cytoplasm and more organelles. This influences the extrachromosomal inheritance or maternal inheritance.

Following are some examples of extrachromosomal inheritance:

1. Plastid Inheritance in Mirabilis jalapa (4 o’clock plant)
(a) Correns, in 1908 worked out on the plastid inheritance in Mirabilis jalapa, which shows two types of branching, i.e., branches with dark green leaves and branches with pale green leaves.

Fig: Leaf Variegation in Mirabilis jalapa

(b) When a dark green leaved plant (male) is crossed with a pale green leaved plant (female), all the F₁ offspring obtained have pale green leaves only, which shows that the inheritance of plastid is a maternal inheritance.

(c) Similarly, when a pale green leaved plant (male) is crossed with a dark green leaved plant (female), all the F₁ offspring obtained have dark green leaves only, which shows that the inheritance of plastid is a maternal inheritance.

(d) From the above cross, it is clear that this inheritance does not involve nuclear genes. It is due to the chloroplast gene found in the ovum of the female plant, which provides the cytoplasm during fertilization as the male gamete contributes only to the nucleus but no cytoplasm.

Fig: Plastid Inheritance in Mirabilis jalapa

2. Cytoplasmic male sterility in maize
(a) The male sterility in maize is inherited through the cytoplasm of the egg in offspring.
(b) When we cross a male sterile plant with a normal fertile plant, all the F₁ plants produced are sterile.
(c) Now, when all the F₁ sterile plants are backcrossed with a normal fertile plant, then all the F₂ plants produced are also sterile.

3. The maternal effect in snail
(a) The coiling of snails is governed by maternal inheritance.
(b) There are two types of snail coiling phenotypes:
(i) Dextral (coiling for right side)
(ii) Sinistral (coiling for left side)
(c) In the development of coiling, the mother’s genotype is exclusively responsible.

Summary

Thus, extrachromosomal or extranuclear inheritance or cytoplasmic inheritance is a form of non-mendelian inheritance, which is defined as the transmission of genes outside the nucleus that is found in most eukaryotes. It commonly occurs in the cytoplasmic organelles such as mitochondria and chloroplasts or in cellular parasites like viruses or bacteria. Extrachromosomal inheritance is also known as maternal inheritance because this type of inheritance is governed by the maternal genes and not the nuclear genes. This inheritance does not follow the Mendelian pattern of inheritance.

FAQs

Q.1: What is extrachromosomal inheritance?
Ans: Extrachromosomal Inheritance is defined as a form of a non-mendelian pattern of inheritance that is governed by the DNA present in the cytoplasm, which refers to the transmission of genes outside the nucleus.

Q.2: What are the criteria for extrachromosomal inheritance?
Ans: The criteria for extrachromosomal inheritance are as follows:
(a) The extrachromosomal DNA does not follow the Mendelian pattern of inheritance, unlike the common nuclear DNA.
(b) The extrachromosomal DNA doesn’t have its own protein synthetic machinery for the process of replication, transcription, and translation. So, it synthesizes its own DNA and makes its own protein.
(c) The extrachromosomal DNA is inherited from the maternal side because the female gamete contains more cytoplasm than the male gamete.

Q.3: Who first reported the extrachromosomal inheritance?
Ans: Carl Correns, in 1908, first reported about the extrachromosomal inheritance in Mirabilis jalapa (Four o’clock plant) plastid DNA for leaf colour.

Q.4: What are the examples of extrachromosomal inheritance?
Ans: The examples of extrachromosomal inheritance are as follows:
1. Cytoplasmic male sterility in maize
2. Maternal effect in snail
3. Inheritance of kappa particles in Paramecium

Q.5. Which parent contributes more to cytoplasmic inheritance?
Ans: The female parent contributes more to cytoplasmic inheritance.