Extra-nuclear inheritance refers to the transmission of genetic information through cellular structures outside of the cell nucleus, such as mitochondria and chloroplasts. These structures contain their own genetic material in the form of DNA, which is inherited from the maternal parent. There are two main systems of extra-nuclear inheritance: maternal inheritance and biparental inheritance. Let's explore each of these systems in more detail:

1. Maternal Inheritance:
Maternal inheritance is a form of extra-nuclear inheritance where the genetic information is transmitted exclusively through the maternal line. This means that the genetic traits carried by mitochondrial DNA (mtDNA) or chloroplast DNA (cpDNA) are inherited only from the mother and passed down to the offspring. The reason behind this pattern is that during fertilization, the sperm contributes only nuclear genetic material to the zygote, while the egg provides both nuclear genetic material and the cytoplasm containing the organelles like mitochondria or chloroplasts.

Example 1: Mitochondrial DNA (mtDNA) Inheritance - Mitochondria are the energy-producing organelles in eukaryotic cells and contain their own small circular DNA. Diseases caused by mutations in mtDNA, such as Leber's hereditary optic neuropathy and mitochondrial encephalomyopathy, are inherited maternally. This is because the zygote primarily receives mitochondria from the cytoplasm of the egg, which contains the mother's mtDNA.

Example 2: Chloroplast DNA (cpDNA) Inheritance - Plants exhibit maternal inheritance of cpDNA, as chloroplasts are inherited