Explanation:

Extranuclear inheritance refers to the inheritance of genetic material from organelles other than the nucleus, such as mitochondria and chloroplasts. These organelles contain their own DNA, which is distinct from the nuclear DNA. The following are the reasons why extranuclear inheritance is due to the presence of genes in mitochondria and chloroplasts.

a) Mitochondria and Chloroplasts contain their own DNA:
Mitochondria and chloroplasts are organelles found in eukaryotic cells that have their own DNA. Mitochondria are the sites of cellular respiration, while chloroplasts are responsible for photosynthesis in plant cells. Both organelles have a double membrane, and the inner membrane contains the DNA. Mitochondrial DNA is circular and codes for some of the proteins involved in cellular respiration. Chloroplast DNA is also circular and codes for some of the proteins involved in photosynthesis.

b) Inheritance of Mitochondrial DNA:
Mitochondrial DNA is inherited from the mother only. During fertilization, the egg contributes the majority of the cytoplasm, including the mitochondria, to the developing zygote. The sperm contributes only a small amount of cytoplasm and no mitochondria. Therefore, all of the mitochondria in the zygote come from the mother, and any mitochondrial DNA mutations are passed down maternally.

c) Inheritance of Chloroplast DNA:
In plants, chloroplast DNA is also inherited maternally. However, the inheritance of chloroplast DNA is more complex than mitochondrial DNA inheritance. Chloroplasts are found in the cytoplasm of plant cells, and they move around during cell division. Therefore, the distribution of chloroplasts in the developing embryo can be uneven. As a result, different cells in the developing plant can have different chloroplast genotypes, which can lead to variegation in the leaves of the plant.

Conclusion:
Extranuclear inheritance is due to the presence of genes in organelles such as mitochondria and chloroplasts. These organelles have their own DNA, which is distinct from the nuclear DNA. Mitochondrial DNA is inherited from the mother only, while chloroplast DNA is also inherited maternally but can be distributed unevenly in the developing plant.