D) Organellar

Inheritance through the mother refers to the transmission of genetic material located in the mitochondria or other organelles within the cytoplasm of the egg cell. This type of inheritance is known as organellar inheritance, which is different from the traditional Mendelian inheritance patterns observed for nuclear genes.

Mitochondria are responsible for producing energy in the form of ATP, and they contain their own set of genes known as mitochondrial DNA (mtDNA). Unlike nuclear DNA, which is inherited from both parents, mtDNA is exclusively inherited from the mother. This is because during fertilization, the egg contributes most of the cytoplasm, including the mitochondria, to the developing embryo, while the sperm's mitochondria are usually eliminated.

Under normal circumstances, both males and females can inherit and pass on mitochondrial DNA to their offspring. However, if a disease-causing mutation occurs in the mitochondrial DNA, it can be passed on only through the mother. This is because sperm cells typically do not contribute mitochondria to the fertilized egg, and therefore, the father does not pass on his mitochondrial DNA.

The inheritance of mitochondrial diseases follows a unique pattern. Since only the mother can transmit the disease-causing mutation, affected individuals will always have an affected mother. In each generation, all the children of an affected mother will inherit the disease-causing mutation, while the offspring of an affected father will not be affected.

Examples of mitochondrial diseases include Leber's hereditary optic neuropathy (LHON), mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS syndrome), and mitochondrial myopathy, among others.

In summary, when a disease is inherited only through the mother, it suggests organellar inheritance, specifically involving the mitochondrial DNA. This mode of inheritance is distinct from autosomal dominant or recessive inheritance, as well as X-linked inheritance, which involve nuclear genes.