Extranuclear Inheritance

Besides the nucleus, genes are also present in the cytoplasm, in the DNA of mitochondria and chloroplasts (plasmagenes).
At fertilisation the egg contributes almost all the cytoplasm of the zygote, while the male gamete contributes practically only its nucleus.
So the cytoplasmic genes of the offspring come almost entirely from the mother.
Characters controlled by them are transmitted only through the female parent; this is called cytoplasmic or maternal inheritance, and reciprocal crosses give different results.
Examples are leaf variegation in Mirabilis jalapa and cytoplasmic male sterility in maize.
In autosomal and X-linked inheritance both parents contribute genes, and Y-linked genes pass only from father to son.
So more maternal influence is expected in cytoplasmic inheritance.

Cytoplasmic male sterility (CMS) is a condition in which the plant fails to produce functional pollen, while the female parts remain normal.
It is inherited only through the female parent (maternal inheritance), which shows that the responsible genes are outside the nucleus.
The genes responsible are located in the mitochondrial genome; rearrangements in mitochondrial DNA produce abnormal proteins that disturb pollen development.
CMS is very useful in hybrid seed production (for example in maize, sorghum and rice), because it avoids the laborious emasculation of flowers.
Nuclear 'restorer of fertility' genes can restore pollen production in the hybrid.
So the genes for cytoplasmic male sterility are generally located in the mitochondrial genome.