Cytoplasmic Inheritance: the genes outside the nucleus
In short
Not all of a plant’s genes sit in the nucleus. A small part lies in two cell components outside it, the chloroplasts and the mitochondria. In most plants these genes pass on through the mother only. That is cytoplasmic inheritance.
Two separate genomes in one cell
Chloroplasts and mitochondria carry their own small rings of DNA, independent of the large genome in the nucleus. In evolutionary terms they descend from bacteria that moved in, and they kept their own DNA. Because these genomes are inherited apart from the nucleus, they follow different rules than classic Mendelian inheritance.
Why usually only the mother passes them on
At fertilisation the egg cell supplies nearly all of the cell fluid, the pollen essentially only the nucleus. Several mechanisms keep the paternal organelles out: plastids are excluded during pollen formation, their DNA is broken down, or the paternal organelles are removed after fertilisation. The result is the same, offspring inherit chloroplasts and mitochondria almost entirely from the mother.
In numbers: more than 80 % of flowering-plant genera pass on their chloroplast DNA strictly maternally, and mitochondrial DNA is passed on maternally throughout the flowering plants studied so far.
The rule has exceptions
Strictly maternal does not mean without exception. In some species such as the evening primrose, inheritance is naturally biparental. And even in otherwise strictly maternal species like tobacco, a small paternal share can slip through under cold stress during pollen formation, so-called paternal leakage. The maternal rule is the norm, not a law of nature.
Why it matters in practice
Because these genomes follow one line only, they serve as markers for the maternal line. In origin and relatedness analyses, chloroplast DNA is used for exactly that. For aroma it plays no part, since the genes for terpenes and cannabinoids sit in the nucleus, not in the organelles.