The unicellular green alga Chlamydomonas
has a single large chloroplast containing many copies of a circular double-stranded DNA.
Its strR (streptomycin resistance) trait exhibits uniparental inheritance:
the phenotype is transmitted only through the mt+ (mating type) parent.
Reciprocal crosses yield
offspring which only express the genotype of the mt+ parent.
After fertilization, the single chloroplasts of the two mating types fuse.
The resulting chloroplast only retains DNA from the mt+ parent.