The unicellular green alga Chlamydomonas has a single large chloroplast containing many copies of a circular double-stranded DNA. Its str^R (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.