AP Syllabus focus:
‘Traits controlled by chloroplast and mitochondrial DNA exhibit non-nuclear inheritance and do not follow simple Mendelian rules.’
Organelle genomes add an important “extra layer” to heredity. Because mitochondria and chloroplasts carry their own DNA and replicate independently, traits they encode often show inheritance patterns that differ from standard nuclear genetics.
What organelle DNA is (and why it matters)
Mitochondria and chloroplasts contain small, usually circular DNA molecules that encode a subset of proteins and RNAs essential for organelle function (for example, components of electron transport or photosynthesis). Most organelle proteins are still encoded by nuclear genes, but organelle-encoded variants can change phenotype.
Practice Questions
FAQ
During germline development, only a small subset of mitochondrial genomes may populate future eggs.
Random sampling can shift mutant loads markedly between siblings.
Some organisms show limited chloroplast recombination.
If recombination occurs, it can reshuffle chloroplast alleles and blur simple lineage-based expectations.
Rarely, organelles from the sperm/pollen contribute DNA to offspring.
This can create exceptions to the usual uniparental pattern and complicate pedigree interpretation.
They often compare reciprocal crosses and track whether offspring phenotypes depend on the cytoplasmic donor.
Molecular genotyping of organelle vs nuclear loci can confirm the source.
Different tissues can end up with different proportions of mutant organelles due to early developmental partitioning.
Energy demand can also amplify effects in particular organs.
