Mitochondria and chloroplasts carry their own genes. They are usually passed down from one parent only, most often the mother, so these traits do not show Mendelian ratios.
As you saw in Where Mitochondria and Chloroplasts Came From, these organelles descend from free-living bacteria, and they still carry their own small circular DNA. Their genes follow different rules from nuclear genes, which shows up clearly in pedigrees.
🔑 Key idea
Genes in mitochondria and chloroplasts are not sorted by meiosis. Offspring usually receive these organelles from one parent only: in many animals, including humans, mitochondria come from the mother, and in many plants chloroplasts come through the seed parent. These traits do not show Mendelian ratios.
💡 Why it works
Meiosis sorts chromosomes in the nucleus. Organelles are in the cytoplasm, and they divide on their own. In animals, the new cell's cytoplasm, and with it the mitochondria, comes almost entirely from the mother's gamete, which is much larger than the father's. So the next generation gets its mitochondria from the mother. No pairs of alleles meet, so there is no dominance and no 3 : 1.
A mitochondrial trait: every child of an affected mother is affected; no child of an affected father is.
Worked examples
✏️ Worked example: Reading the pedigree
In the pedigree above, the affected woman in generation 2 has two affected children. Her affected brother has two unaffected children. What pattern fits?
Both of an affected mother's children are affected.
Neither of an affected father's children is.
Males and females are both affected, so this is not about the X chromosome.
Answer: Maternal (mitochondrial) inheritance.
✏️ Worked example: Variegated leaves
A plant has some branches with green leaves and some with white-streaked leaves, caused by faulty chloroplasts. Seeds from flowers on a streaked branch are grown, using pollen from a green branch. What do the seedlings look like?
Chloroplasts reach the seed through the seed parent (here, the flower on the streaked branch).
The pollen supplies no chloroplasts.
So the seedlings get the streaked branch's mix of faulty and normal chloroplasts.
Answer: Streaked or pale, like the branch that made the seeds, whatever the pollen source.
Because mitochondrial DNA passes down the maternal line almost unchanged, except for occasional mutations, it is also a useful tool for tracing ancestry and for comparing populations and species, as you will see in Molecular Clocks. A cell holds hundreds of mitochondria, and they need not all be identical, so the severity of a mitochondrial trait can vary among children of the same mother.
⚠️ Common mistake
Expecting a 3 : 1 ratio, or a pattern of carriers, for a mitochondrial trait. These genes are not paired and not sorted by meiosis. The test is simple: affected mother, affected children (often all); affected father, unaffected children.
Practice
Pass with 3 right answers, the last 2 in a row. Each right answer earns 2 XP.