Inheritance of mitochondrial and chloroplast DNA
Mitochondrial and chloroplast DNA
How is non-nuclear DNA inherited?
- High copy number. A mitochondrion or chloroplast has multiple copies of its DNA, and a typical cell has many mitochondria (and, in the case of a plant cell, chloroplasts). As a result, cells usually have many copies – often thousands – of mitochondrial and chloroplast DNA.
- Random segregation. Mitochondria and chloroplasts (and the genes they carry) are randomly distributed to daughter cells during mitosis and meiosis. When the cell divides, the organelles that happen to be on opposite sides of the cleavage furrow or cell plate will end up in different daughter cells.
- Single-parent inheritance. Non-nuclear DNA is often inherited uniparentally, meaning that offspring get DNA only from the male or the female parent, not both. In humans, for example, children get mitochondrial DNA from their mother (but not their father).
Chloroplast inheritance: Early experiments
- The color of the egg cell-donating branch (female parent) determined the color of the offspring.
- Female parent branches that were pure green or pure white produced only pure green or pure white offspring, respectively.
- Female parent branches that were variegated could produce all three types of offspring, but not in any predictable ratios.
Explaining Correns' results
|Female branch||Egg cells||Zygotes||Offspring|
|Variegated branch||Egg cell with green chloroplasts, egg cell with white chloroplasts, or egg cell with mixed chloroplasts||Egg cell with white chloroplasts leads to zygote with white chloroplasts; egg cell with green chloroplasts leads to zygote with green chloroplasts; egg cell with mixture of chloroplasts leads to zygote with mixture of chloroplasts||Variegated plant|
Maternal inheritance of mitochondria in humans
Mitochondrial mutations and human disease
- A person with a disease caused by a mitochondrial mutation may lack normal mitochondria (and have only abnormal, mutation-bearing ones). In this case, an affected mother will always pass on mutation-bearing mitochondria to her children.
- A mitochondrial disorder may occur when a person has a mix of normal and abnormal mitochondria her body. In this case, normal and mutation-bearing mitochondria may go randomly into eggs during meiosis. Children who get a large proportion of mutant mitochondria may have severe disease, while those with few mutant mitochondria may have mild or no disease.