AP Syllabus focus:
‘In animals and plants, mitochondria and chloroplasts are usually inherited maternally through eggs or ovules, not sperm or pollen.’
Maternal inheritance explains why some traits track through mothers regardless of offspring sex. It arises because cytoplasm-rich eggs contribute most organelles to the zygote, shaping patterns of inheritance in animals and plants.
Core idea: cytoplasmic (maternal) transmission
What “maternal inheritance” means
Maternal inheritance occurs when genetic information is passed primarily from the mother because it resides in organelles found in the egg/ovule cytoplasm.
Practice Questions
FAQ
Rarely, paternal mitochondrial transmission has been reported in a few cases.
This typically requires failure of the usual mechanisms that prevent retention of sperm mitochondria after fertilisation.
No. Some species exhibit paternal or biparental chloroplast inheritance.
The pattern depends on how plastids are packaged or excluded during pollen formation and fertilisation.
Variation often depends on heteroplasmy.
If a mother is nearly homoplasmic (one organelle genotype), offspring tend to be similar; mixed organelle populations can produce variable outcomes.
Clonal propagation (e.g., cuttings) can preserve the same cytoplasmic organelles across generations of clones.
This can maintain chloroplast/mitochondrial trait patterns without involving seeds or pollen.
Yes, because organelle genomes are often uniparentally inherited and have different effective population sizes.
This can alter how quickly variants spread and how strongly harmful variants are removed across maternal lineages.
