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.
Maternal inheritance: A non-Mendelian inheritance pattern in which offspring inherit certain genes primarily from the mother, usually because the genes are located in mitochondria or chloroplasts transmitted via eggs or ovules.
This differs from nuclear inheritance because organelle genomes are typically not delivered equally by both gametes.
Why eggs/ovules transmit organelles
Eggs and ovules contain abundant cytoplasm, including many mitochondria (and in plants, plastids that develop into chloroplasts).
Sperm and pollen contribute comparatively little cytoplasm:
Animal sperm usually deliver a nucleus and minimal cytoplasm, so their mitochondria are not typically retained in the embryo.
In many flowering plants, pollen primarily delivers sperm nuclei to the ovule; plastids are usually excluded from the male contribution.
As a result, organelles (and their DNA) are usually inherited maternally through eggs or ovules, not sperm or pollen.
This figure summarizes organelle-genome inheritance using reciprocal crosses, illustrating how an offspring’s organelle genotype can match one parent consistently even when nuclear alleles follow standard Mendelian patterns. It helps distinguish cytoplasmic inheritance (organelle DNA) from nuclear inheritance and shows why reciprocal crosses are informative for detecting maternal transmission. Source
Key organelles involved
Mitochondria in animals and plants
Mitochondria carry small genomes that encode some components needed for cellular respiration. Because the zygote’s mitochondria come mostly from the egg, traits caused by mitochondrial variants often show maternal-line transmission.
Mitochondrial inheritance: Transmission of mitochondrial DNA variants to offspring predominantly through the maternal line because mitochondria are typically supplied by the egg.
A practical consequence is that an affected mother can pass a mitochondrial trait to all her children, whereas an affected father typically does not pass it on.
Chloroplasts in plants
In plants, chloroplasts (and their precursor plastids) also contain DNA, and these organelles are usually contributed by the ovule rather than pollen. Thus, chloroplast-encoded traits often show maternal inheritance in many species, especially angiosperms.
Recognising maternal inheritance patterns
Hallmarks in family patterns (conceptual)
Transmission through mothers: Offspring phenotypes track with the mother more reliably than the father.
Both sexes can be affected: Sons and daughters may show the trait, but only mothers typically pass it onward.
No Mendelian ratios required: Because inheritance depends on organelle transmission, expected nuclear ratios (like 3:1) may not apply.
Important complication: mixed organelle populations
Some individuals carry more than one organelle genotype in their cells, which can make expression variable among siblings.
Heteroplasmy: The presence of more than one mitochondrial (or chloroplast) DNA type within a cell or individual, potentially causing variation in trait severity among offspring.
Heteroplasmy helps explain why maternal inheritance can show variable expressivity, even within the same family, if eggs/ovules differ in their organelle composition.
This schematic models the mitochondrial “genetic bottleneck,” showing how a heteroplasmic mother can produce eggs with different mutant vs. wild-type mtDNA proportions. The resulting variance helps explain why siblings can inherit the same mitochondrial mutation but show different severity or even different phenotypes. Source
Plant vs animal emphasis
Animals
Maternal inheritance is most commonly discussed for mitochondria.
The egg’s cytoplasm supplies nearly all mitochondria to the embryo, reinforcing maternal transmission.
Plants
Maternal inheritance can involve mitochondria and chloroplasts, depending on the species.
Because ovules provide most cytoplasm to the zygote, organelles (especially plastids) are typically maternally transmitted, aligning with the syllabus focus on eggs or ovules vs sperm or pollen.
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.
Practice Questions
State why mitochondrial traits are usually inherited from the mother rather than the father in animals. (2 marks)
Egg contributes most cytoplasm/organelles (1)
Sperm contributes little cytoplasm; paternal mitochondria are typically not transmitted/retained (1)
Describe how maternal inheritance can be identified in a family and explain why offspring from the same mother might show different levels of a mitochondrially inherited trait. (5 marks)
Trait can affect both sexes (1)
Affected mothers can pass the trait to all offspring; affected fathers typically do not (1)
Pattern does not follow typical Mendelian ratios because genes are in organelles (1)
Explanation of heteroplasmy: more than one mtDNA type can be present (1)
Different eggs may receive different proportions of mtDNA types, causing variable severity among offspring (1)
