Nuclear and Non-Nuclear Inheritance
A mother in Galway has a rare muscle disorder. Her son and daughter may both inherit it (the severity can vary), yet when her son has children, none of them are affected. Mendel's laws do not apply here, because the faulty gene is not in the nucleus. To solve the puzzle, we need to look for DNA outside the nucleus.
Where is the DNA in a cell?
Most of a eukaryotic cell's DNA is in the nucleus, packaged into chromosomes. This is nuclear DNA. Half of it comes from each parent, and it follows Mendel's Laws of Segregation and Independent Assortment.
Two organelles also contain a small amount of their own DNA:
| Organelle | Found in | DNA |
|---|---|---|
| Mitochondrion | Almost all eukaryotes; kingdoms Animalia, Plantae, Fungi and Protista | mitochondrial DNA (mtDNA) |
| Chloroplast | Plants and algae (kingdom Plantae and some Protista) | chloroplast DNA (cpDNA) |
Animals and fungi have mtDNA but no chloroplasts, so they have no cpDNA.
Name two organelles in a eukaryotic cell that contain their own DNA, and state which kingdoms of organisms have each.
Name two organelles in a eukaryotic cell that contain their own DNA, and state which kingdoms of organisms have each.
Distinguish between nuclear* inheritance and non-nuclear* inheritance.
Distinguish between nuclear inheritance and non-nuclear inheritance.
Features of mtDNA and cpDNA
Organelle DNA is quite different from nuclear DNA, and it resembles the DNA of bacteria.
| Feature | Nuclear DNA | mtDNA / cpDNA |
|---|---|---|
| Shape | Linear chromosomes | Small, circular molecule |
| Histones | Wrapped around histone proteins | Not packaged with histones (like bacteria) |
| Amount | Very large, thousands of genes | Very small, few genes (about 37 in human mtDNA) |
| Copies per cell | Two sets (diploid) | Many copies per cell, because each cell has many organelles and each organelle has several copies |
| Non-coding DNA | Large amounts | Very little |
Organelle genes code for only some of the proteins the organelle needs, mostly for the electron transport chain (mitochondria) or photosynthesis (chloroplasts). The rest are coded by nuclear genes. The organelles also have their own 70S ribosomes, like prokaryotes.
Evidence for the endosymbiotic theory
The endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by an ancestral eukaryotic cell and lived inside it. Over time they became permanent organelles. The evidence is:
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their DNA is circular and not bound to histones, as in bacteria
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they have 70S ribosomes, like bacteria
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they divide by splitting in two, independently of the cell, like bacteria
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they have a double membrane
Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule. a) State two ways in which mtDNA differs from nuclear DNA in structure. b) State one way in…
Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule.
- a) State two ways in which mtDNA differs from nuclear DNA in structure.
- b) State one way in which the mtDNA is similar to the DNA of prokaryotes.
- c) Explain how this similarity supports the endosymbiotic theory.
Maternal inheritance
In most species, mitochondria and chloroplasts are inherited from the mother only. This is called maternal inheritance (or uniparental inheritance).
Why? The egg cell is large and full of cytoplasm, with hundreds of thousands of mitochondria. The sperm is tiny. In humans, only the head of the sperm, with the nucleus, enters the egg, and the mitochondria in the sperm's tail are left outside or broken down. So the zygote gets its mitochondria almost entirely from the egg. In plants, the male gamete (the pollen nucleus) brings little or no cytoplasm, so chloroplasts usually come from the seed parent (the female).
The pattern for a mitochondrial disorder is therefore:
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an affected mother passes the mutated mtDNA to all her children (sons and daughters), though severity can vary
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an affected father passes it to none of his children
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only daughters pass the mutation on to the next generation
In humans, mitochondria in the zygote are inherited almost entirely from one parent. a) State which parent contributes the mitochondria. b) Explain briefly why this is…
In humans, mitochondria in the zygote are inherited almost entirely from one parent.
- a) State which parent contributes the mitochondria.
- b) Explain briefly why this is the case.
A mother from Galway has a rare disorder caused by a mutation in her mtDNA. She has two children, a son and a daughter, with a healthy father. a) State whether each…
A mother from Galway has a rare disorder caused by a mutation in her mtDNA. She has two children, a son and a daughter, with a healthy father.
- a) State whether each child is likely to inherit the mutation, giving a reason.
- b) State whether the grandchildren of the daughter would be likely to inherit the mutation, and of the son, giving reasons.
- c) Explain why the pattern of inheritance cannot be explained by Mendel's laws.
Chloroplast inheritance in plants
Some plants have variegated leaves with green and white patches. The white patches have a mutation in the chloroplast DNA, so the chloroplasts there cannot make chlorophyll.
Chloroplast genes are usually inherited from the seed parent (female), so the offspring resemble the female parent.
White tissue cannot make chlorophyll, so it cannot carry out photosynthesis, which is the process impaired. Without it, the tissue cannot make glucose and relies on the green tissue to supply it, so it could not survive on its own. A seedling that is completely white dies once the food reserves in the seed are used up.
In some plants, leaves are variegated (patches of green and white). A mutation in chloroplast DNA causes white patches, where the chloroplasts cannot make chlorophyll.…
In some plants, leaves are variegated (patches of green and white). A mutation in chloroplast DNA causes white patches, where the chloroplasts cannot make chlorophyll. In a cross, the offspring always resemble the seed parent (the female parent) in leaf colour.
- a) Explain this pattern of inheritance.
- b) State why white tissue cannot survive on its own in a plant.
- c) Name the process in the plant that is impaired in white patches.
Reciprocal crosses are carried out between two varieties of a plant: variety X (green leaves) and variety Y (variegated leaves, caused by a chloroplast mutation). a)…
Reciprocal crosses are carried out between two varieties of a plant: variety X (green leaves) and variety Y (variegated leaves, caused by a chloroplast mutation).
- a) State the expected phenotype of the offspring when X is the female parent and Y is the male parent, and when Y is the female parent and X is the male parent.
- b) Explain why the results of reciprocal crosses would be identical if the trait were controlled by a nuclear gene on an autosome.
- c) Explain how reciprocal crosses can be used to distinguish nuclear from non-nuclear inheritance.
Uses of mtDNA
Tracing maternal lineages and forensics
Because mtDNA is passed unchanged down the female line (apart from rare mutations), people who share a maternal ancestor share nearly identical mtDNA. It can be compared across many generations to trace maternal ancestry, or to identify remains by comparing them with a living maternal relative.
mtDNA can be recovered from degraded samples such as old bones, teeth and hair shafts, because each cell has hundreds or thousands of copies, but only two copies of each nuclear gene. More copies means a greater chance that some survive.
Mitochondrial replacement therapy
If a mother carries a faulty mtDNA, her children are at risk. In mitochondrial replacement therapy, the nucleus of the mother's egg (or zygote) is moved into a donor egg whose own nucleus was removed. The donor egg has healthy mitochondria. The embryo is then made with the father's sperm.
The child inherits nuclear DNA from the mother and father, and mtDNA from the donor. Since the faulty mitochondria are left behind, the disorder should not be passed on.
mtDNA is used in forensic science and in tracing maternal ancestry, for example in the identification of remains. a) Explain why mtDNA can be used to trace maternal…
mtDNA is used in forensic science and in tracing maternal ancestry, for example in the identification of remains.
- a) Explain why mtDNA can be used to trace maternal lineages over many generations.
- b) Explain why mtDNA can sometimes be recovered from degraded samples when nuclear DNA cannot.
- c) Explain one limitation of using mtDNA alone to identify an individual.
Mitochondrial replacement therapy (sometimes called "three-parent baby" techniques) replaces a mother's faulty mitochondria with healthy donor mitochondria. a) Explain…
Mitochondrial replacement therapy (sometimes called "three-parent baby" techniques) replaces a mother's faulty mitochondria with healthy donor mitochondria.
- a) Explain how this technique could prevent a child inheriting a mitochondrial disorder.
- b) State which genetic information the child inherits from the donor and which from the parents.
- c) Discuss one ethical issue raised by this technique.
Summary
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Nuclear DNA is linear, wrapped in histones and inherited from both parents following Mendel's laws.
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Mitochondria (in almost all eukaryotes) and chloroplasts (in plants and algae) contain small, circular DNA without histones, like bacteria. This supports the endosymbiotic theory.
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Organelle DNA is inherited from the mother, because the egg supplies the cytoplasm.
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Non-nuclear inheritance does not follow Mendel's ratios. Reciprocal crosses give different results, which distinguishes it from nuclear inheritance.
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mtDNA is used to trace maternal ancestry and identify degraded remains, but it cannot identify an individual alone.
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Mitochondrial replacement therapy uses donor mitochondria and raises ethical issues.
Back to the start: the Galway mother passes her mtDNA mutation to both her children, but only her daughter can pass it on, so her son's children are unaffected.