Bio Passage

[663697]

Maternal inheritance is one pattern of inheritance which does not follow the rules of Mendelian genetics. It is an example of uniparental inheritance in which all progeny have the genotype and phenotype of the female parent.

Maternal inheritance can be demonstrated in the haploid fungus Neurospora by crossing the fungi in such a way that one parent contributes the bulk of the cytoplasm to the progeny. This cytoplasm-contributing parent is called the female parent, even though no true sexual reproduction occurs. The inheritance patterns of a mutant strain of Neurospora called poky have been studied using such crosses. Poky differs from the wild-type in that it is slow-growing and has abnormal quantities of cytochromes.

Investigators suspected that the poky mutation was carried in the mitochondria, instead of in the nuclear genome. The following experiments were designed to test this hypothesis.

Step 1:

Mitochondria were extracted from poky Neurospora mutants.

Step 2:

Using an ultrafine needle and syringe, these mitochondria were injected into wild-type Neurospora cells.

Step 3:

These recipient cells were cultured for several generations, and the phenotypes were examined.

Results:

The poky phenotype was observed in some of the cultured fungi.

The experiment supports which of the following conclusions?

A) The poky mutation resides in the mitochondria.
B) Poky mitochondria induce a genomic mutation which results in the poky phenotype.
C) Maternal inheritance does not apply to poky Neurospora mutants.
D) The slow growth of poky mutants increases their mutation rate.

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Edit: The answer was A for this.. can somebody please explain why? I am having a hard time with it.

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[desperadoflakes]

Maternal inheritance is one pattern of inheritance which does not follow the rules of Mendelian genetics. It is an example of uniparental inheritance in which all progeny have the genotype and phenotype of the female parent.

Maternal inheritance can be demonstrated in the haploid fungus Neurospora by crossing the fungi in such a way that one parent contributes the bulk of the cytoplasm to the progeny. This cytoplasm-contributing parent is called the female parent, even though no true sexual reproduction occurs. The inheritance patterns of a mutant strain of Neurospora called poky have been studied using such crosses. Poky differs from the wild-type in that it is slow-growing and has abnormal quantities of cytochromes.

Investigators suspected that the poky mutation was carried in the mitochondria, instead of in the nuclear genome. The following experiments were designed to test this hypothesis.

Step 1:

Mitochondria were extracted from poky Neurospora mutants.

Step 2:

Using an ultrafine needle and syringe, these mitochondria were injected into wild-type Neurospora cells.

Step 3:

These recipient cells were cultured for several generations, and the phenotypes were examined.

Results:

The poky phenotype was observed in some of the cultured fungi.

The experiment supports which of the following conclusions?

A) The poky mutation resides in the mitochondria.
B) Poky mitochondria induce a genomic mutation which results in the poky phenotype.
C) Maternal inheritance does not apply to poky Neurospora mutants.
D) The slow growth of poky mutants increases their mutation rate.

-----

Edit: The answer was A for this.. can somebody please explain why? I am having a hard time with it.

Yeah, they're trying to investigate if this mutation is carried on mitochondrial DNA or nuclear DNA. They hypothesized its carried on mitochondrial DNA. So they take the mitochondria out of a mutant strain (a strain of fungi that exhibits the mutant phenotype), and they injected that mitochondria (with its mitochondrial DNA) into a normal fungus. They then see that some of those previously normal fungi now have the disease phenotype. This can be explained by the mitochondrial DNA they received carrying the mutation. If they didn't show a disease phenotype, this would imply that the mutant phenotype is carried on nuclear DNA.

 

[aldol16]

Yeah, they're trying to investigate if this mutation is carried on mitochondrial DNA or nuclear DNA. They hypothesized its carried on mitochondrial DNA. So they take the mitochondria out of a mutant strain (a strain of fungi that exhibits the mutant phenotype), and they injected that mitochondria (with its mitochondrial DNA) into a normal fungus. They then see that some of those previously normal fungi now have the disease phenotype. This can be explained by the mitochondrial DNA they received carrying the mutation. If they didn't show a disease phenotype, this would imply that the mutant phenotype is carried on nuclear DNA.

Great explanation, but missing one key point - they're not looking at the "previously normal fungi." These "previously normal fungi" would likely still appear quite normal because they have normal mitochondria too. If they do show some abnormalities, it would only show that the mitochondria is the seat of the disease - not that it's heritable. To show that it's in the mitochondrial DNA and thus heritable, the researchers are looking at the recipient cells after several generations. If it's not heritable, you shouldn't be able to find poky mitochondria. If you do, then it means that it's not only in the mitochondria, but it's also inherited via the mitochondria, i.e. it's in the mitochondrial DNA.

 

[663697]

Great explanation, but missing one key point - they're not looking at the "previously normal fungi." These "previously normal fungi" would likely still appear quite normal because they have normal mitochondria too. If they do show some abnormalities, it would only show that the mitochondria is the seat of the disease - not that it's heritable. To show that it's in the mitochondrial DNA and thus heritable, the researchers are looking at the recipient cells after several generations. If it's not heritable, you shouldn't be able to find poky mitochondria. If you do, then it means that it's not only in the mitochondria, but it's also inherited via the mitochondria, i.e. it's in the mitochondrial DNA.

This was my line of thinking. However, why is the poky phenotype only observed in some, not all, of the cultured recipient cells?

 

[aldol16]

This was my line of thinking. However, why is the poky phenotype only observed in some, not all, of the cultured recipient cells?

Remember you're putting in some poky mitochondria into normal cells that also contain normal mitochondria. The normal mitochondria will also replicate. Whether you get the poky phenotype likely depends on how many poky mitochondria you have versus normal mitochondria. There's likely a threshold - for instance, you probably won't get the poky phenotype of a cell has only one poky mitochondria and many normal ones.

 

[desperadoflakes]

Remember you're putting in some poky mitochondria into normal cells that also contain normal mitochondria. The normal mitochondria will also replicate. Whether you get the poky phenotype likely depends on how many poky mitochondria you have versus normal mitochondria. There's likely a threshold - for instance, you probably won't get the poky phenotype of a cell has only one poky mitochondria and many normal ones.

Right, and this is because mendelian laws of inheritance really don't apply to mitochondrial DNA. When the cell divides, the nuclear chromosomes double and the chromatids split up evenly into the two daughter cells. This does not happen in mitochondria. When the cell splits in cytokinesis, it'll just take whatever mitochondria are present in its half of the cytoplasm at the time. So some of the progeny will have normal mitochondria, some will have mutant, and the proportions really could vary a lot.