8. In Neurospora, the mutant stp exhibits erratic stop-start growth. The mutant site is known to be in the mitochondrial DNA. If an stp strain is used as the female parent in a cross to a normal strain acting, as the male, what type of progeny can be expected?
(1) All Start and stop mutant
(2) All wild type
(3) Majority of Start and stop mutant
(4) Majority of wild type
In Neurospora, the mitochondrial mutant stp exhibiting erratic stop-start growth is inherited maternally when used as the female parent in a cross with a normal male parent, so all progeny will show the stop-start mutant phenotype. This is because mitochondrial DNA, which carries the mutation, is inherited exclusively from the female parent cytoplasm.
Explanation of the Inheritance Pattern
Neurospora mitochondria are inherited uniparentally (non-Mendelian inheritance), predominantly from the female parent. The mitochondria and their DNA come from the cytoplasm of the female gamete, whereas the male parent contributes very little or no mitochondria to the zygote. Therefore, if the female parent harbors the mitochondrial mutant stp allele, all the progeny will inherit the mutation irrespective of the normal genotype present in the male parent.
Detailed Explanation of Each Option
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All Stop and start mutant: This is the correct answer because the mutant mitochondria from the female stp strain are transmitted to all the offspring. The erratic stop-start growth phenotype controlled by mitochondrial DNA manifests in all progeny.
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All wild type: This is incorrect because the female parent’s cytoplasm, containing the mutant stp mitochondrial DNA, determines mitochondrial inheritance; male mitochondria do not influence progeny phenotype. Thus, offspring do not revert to wild type if the female is mutant.
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Majority of Stop and start mutant: This is incorrect since mitochondrial inheritance in Neurospora is not segregating randomly or biparentally. It is strictly uniparental (maternal), so all progeny should show the mutant phenotype, not just a majority.
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Majority of wild type: Similarly, this is incorrect because the wild-type mitochondria from the male parent are not typically inherited or do not replicate in progeny. Majority wild type progeny would only happen if the mutation was nuclear or predominantly segregated, which is not the case here.
Summary
When the stp mutant Neurospora strain (carrying the mitochondrial mutation) is used as the female parent in a cross with a normal male strain, all progeny inherit the mutant mitochondrial genome from the female parent and show the erratic stop and start growth phenotype. This is a model example of maternal (cytoplasmic) inheritance of mitochondrial traits in fungi.
Introduction: The stp mutant in Neurospora is characterized by erratic stop-start growth and is caused by a mutation in the mitochondrial DNA. When a female stp strain is crossed with a normal male, all progeny inherit the mutant mitochondrial phenotype, demonstrating the maternal inheritance pattern typical of mitochondrial genes in Neurospora.
This pattern of uniparental inheritance explains why all progeny from a female mutant cross show the phenotype, whereas a reciprocal cross with a male mutant results in wild-type offspring.
This knowledge highlights the fundamental principles of mitochondrial genetics in fungi and aids in understanding cytoplasmic inheritance mechanisms in eukaryotes.
References:
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Mitochondrial inheritance in Neurospora is maternal, with female-derived mitochondria determining progeny phenotype.
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The stp mutant mitochondria cause the characteristic stop-start growth phenotype and are retained exclusively from the female parent.
