9. In Neurospora, the mutant stp exhibits erratic stop-and-start growth. When a female of stp strain is crossed with a normal strain acting as a male, all progeny individuals showed stp mutant phenotype. However, the reciprocal cross resulted in all normal progeny, individuals. These results can be explanined on the basis of
A. maternal inheritance
B. sex limited inheritance
C. sex influenced inheritance
D. stp mutation may be located in mitochondrial DNA
The most appropriate statement or combination of the above statements for explaining the experimental results is:
(1) A and C (2) C only
(3) A and D (4) B and D
Introduction to Neurospora stp Mutant Inheritance
The stp mutant in Neurospora exhibits an erratic stop-and-start growth pattern, which is intriguing from a genetic inheritance perspective. When a female Neurospora with the stp mutation is crossed with a normal male, all the progeny show the stp phenotype; however, the reciprocal cross results in all normal progeny. This pattern suggests an inheritance mode tied to maternal influence or extranuclear genetic elements such as mitochondrial DNA.
Explanation of Experimental Results and Options
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Maternal Inheritance (Option A): This refers to traits inherited exclusively through the mother, often due to genes present in organelles like mitochondria. In Neurospora, mitochondrial DNA (mtDNA) is passed down maternally because mitochondria in the zygote mostly come from the egg rather than the sperm. The stp phenotype passing exclusively through the female parent aligns with maternal inheritance.
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Sex-Limited Inheritance (Option B): Traits limited strictly to one sex regardless of genotype. This is unlikely in Neurospora because the trait’s expression depends on the parental origin, not on sex-limited expression.
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Sex-Influenced Inheritance (Option C): Traits whose expression differs between sexes but are not limited to one sex. This doesn’t fit the described inheritance since stp inheritance is all-or-none based on maternal versus paternal origin, not influenced by sex.
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Mitochondrial DNA Mutation (Option D): The stp mutant’s erratic growth correlates with the presence of defective mitochondrial DNA, which affects mitochondrial function and growth patterns. Studies confirm that stp mutants have deletions or defective segments in mtDNA, leading to the phenotype being maternally inherited.
The results show progeny phenotype depends strictly on which parent is female, strongly supporting both maternal inheritance and mitochondrial DNA involvement. Therefore, options A and D together best explain the findings.
Conclusion on the Most Appropriate Explanation
The inheritance pattern of the stp mutant phenotype in Neurospora is best explained by a combination of maternal inheritance (A) and the mutation being located in mitochondrial DNA (D). This means the trait follows extranuclear inheritance due to defective mitochondria passed from the mother, consistent with the observed pattern in crosses. Sex-limited or sex-influenced inheritance (B and C) do not align well with the data.
This detailed analysis clarifies that the stp mutant’s erratic growth phenotype in Neurospora is an example of maternal inheritance caused by mitochondrial DNA mutation, making option (3) A and D the correct choice for the genetic explanation of this experimental result.
