6. A poky Neurospora was crossed with normal Neurospora and following results were obtained

   ♀ Poky X ♂ Normal à all poky

♀Normal X ♂ Poky à all Normal

The mode of inheritance is
(1) Maternal Inheritance  (2) Maternal effect
(3) X-Linked                   (4) Sex influenced

Introduction:

The inheritance pattern of the slow-growing “Poky” mutant in Neurospora crassa demonstrates a fascinating example of non-Mendelian genetics. When crossed with normal Neurospora, the progeny outcome varies depending on the maternal strain, indicating extranuclear or cytoplasmic inheritance rather than traditional chromosomal transmission. This article explores the modes of inheritance involved in this classic genetic cross, explaining key concepts like maternal inheritance, maternal effect, X-linked inheritance, and sex-influenced traits.

Detailed Explanation of the Cross:

The cross results summarized are:

• ♀ Poky × ♂ Normal → all Poky offspring

• ♀ Normal × ♂ Poky → all Normal offspring

This pattern shows that the phenotype of the offspring depends entirely on the genotype of the mother (the female parent), not the father. This suggests the inheritance is not linked to nuclear genes on chromosomes but rather to genetics inherited through the cytoplasm in the egg.

Explanation of Options:

1. Maternal Inheritance
   Maternal inheritance means the trait is passed from the mother to all offspring regardless of the father’s genotype because the trait’s determinant is located in the cytoplasm (e.g., mitochondria) which is inherited predominantly from the egg. The Poky mutant in Neurospora is a classic case of maternal inheritance due to mutations in mitochondrial DNA leading to slow growth. In this case, only progeny from a poky mother display the poky trait, confirming maternal inheritance.​

2. Maternal Effect
   Maternal effect refers to a situation where the mother’s genotype or environment affects the phenotype of the offspring, but the offspring’s genotype determines its own inherited traits. It is usually due to substances like mRNA or proteins deposited into the egg by the mother. However, in the poky Neurospora example, the trait is inherited through mitochondrial DNA, not just influenced by maternal substances, so this is different from a maternal effect.​

3. X-Linked Inheritance
   X-linked inheritance involves genes located on the X chromosome, affecting males and females differently, often showing criss-cross inheritance or sex bias. The poky phenotype does not follow this pattern and is unrelated to sex chromosomes or males versus females.​

4. Sex Influenced
   Sex-influenced traits exhibit different expressions depending on the sex of the individual but are inherited through autosomal chromosomes. Since the poky trait strictly depends on the mother’s cytoplasm and not the offspring’s sex, this mode is not applicable.​

This detailed analysis confirms that the mode of inheritance for the poky Neurospora mutant is maternal inheritance because the offspring phenotype is determined by the mother’s genotype transmitted through mitochondrial DNA in the cytoplasm, not by nuclear genes or sex chromosomes.​