5. Yeast with petite colony when crossed with wild type generates no petite colony. The most probable mode of inheritance is
(1) Chloroplast      (2) Mitochondria
(3) Episomal         (4) Nuclear

Introduction:
Petite colonies in yeast are small-sized colonies resulting from defects in mitochondrial function, leading to respiratory deficiency. The inheritance of petite characteristics in yeast provides an excellent example of extranuclear inheritance, involving mitochondrial DNA rather than nuclear genes. This article explains the possible inheritance modes of petite colonies in yeast, clarifies why neutral petites crossed with wild type produce no petite offspring, and discusses related genetic mechanisms.

Explanation of Each Option:

1. Chloroplast Inheritance:
   Chloroplast genes are involved in photosynthesis and are predominantly found in plant cells, not yeast. Yeast does not contain chloroplasts; thus, chloroplast inheritance is irrelevant in yeast petite colony formation.

2. Mitochondrial Inheritance:
   The petite phenotype in yeast is primarily caused by mutations or deletions in mitochondrial DNA (mtDNA), leading to defective aerobic respiration and small colony size. When a neutral petite yeast (with defective or deleted mitochondrial DNA) is crossed with a wild-type yeast, the offspring inherit the normal mitochondrial DNA from the wild-type parent, producing no petite colonies. This pattern illustrates extranuclear inheritance through mitochondria.

3. Episomal Inheritance:
   Episomal inheritance refers to genes carried on plasmids or other extra-chromosomal elements in the cytoplasm. While yeast can carry plasmids, petite colony traits specifically involve mitochondrial genome mutations, not plasmid-based inheritance.

4. Nuclear Inheritance:
   Some petite mutants arise from nuclear gene mutations (called segregational petites) and follow Mendelian inheritance with ratio segregation. However, these show petite colonies in offspring crosses with wild-type yeast. Since neutral petites produce only wild-type progeny when crossed, nuclear inheritance is not the most probable mode here.

Conclusion:
The scenario where yeast with petite colony phenotype crossed with wild type produces no petite colonies aligns with neutral petite mutations caused by mitochondrial DNA defects, inherited extranuclearly through mitochondria. Thus, mitochondrial inheritance is the most probable mode for the petite colony trait described in this question.

References:

• Petite mutations are respiratory-deficient and result from mitochondrial DNA deletion or damage, showing extranuclear inheritance.​

• Neutral petites crossed with wild type yield all wild-type progeny due to mitochondrial DNA inheritance from wild type.​

• Nuclear petite mutants show Mendelian segregation, unlike neutral petites.​

This article explains the genetics behind petite colony inheritance, valuable for students and researchers studying yeast genetics and extranuclear inheritance patterns.

Keywords: petite colony inheritance yeast, mitochondrial inheritance yeast, neutral petite yeast, extranuclear inheritance, yeast genetics