Ch. 9 - Extranuclear Inheritance

Klug - Concepts of Genetics 12th Edition

Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook

Klug 12th Edition

Ch. 9 - Extranuclear Inheritance

Problem 6

Chapter 9, Problem 6

Predict the results of a cross between ascospores from a segregational petite strain and a neutral petite strain. Indicate the phenotype of the zygote and the ascospores it may subsequently produce.

Verified step by step guidance

Understand the genetic basis of petite mutations in yeast. Segregational petites are caused by nuclear mutations, while neutral petites are caused by mitochondrial mutations. This distinction is crucial for predicting the outcomes of the cross.

Recognize that the zygote formed from the cross will inherit nuclear DNA from both parents and mitochondrial DNA from the neutral petite strain. Mitochondrial DNA is typically inherited maternally in yeast.

Determine the phenotype of the zygote. Since the neutral petite strain has functional mitochondria, the zygote will likely have functional mitochondria and exhibit a wild-type phenotype (non-petite).

Predict the segregation of nuclear alleles during meiosis. The nuclear alleles from the segregational petite strain will segregate according to Mendelian principles, producing ascospores with either wild-type or petite nuclear genotypes.

Consider the mitochondrial inheritance in the ascospores. Since the neutral petite strain contributes functional mitochondria, all ascospores will inherit functional mitochondria and exhibit a wild-type phenotype, regardless of their nuclear genotype.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Segregational Petite Strain

A segregational petite strain refers to a type of yeast that exhibits a petite phenotype due to mutations affecting mitochondrial function. These strains can produce offspring with varying phenotypes when crossed, as they segregate their genetic material during meiosis. Understanding this concept is crucial for predicting the outcomes of genetic crosses involving petite strains.

Neutral Petite Strain

A neutral petite strain is a yeast strain that also displays a petite phenotype but does not have the same genetic mutations as a segregational petite strain. This type of strain typically does not affect mitochondrial function in a way that leads to segregation of phenotypes. Recognizing the differences between these strains is essential for predicting the phenotypic outcomes of their crosses.

Zygote and Ascospores

In yeast, a zygote is formed when two haploid cells fuse, resulting in a diploid cell that can undergo meiosis to produce ascospores. Ascospores are the haploid spores produced by the zygote, which can exhibit various phenotypes depending on the genetic contributions from the parent strains. Understanding the formation and characteristics of zygotes and ascospores is vital for predicting the results of genetic crosses.

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Textbook Question

A plant may have green, white, or green-and-white (variegated) leaves on its branches, owing to a mutation in the chloroplast that prevents color from developing. Predict the results of the following crosses:

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Textbook Question

In diploid yeast strains, sporulation and subsequent meiosis can produce haploid ascospores, which may fuse to reestablish diploid cells. When ascospores from a segregational petite strain fuse with those of a normal wild-type strain, the diploid zygotes are all normal. Following meiosis, ascospores are petite and normal. Is the segregational petite phenotype inherited as a dominant or a recessive trait?

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