3. Extensions to Mendelian Inheritance

Because offspring inherit the mitochondrial genome only from the mother, evolutionarily the mitochondrial genome in males encounters a dead end. The mitochondrial genome in males has no significant impact on the genetic information of future generations. Scientists have proposed that this can result in an accumulation of mutations that have a negative impact on genetic fitness of males but not females. Experiments with Drosophila support this possibility. What experimental data or evidence would you want to evaluate or consider to determine if an accumulation of mtDNA mutations negatively impacts the fitness of males of any species?

In this chapter, we focused on extranuclear inheritance and how traits can be determined by genetic information contained in mitochondria and chloroplasts, and we discussed how expression of maternal genotypes can affect the phenotype of an organism. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions?

What observations support the endosymbiotic theory?

In this chapter, we focused on extranuclear inheritance and how traits can be determined by genetic information contained in mitochondria and chloroplasts, and we discussed how expression of maternal genotypes can affect the phenotype of an organism. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions?

How did the discovery of three categories of petite mutations in yeast lead researchers to postulate extranuclear inheritance of colony size?

How are some of the characteristics of the organelles (the mitochondria and chloroplasts) explained by their origin as ancient bacterial endosymbionts?

Streptomycin resistance in Chlamydomonas may result from a mutation in either a chloroplast gene or a nuclear gene. What phenotypic results would occur in a cross between a member of an mt⁺ strain resistant in both genes and a member of a strain sensitive to the antibiotic? What results would occur in the reciprocal cross?

What is the evidence that transfer of DNA from the organelles to the nucleus continues to occur?

Draw a graph depicting the relative amounts of nuclear DNA present in the different stages of the cell cycle (G₁,S,G₂,M). On the same graph, plot the amount of mitochondrial DNA present at each stage of the cell cycle.

What is the evidence that the ancient mitochondrial and chloroplast endosymbionts are related to the alphaproteobacteria and cyanobacteria, respectively?

Outline the steps required for a gene originally present in the endosymbiont genome to be transferred to the nuclear genome and be expressed, and for its product to be targeted back to the organelle of origin.

Consider the phylogenetic tree presented in Figure 17.17. How were the origins of secondary endosymbiosis in the brown algae determined?

You are a genetic counselor, and several members of the family whose pedigree for an inherited disorder is depicted in Genetic Analysis 17.2 consult with you about the probability that their progeny may be afflicted. What advice would you give individuals III-1, III-2, III-4, III-6, III-8, and III-9?

A mutation in Arabidopsis immutans results in the necrosis (death) of tissues in a mosaic configuration. Examination of the mitochondrial DNA detects deletions of various regions of the mitochondrial genome in the tissues that are necrotic. When immutans plants are crossed with wild-type plants, the  are wild type, and the  are wild type and immutans in a 3:1 ratio. Explain the inheritance of the immutans mutation and a possible origin of the mitochondrial DNA deletions.

What type or types of inheritance are consistent with the following pedigree? 

What is the endosymbiotic theory, and why is this theory relevant to the study of extranuclear DNA in eukaryotic organelles?

A 50-year-old man has been diagnosed with MELAS syndrome (see Figure 17.6). His wife is phenotypically normal, and there is no history of MELAS syndrome in either of their families. The couple is concerned about whether their children will develop the disease. As a genetic counselor, what will you tell them? Would your answer change if it were the mother who exhibited disease symptoms rather than the father?