(a) In humans the mitochondrial genome encodes a low number of proteins, rRNAs, and tRNAs but imports approximately 1100 proteins encoded by the nuclear genome. Yet, with such a small proportion from the mitochondrial genome encoding proteins and RNAs, a disproportionately high number of genetic disorders due to mtDNA mutations have been identified [Bigger, B. et al. (1999)]. What inheritance pattern would you expect in a three-generation pedigree in which the grandfather expresses the initial mtDNA defect? What inheritance pattern would you expect in a three-generation pedigree in which the grandmother expresses the initial mtDNA defect?
(b) Considering the description in part (a) above, how would your pedigrees change if you knew that the mutation that caused the mitochondrial defect was recessive and located in the nuclear genome, was successfully transported into mitochondria, and negated a physiologically important mitochondrial function?

The accompanying pedigree shows the transmission of albinism (absence of skin pigment) in a human family.

The female I-1 and her mate, male I-2, had four children, one of whom has albinism. What is the probability that they could have had a total of four children with any other outcome except one child with albinism and three with normal pigmentation? 

The accompanying pedigree shows the transmission of albinism (absence of skin pigment) in a human family.

Using allelic symbols of your choice, identify the genotypes of the male and his two mates in generation I. 

The accompanying pedigree shows the transmission of albinism (absence of skin pigment) in a human family.

What is the most likely mode of transmission of albinism in this family? 

Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?

The following pedigree shows a family in which several individuals exhibit symptoms of the mitochondrial disease MERRF. Two siblings (II-2 and II-5) approach you to inquire about whether their children will also be afflicted with MERRF. What do you tell them? 

What is the most likely mode of inheritance for the trait depicted in the following human pedigree? 

Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.

In what way does a condition caused by mtDNA differ in expression and transmission from a mutation that causes albinism?

Researchers examined a family with an interesting distribution of Leigh syndrome symptoms. In this disorder, individuals may show a progressive loss of motor function (ataxia, A) with peripheral neuropathy (PN, meaning impairment of the peripheral nerves). A mitochondrial DNA (mtDNA) mutation that reduces ATPase activity was identified in various tissues of affected individuals. The accompanying table summarizes the presence of symptoms in an extended family.

How can some individuals in the same family show such variation in symptoms? What term, as related to organelle heredity, describes such variation?