(Choices A and E) Genomic imprinting is a normal process that refers to selective activation of gene expression depending on the parent of origin. Aberrant imprinting occurs with uniparental disomy, or when a person receives 2 copies of a chromosome from the same parent and no copy from the other parent. Prader-Willi syndrome and Angelman syndrome (15q) are examples of conditions caused by dysfunctional imprinting due to uniparental disomy.
(Choice C) Many genetic syndromes are caused by deletions (loss of genetic material). Cri du chat syndrome (5p deletion) is an example of a syndrome caused by a partial deletion of chromosome 5.
(Choice D) Increased trinucleotide repeats (triplet expansion) on certain genes can lead to silencing of a gene or synthesis of an abnormal gene product. Huntington disease and fragile X syndrome are examples of conditions caused by triplet expansion.

(Choice A) Heteroplasmy describes the presence of different mitochondrial genomes (eg, mutated and wild type) within a single cell. The severity of mitochondrial diseases is often related to the proportion of abnormal to normal mitochondria.
(Choice B) Penetrance is the proportion of people with a given genotype who express its associated phenotype. If all individuals with a given gene express its phenotype, that gene is said to have full penetrance.
(Choice D) Pleiotropy is the occurrence of multiple phenotypic manifestations, often in different organ systems, which result from a mutation in a single gene.
(Choice E) The law of segregation describes the phenomenon in which gametogenesis results in the separation of paired chromosomes so that the offspring inherit only half of each parent’s genetic composition.

(Choice A) Aconitase catalyzes the isomerization of citrate to isocitrate in the citric acid cycle.
(Choice B) Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate in glycolysis.
(Choice C) Glucose-6-phosphate dehydrogenase catalyzes the initial and rate-l imiting step of the pentose phosphate pathway. Deficiency of this enzyme results in hemolytic anemia due to the inability to generate NADPH in the oxidative portion of the pathway. However, nonoxidative reactions are responsible for conversion of fructose6-phosphate to ribose-5-phosphate.
(Choice D) Glutathione reductase catalyzes the reduction of glutathione disulfide to glutathione using NADPH. Glutathione aids red blood cells in resisting oxidative stress.