For decades scientists have been perplexed by different circumstances surrounding families with rare, early-onset auditory neuropathy (deafness). In some families, parents and grandparents of the proband have normal hearing, while in other families, a number of affected (deaf) family members are scattered throughout the pedigree, appearing in every generation. Assuming a genetic cause for each case, offer a reasonable explanation for the genetic origin of such deafness in the two types of families.

Verified step by step guidance

1. Identify the two patterns of inheritance described in the problem: one where the condition appears in every generation and another where it skips generations.
2. For the pattern where the condition appears in every generation, consider autosomal dominant inheritance, where a single copy of the mutant allele can cause the condition.
3. For the pattern where the condition skips generations, consider autosomal recessive inheritance, where two copies of the mutant allele are necessary for the condition to manifest.
4. In the autosomal dominant scenario, affected individuals have a 50% chance of passing the mutant allele to their offspring, leading to the condition appearing in every generation.
5. In the autosomal recessive scenario, carriers (heterozygous individuals) do not show symptoms, but if two carriers have children, there is a 25% chance for each child to be affected, explaining the skipping of generations.

Key Concepts

Autosomal Dominant and Recessive Inheritance

Autosomal dominant inheritance occurs when a single copy of a mutated gene from one parent can cause a trait or disorder, leading to affected individuals in every generation. In contrast, autosomal recessive inheritance requires two copies of the mutated gene, often resulting in affected individuals appearing sporadically in a pedigree, typically when both parents are carriers. Understanding these patterns helps explain the differing occurrences of auditory neuropathy in families.

Penetrance and Expressivity

Penetrance refers to the proportion of individuals with a specific genotype that actually express the associated phenotype, while expressivity describes the degree to which a genotype is expressed in an individual. In the context of auditory neuropathy, variable penetrance could explain why some family members with the same genetic mutation may have normal hearing, while others are affected, indicating that not all individuals with the mutation will exhibit the disorder.

Mitochondrial Inheritance

Mitochondrial inheritance involves genes located in the mitochondria, which are passed exclusively from mother to offspring. This type of inheritance can lead to conditions that affect multiple family members across generations, particularly if the mother carries a mutation. In cases of auditory neuropathy, if the condition is linked to mitochondrial DNA, it could explain the presence of deafness in some family members while others remain unaffected.