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?

Key Concepts

Mitochondrial Inheritance

MELAS syndrome is a mitochondrial disorder, meaning it is caused by mutations in mitochondrial DNA (mtDNA), which is inherited exclusively from the mother. This type of inheritance pattern is crucial for understanding the risk of passing on the condition, as only the mother can transmit mitochondrial mutations to her offspring.

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 case of MELAS, even if a child inherits the mtDNA mutation, the severity and presence of symptoms can vary, making it important to discuss these concepts with the couple.

Risk Assessment in Genetic Counseling

In genetic counseling, assessing the risk of passing on genetic conditions involves evaluating family history, understanding inheritance patterns, and discussing potential outcomes. If the father has MELAS, the risk to children is low, as the condition is maternally inherited. However, if the mother were affected, the risk would be significant, as all children would inherit her mtDNA.