Deviation from Mendelian Inheritance: Incomplete Penetrance and Variable Expressivity

Incomplete Penetrance

• Definition: Occurs when an individual has the genotype associated with a phenotype but does not express the phenotype.
• Example: BRCA1 gene mutation linked to female breast cancer.
  • Has 80% penetrance: means 80% with the mutation exhibit the phenotype (breast cancer) over their lifetime.
• Penetrance Measurement:
  • Typically measured over a person's lifetime.
  • Can be specific to age spans (e.g., 30-year or 40-year penetrance).
• Challenges:
  • Makes genetic analyses complex as not everyone with the genotype expresses the phenotype.

Incomplete Dominance

• Definition: The dominant allele is not completely dominant over the recessive allele, resulting in a blended phenotype.
• Example: Red (dominant) and white (recessive) alleles result in a pink phenotype in heterozygous individuals.

Distinction Between Incomplete Penetrance and Incomplete Dominance

• Incomplete Penetrance: You may not express the phenotype despite having the genotype.
• Incomplete Dominance: The phenotype is a blend of dominant and recessive alleles.

Variable Expressivity

• Definition: Different individuals with the same genotype express the phenotype in varied forms.
• Example: Marfan Syndrome.
  • Symptoms may include being tall and thin with long fingers, cardiac problems, or other physiological issues.
  • Variation in expression: some may just be tall and thin, while others may have additional symptoms.

Key Considerations

• Both incomplete penetrance and variable expressivity introduce complexity in genetic analyses and deviate from Mendelian principles.
• They highlight the variability and unpredictability in genetic expression even among individuals with similar genotypes.