Question

Which pedigree pattern best characterises a trait showing INCOMPLETE PENETRANCE?

• A. A generation appears skipped - an obligate carrier is unaffected so affected individuals occur in alternate generations
• B. Only one gender is ever affected
• C. None of the family members are affected
• D. Every individual in all generations is affected

Hint:

An obligate carrier who shows no phenotype makes the disease appear to skip them.

Answer 1

The Correct Option is A

Step 1: Define penetrance. Penetrance is the proportion of individuals carrying a particular genotype who express the corresponding phenotype. With complete penetrance, every carrier shows the trait; with incomplete (reduced) penetrance, some carriers are genotype-positive but phenotype-negative.

Step 2: Translate to a pedigree. When an obligate gene carrier does not manifest the disease, the trait appears to 'skip' that person - their affected parent and affected child are linked through them. On the pedigree this looks like affected members in alternate generations with an apparently unaffected (but carrier) individual in between.

Step 3: Eliminate distractors. • 'Only one gender affected' suggests X-linked/sex-limited inheritance, not penetrance. • 'None affected' would mean the trait is absent, not reduced penetrance. • 'All generations/everyone affected' describes complete penetrance with vertical transmission.

Note: A handwritten 'anticipation' annotation on the recall is a distractor - anticipation refers to worsening severity/earlier onset over generations (triplet-repeat disorders), which is a different concept from penetrance.

Key fact: Incomplete penetrance to an unaffected obligate carrier to the disease appears to skip a generation (affected in alternate generations).