Question

If an individual does not show a trait even though they have the appropriate genotype, the trait is said to exhibit

• A. Variable expressivity
• B. Incomplete penetrance
• C. Partial dominance
• D. Co-dominance

Hint:

To remember the difference:
- Penetrance is a qualitative measure (Does the phenotype appear? Yes or No).
- Expressivity is a quantitative measure (How severe is the phenotype? Mild to Severe).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Penetrance and expressivity describe how a genotype is expressed as an observable phenotype.
They help explain variations in how inherited traits are displayed in individuals.

Step 3: Detailed Explanation:
Let us review the definitions of the options:
Incomplete penetrance: This occurs when an individual inherits a specific genotype but does not display the expected phenotype.
For example, if a dominant allele for polydactyly (extra fingers) has \(80%\) penetrance, \(20%\) of carriers will have normal hands and feet.
This is an "all-or-nothing" metric evaluating whether a trait is expressed or not.
Variable expressivity: This occurs when a genotype is expressed as a phenotype, but its severity varies among individuals.
In polydactyly, variable expressivity would mean one carrier has a fully formed extra finger, while another has only a small skin tag.
Partial dominance (incomplete dominance): This describes a heterozygote displaying an intermediate phenotype between the two homozygotes (e.g., pink flowers from red and white parents).
Co-dominance: This is when both alleles in a heterozygote are fully and independently expressed (e.g., AB blood group).
Because the question describes an individual who does not show the trait at all despite having the genotype, it is incomplete penetrance.

Step 4: Final Answer:
The failure to express a trait despite having the corresponding genotype is called incomplete penetrance, matching Option (B).