Genetic Penetrance in Polydactyly: Calculating 45% Extent with Explanation

Penetrance measures the proportion of individuals carrying a specific genotype who express the associated phenotype. In this genetic study on polydactyly, the extent of penetrance is 45%.​

Correct Answer and Calculation

Penetrance percentage equals the number of individuals expressing the phenotype divided by those carrying the genotype, multiplied by 100. Here, 36 out of 80 people with polydactyly alleles were polydactylous, so penetrance is (36/80) × 100 = 45%.​
This formula applies universally in genetics, reflecting environmental or genetic modifiers preventing full expression.​
Polydactyly, a dominant trait for extra digits, often shows incomplete penetrance like this case.​

Understanding Penetrance Types

Penetrance varies from complete (100% expression) to incomplete (less than 100%).

• Complete penetrance: All genotype carriers show the phenotype, e.g., achondroplasia where every heterozygote expresses dwarfism.​

• Incomplete penetrance: Only a fraction express it, as in this 45% scenario or BRCA1 mutations (~70% lifetime risk).​
  If options included 100%, that assumes full expression (incorrect here).​

Why Not Other Percentages?

Common exam options might be 45%, 55%, 64%, or 80%, but only 45% fits the data.

• 55% or 64%: Wrong ratios, e.g., 44/80 ≈55%, but data specifies 36.​

• 80%: Misreads total as affected or inverts formula (80/36 ×100).​
  Calculation uses exact 36/80 = 0.45, confirming 45% as precise.​

Implications for Genetic Studies

Incomplete penetrance complicates inheritance predictions, vital for counseling in traits like polydactyly. Factors like modifier genes or environment reduce expression from 80 carriers to 36 affected.​
This 45% highlights why not all allele carriers show traits, aiding research in biotechnology and molecular genetics.​