​Human polydactyly traits having extra fingers or toes are caused by a dominant allele. In a screening it was found that out of 42 individuals having an allele for polydactyly, only 38 of them were polydactylus. Which of the following is the correct interpretation of the observation?​

Correct option is A

Explanation-

Polydactyly is caused by a dominant allele. Out of 42 individuals with the allele, only 38 showed the trait.

So, 38 out of 42 had the phenotype →

$$\text{Penetrance} = \frac{38}{42} \times 100 \approx 90\%$$

Incorrect options-

​Option b: "The expressivity of polydactyly is 90%"
Expressivity refers to degree or severity of a phenotype, not the number of individuals showing it. This data doesn't describe variation in how polydactyly is expressed (e.g., size or number of extra fingers), only whether it is present or not.
Option c: "This is an example of variable expressivity"
Again, no information is given about variability in trait severity — just its presence/absence. So this is about penetrance, not expressivity.
Option d: "The polydactyly trait is showing complete penetrance"
Complete penetrance means 100% of individuals with the allele show the trait. Here, only 90% do, so it is incomplete penetrance.

Correct Answer - Option a – "The penetrance of polydactyly is estimated to be 90%"
This directly explains the observed data. Penetrance refers to the proportion of individuals with the genotype who actually express the phenotype. Since not all carriers showed the phenotype, penetrance is less than 100% — here, ~90%.​​