62. 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?
(1) The penetrance of polydactyly is estimated to be 90%
(2) The expressivity of polydactyly is 90%
(3) This is an example of variable expressivity
(4) The polydactyly trait is showing complete penetrance.

Introduction:
Polydactyly, the condition of having extra fingers or toes, is caused by a dominant allele in humans. However, not everyone carrying this allele necessarily shows the trait. This brings into focus the concept of penetrance in genetics—the percentage of individuals with a particular genotype who actually express the associated phenotype. In a screening, if 42 individuals have the allele for polydactyly but only 38 show the trait, interpreting this data correctly requires understanding penetrance and expressivity.

Detailed Solution and Explanation:

1. Penetrance of Polydactyly Is Estimated to Be 90%
   Penetrance measures the proportion of individuals with the gene who actually show the trait. Here, 38 out of 42 individuals with the polydactyly allele exhibit the phenotype, so penetrance is:

3842×100%≈90%4238×100%≈90%

This means 90% penetrance, indicating incomplete penetrance since it is less than 100%. This is the correct interpretation of the observation.

2. Expressivity of Polydactyly Is Not 90%
   Expressivity refers to the degree or severity of the trait in individuals who show it, not the percentage expressing it. For polydactyly, expressivity might describe whether the extra digit is fully formed or a small stub, or if the number of extra digits varies. Since the data only specifies presence or absence of the trait, expressivity is not measured here.

3. Variable Expressivity Is Not Demonstrated
   Variable expressivity means the trait shows different forms or severity among individuals with the genotype. The screening data only tells how many individuals show the trait, not how the trait varies in appearance. Therefore, this example does not demonstrate variable expressivity.

4. Polydactyly Trait Does Not Show Complete Penetrance
   Complete penetrance means 100% of individuals with the allele express the trait. Since only about 90% exhibit polydactyly, the penetrance is incomplete, ruling out complete penetrance.

In summary, the best explanation is that the polydactyly trait shows about 90% penetrance, meaning not all carriers express the trait, which is a case of incomplete penetrance rather than variable expressivity or complete penetrance.