61. Pentadactylity is a dominant trait, yet many individuals having single dominant alleles does not show any sign of polydactylity due to
(1) Incomplete penetrance
(2) Variable expressivity
(3) Co dominance
(4) Incomplete dominance

Introduction:
Pentadactylity, the condition of having five digits on each limb, is a dominant genetic trait. However, not all individuals carrying the dominant allele show signs of polydactyly, the presence of extra digits. This phenomenon can be explained by concepts such as incomplete penetrance and variable expressivity. Understanding these genetic terms provides clarity on why some carriers do not display the trait despite having the allele.

Detailed Explanation:

1. Incomplete Penetrance:
   Incomplete penetrance occurs when individuals carry a dominant allele but do not express the associated trait phenotypically. In the case of pentadactylity (or polydactyly), although the trait is dominant, some individuals with one dominant allele fail to show extra digits. This means the gene is present but not always “penetrant” in causing the trait, likely due to other genetic, environmental, or stochastic factors influencing gene expression.

2. Variable Expressivity:
   Variable expressivity refers to the variation in the degree or intensity of a phenotype among individuals who have the same genotype. A person carrying the polydactyly gene might have a wide range of phenotypic outcomes, from a fully formed extra digit to a smaller or less noticeable extra nub. Thus, even if the gene is expressed, the trait may show different severities.

3. Co-Dominance:
   Co-dominance is when both alleles in a heterozygous organism contribute equally and visibly to the phenotype. In pentadactylity or polydactyly, the trait is not co-dominant because the presence of one dominant allele either leads to the trait’s expression or not, rather than a combined phenotype from both alleles.

4. Incomplete Dominance:
   Incomplete dominance results in a blended or intermediate phenotype between two different alleles. This is not applicable to pentadactylity because the trait is either expressed (extra digit) or not, without an intermediate form resulting from allele interaction.

In summary, the reason many individuals with a single dominant allele for pentadactylity do not show polydactyly is due to incomplete penetrance, where the gene does not always express the phenotype. Variable expressivity can explain differences in how the trait manifests when it does appear, while co-dominance and incomplete dominance are not relevant explanations for this trait’s expression.

References:

• Incomplete penetrance describes the presence of a dominant allele without phenotype expression due to genetic and environmental factors.

• Variable expressivity reflects the range of manifestations of a trait among individuals with the same genotype.​