Dr. ANJU BATTA SEHGAL
In Genetics, as per laws of father of Genetics Gregor Mendel, interaction of alleles produce a particular genotype and hence express the expected phenotype (External Morphology). We assume e.g that Rr genotype will produce round seeds and rr genotype will produce wrinkled seeds. For some characters such calculations are incorrect, But in some cases genotype does not always produce the expected phenotype (External Morphology), the phenomenon is called as incomplete penetrance.
Incomplete penetrance is seen in Human Polydactyly, condition of having extra fingers and toes. In these cases, the gene for polydactyly is not fully penetrant. Penetrance is defined as percentage of individual organisms having a particular genotype that express the expected phenotype. For example, if it’s found that 42 people having the allele for polydactyly and found that only 38 of them were polydactylous, the penetrance would be 38/42= 0.90%.
A related concept is that of expressivity, the degree to which a character is expressed. In addition to incomplete penetrance, polydactyly exhibits variable expressivity. Some polydactylous persons posses extra fingers and toes that are fully functional, whereas others posses only a small tag of extra skin.
Incomplete penetrance and variable expressivity are due to the effects of other genes and environmental factors that can alter or completely supress the effect of particular gene. For example a gene may encode an enzyme that produces a particular phenotype only within a limited temperature range. At higher or lower temperature, the enzyme does not function and the phenotype is not expressed: the allele coding such a particular enzyme is therefore penetrant only within a particular temperature range. Many characters exhibit incomplete penetrance and variable expressivity: thus the mere presence of a gene does not guarantee its expression.
This situation was described as “superfluous fingers” in the 16th century by Ambrose Parey(Bell, 1953). It is among the most common congenital limb anomaly observed immediately at birth, manifesting in a variety of forms, ranging from complete or incomplete duplication of digits. Having extra fingers or toes (6 or more) can occur on its own. There may not be any other symptoms or disease present. Polydactyly may be passed down in families. This trait involves only one gene that can cause several variations.
(Author is Principal (Retd). NSCBM Govt. College Hamirpur and former Professor of Botany)
