Chapter 4, Problem 1P

Define and distinguish incomplete penetrance and variable expressivity.

Summary Introduction

To analyze:

The differences and definitions for the given terms – Incomplete penetrance and Variable expressivity.

Introduction:

The phenotypic variation seen in the population is due the genotypic difference that results in that phenotype. If a particular genotype produces the expected phenotype, it said to be complete penetrance. On the other hand, if a genotype does not produce the expected phenotype, but rather results in a variable phenotype, the reason behind this is gene-environmental interactions between the alleles of other genes in the genome.

Explanation of Solution

Explanation:

Penetrance- If a phenotype is accordant with the genotype, the organism is said to be “Penetrant” for the trait whereas if the phenotype is inconsistent with the genotype, it said to be “Non-penetrant.” When the traits for a genotype result in the estimated phenotype, it is called “Fully penetrant” while if the organism has the effective gene but still fails to show the expected phenotype, it is called an incomplete penetrance. The highly penetrated alleles are studied by clinicians and geneticist. The importance of studying the penetrance benefits in identifying the individuals who are prone to a particular disease.

Expressivity- Expressivity is the extent to which an individual shows a phenotype or a trait or mutation. The variable expressivity refers to the phenotypic expression to a different degree with the same genotype.

a. Incomplete penetrance- If the organism has the effective gene still it fails to show the expected phenotype, it is called incomplete penetrance.

b. Variable expressivity- The degree of phenotypic expression varies in different individual with the same genotype.

Sr No.	Incomplete penetrance	Variable expressivity
1.	Individual do not express the phenotype; however they have the allele.	Symptoms contrast in intensity in different individuals.
2.	Example- Polydactyly.	Example- Waardenburg syndrome.

Conclusion

Thus it is concluded that incomplete penetrance is a condition in which the organism has the effective gene still it fails to show the expected phenotype. Variable expressivity is the degree of phenotypic expression that varies in different individual with the same genotype.