4. PENETRANCE:
•The percentage of individuals that show at least
some degree of expression of a mutant genotype
defines the penetrance of the mutation.
5. EXAMPLE:
• For example, the phenotypic expression of many of the mutant
alleles found in drosophila can overlap with wild-type expression. If
15 percent of flies with a given mutant genotype show the wild-type
appearance, the mutant gene is said to have a penetrance of 85
percent.
7. EXAMPLE:
•Flies homozygous for the recessive mutant gene eyeless exhibit phenotypes
that range from the presence of normal eyes to a partial reduction in size to the
complete absence of one or both eyes. ). Although the average reduction of eye
size is one-fourth to one-half, expressivity ranges from complete loss of both
eyes to completely normal eyes. Examples such as the expression of the eyeless
gene have provided the basis for experiments to determine the causes of
phenotypic variation
8. USES OF PENETRANCE AND EXPRESSIVITY
•Some mutant genotypes are always expressed as a distinct phenotype, whereas
others produce a proportion of individuals whose phenotypes cannot be
distinguished from normal (wild type). The degree of expression of a particular
trait can be studied quantitatively by determining the penetrance and
expressivity of the genotype under investigation
9. REASONS OF PHENOTYPIC MUTATIONS
OTHER THAN GENOTYPE:
•If the laboratory environment is held constant and extensive variation is still
observed, other genes may be influencing or modifying the phenotype. On the
other hand, if the genetic background is not the cause of the phenotypic
variation, environmental factors such as temperature, humidity, and nutrition
may be involved. In the case of the eyeless phenotype, experiments have shown
that both genetic background and environmental factors influence its
expression.
