The document discusses expressivity, describing it as the degree to which trait expression varies among individuals with the same genotype. It contrasts penetrance, which measures the frequency of phenotype generation, with expressivity's focus on qualitative or quantitative variation. Examples include uniform and variable expressivity, with the latter illustrated by conditions like Waardenburg syndrome and neurofibromatosis, highlighting how genetic and environmental factors influence phenotypic outcomes.

2. Expressivity
By
Rani Summeya Seme 33
Zahra Fayyaz 12

3. Outline
• Definition
• Penetrance vs. Expressivity
• Example
• Quantified Example
• Qualitative Example
• Types
• Uniform Expressivity
• Variable Expressivity
• Waardenburg Syndrome

4. Definition
“The degree to which trait expression differs among individuals”
OR
‘’Expressivity quantifies variation in a non-binary phenotype across
individuals carrying a particular genotype”
OR
“The relative capacity of a gene to affect the phenotype of an organism
of which it is a part”
It is equal to the proportion of individual carriers of a genotype for a
trait who show the trait to a specifiable extent.

5. Penetrance vs expressivity
Penetrance measures how often a gene generates its associated
phenotype to any extent that makes an individual carrier different from
the wild type.
Expressivity characterizes non-binary qualitatively or quantitatively the
extent of phenotypic variation within a particular genotype.
‘’With binary phenotypes expressivity and penetrance are the same.’’

8. Quantified Example
• volume of blood ejected from the pumping heart
• with each contraction
• relative to the total amount of blood contained in the heart's
chamber
• can be quantified by echocardiography
• called the ejection fraction

9. Qualitative Example
• in the hypothetical example of a "blue" gene
• the gene might have an expressivity of 25% for individuals
• that express the "blue" gene & appear light blue
• while individuals who express the "blue" gene & appear dark blue
• the expressivity might be 75%.

10. Types
• Uniform expressivity
• Variable Expressivity

11. Uniform Expressivity
“ When a phenotypic expression of a gene is identical or similar in all
the individuals which carry such genes.”
Most of the qualitative characters exhibit it.

12. Variable expressivity
“it occurs when a phenotype is expressed to a different degree among
individuals with the same genotype..”.”
Example:-
• 2 extra digits vs 3 in polydactyly
• The digit can be full size or just a stub

15. WAARDENBURG SYNDROME

18. Example
• Individuals with the same allele for a gene involved in a quantitative
trait like body height might have large variance (some are taller than
others), making prediction of the phenotype from a particular
genotype alone difficult. The expression of a phenotype may be
modified by the effects of aging, other genetic loci, or environmental
factors
• Neurofibromatosis, where patients with the same genetic mutation
show different signs and symptoms of the disease.

20. Reasons
Variable expressivity may be due to the following reasons.
• 1) Modifying effect of the genetic background or modifying genes.
• 2) The effect of various environmental factors on the gene expression.