Summary
Penetrance is the fraction of carriers showing a phenotype; expressivity is how strongly it appears.
In depth
Incomplete penetrance means some people with a genotype show no phenotype (BRCA1 carriers have about 70 percent breast cancer risk by 80); variable expressivity means that those who show it differ in severity (neurofibromatosis). Modifier genes and the environment explain this.
Penetrance is the proportion of people with a genotype who show any phenotype, and expressivity is the degree of the effect among those who do. Polydactyly can show in one hand but not the other, in the same person, which is variable expressivity. Neurofibromatosis type 1 has complete penetrance but very variable expressivity.
Causes include modifier genes, environment, stochastic effects in development, epigenetics and age. Age-dependent penetrance is seen in Huntington disease, with symptoms typically beginning in mid-adulthood. Risk estimates for relatives must take reduced penetrance into account.
Common mistake: concluding that a person lacking the phenotype did not inherit the allele; with incomplete penetrance they may still carry and pass it on.
Frequently asked questions
What is Penetrance and Expressivity?
Penetrance is the fraction of carriers showing a phenotype; expressivity is how strongly it appears.
Which area of biology covers Penetrance and Expressivity, and how advanced is it?
Penetrance and Expressivity is one of the 54 topics in Genetics. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Dominant and Recessive, Allele, Down Syndrome.
Can you give an example of Penetrance and Expressivity?
Polydactyly can appear with variable numbers of extra digits.
What should I learn before Penetrance and Expressivity?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Penetrance and Expressivity. Advanced topics assume the Intermediate vocabulary.
What topics are related to Penetrance and Expressivity?
The closest topics to Penetrance and Expressivity are Dominant and Recessive, Allele, Down Syndrome, Epistasis. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Penetrance and Expressivity?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Penetrance and Expressivity. To practise the whole of Genetics, use the Genetics quiz.
How is Penetrance and Expressivity different from Dominant and Recessive?
Penetrance and Expressivity: Penetrance is the fraction of carriers showing a phenotype; expressivity is how strongly it appears. Dominant and Recessive: A dominant allele shows its effect when present in one copy; a recessive allele only shows when two copies are present. Read both side by side and note what each one explains.
What is a common mistake about Penetrance and Expressivity?
Concluding that a person lacking the phenotype did not inherit the allele; with incomplete penetrance they may still carry and pass it on.
How should I study Penetrance and Expressivity step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Dominant and Recessive and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.