Expressivity and Penetrance

Question 1

Penetrance

Answer 1

The proportion of individual organisms having a particular genotype that express the expected phenotype

Question 2

Expressivity

Answer 2

The degree to which a phenotype is expressed (mild to severe); variation in the individual

Question 3

Complete penetrance

Answer 3

Identical known genotypes yield 100% expected phenotype

Question 4

Incomplete penetrance

Answer 4

Identical known genotype yield <100% expected phenotype. E,g. Polydactyly

Question 5

Constant expressivity

Answer 5

Identical known genotypes with no expressivity effect yield 100% expected phenotype

Question 6

Variable expressivity

Answer 6

Identical known genotypes with an expressivity effect yield a range of phenotypes

Question 7

Expressivity examples

Answer 7

Split Hand foot syndrome (rare autosomal dominant) - Variable expressivity

Piebaldism (rare autosomal dominant) - Variable expressivity

Huntington Disease (rare autosomal dominant) - Variable expressivity

Question 8

Causes of Incomplete penetrance and expressivity

Answer 8

It is due to the effects of other genes and to environmental factors that can completely suppress the effect of a particular gene

Question 9

Environmental Factors that affect phenotypic expression

Answer 9

• Age
• Sex
• Temperature
• Chemicals

Question 10

Norm of reaction

Answer 10

The range of phenotypes expressed by a single genotype under different environmental conditions