Mendel's First Law

Variations to Mendel's First Law

Pedigree Analysis

Mendel's Second Law

Chi-Square Test



Modifier Genes

Penetrance and Expressivity

Study Questions

Mendelian Genetics Overheads

Mendelian Genetics WWW Links

Genetic Topics

Variation in Gene Expression

Not all traits are expressed 100% of the time even though the allele is present. For example the dominant allele P produces polydactyly in humans, a trait that is characterized by extra toes and/or fingers. Two normal appearing adults have been known to mate and produce offspring that express polydactyly. Thus one parent must carry at least one dominant allele (P allele) and its genotype is probably Pp. This parent with the Pp genotype exhibits reduced penetrance for the P allele.

The hand of blues guitar player Hound Dog Taylor exhibiting polydactyly.

Penetrance - the frequency of expression of an allele when it is present in the genotype of the organism (if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant)

Not all phenotypes that are expressed are manifested to the same degree. For polydactyly, an extra digit may occur on one or more appendages, and the digit can be full size or just a stub. Therefore, when the P allele is present it expresses variable expressivity.

Expressivity - variation in allelic expression when the allele is penetrant.

Copyright © 2000. Phillip McClean