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

Pedigree Analysis

All of the conclusions regarding gene action (dominant/recessive; codominant) we have discussed so far have been obtained from analyzing the results of controlled crosses. In some situations, we do not have the opportunity to perform controlled crosses. Rather we need to analysis an existing population. This is always the case when studying human genetics. Scientists have devised another approach, called pedigree analysis, to study the inheritance of genes in humans. Pedigree analysis is also useful when studying any population when progeny data from several generations is limited. Pedigree analysis is also useful when studying species with a long genration time.

A series of symbols are used to represent different aspects of a pedigree. Below are the principal symbols used when drawing a pedigree.

Once phenotypic data is collected from several generations and the pedigree is drawn, careful analysis will allow you to determine whether the trait is dominant or recessive. Here are some rules to follow.

For those traits exhibiting dominant gene action:

  • affected individuals have at least one affected parent
  • the phenotype generally appears every generation
  • two unaffected parents only have unaffected offspring
The following is the pedigree of a trait contolled by dominant gene action.

And for those traits exhibiting recessive gene action:

  • unaffected parents can have affected offspring
  • affected progeny are both male and female
The following is the pedigree of a trait contolled by recessive gene action.

Copyright © 2000. Phillip McClean