Basics of 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 controlled 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 controlled by recessive gene action.