What is a Pedigree?

A pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships. These diagrams make it easier to visualize relationships within families, particularly large extended families. Pedigrees are often used to determine the mode of inheritance (dominant, recessive, etc.) of genetic diseases. A sample pedigree is below.

In a pedigree, squares represent males and circles represent females. Horizontal lines connecting a male and female represent mating. Vertical lines extending downward from a couple represent their children. Subsequent generations are therefore written underneath the parental generations and the oldest individuals are found at the top of the pedigree.

If the purpose of a pedigree is to analyze the pattern of inheritance of a particular trait, it is customary to shade in the symbol of all individuals that possess this trait.

In the pedigree above, the grandparents had two children, a son and a daughter. The son had the trait in question. One of his four children also had the trait.

What is a cladogram?

A cladogram is a tree-like diagram showing evolutionary relationships. Any two branch tips sharing the same immediate node are most closely related. All taxa that can be traced directly to one node (that is they are "upstream of a node") are said to be members of a monophyletic group.

A cladogram is a diagram much like a family tree showing the phylogenic tree of different species and demonstrating where they evolved from common ancestors. Once taxonomists based cladograms on physical, easily-observed characteristics; today, they can use more reliable information like genetic and biochemical analysis to determine the relationships between different species.

Each branch on a cladogram is referred to as a "clade" and can have two or more arms. Taxa sharing arms branching from the same clade are referred to as "sister groups" or "sister taxa." Synapomorphies are characteristics shared by the taxa branching from the same clade, but not shared by taxa on other branches – for instance, vertebrates share certain synapomorphies that

are not shared by invertebrates.

The two basic principles behind assembly of a cladogram are that the process from ancestor to descendant should be shown as simply as possible in the tree; and the taxonomic categories applied to the branches in a clade should be monophyletic, or cover an ancestor and all its descendants. Cladograms should not be seen as evolutionary fact, but only as a possible path for speciation.