A pedigree is a family tree, a graphical representation of biological descent (Fig. 1). Each generation is represented in a separate row. Males are represented by squares and females are represented by circles; alternative respective symbols are 3, representing the shield and spear of Mars, and the mirror of Venus. Horizontal lines join parents and sets of sibs; vertical lines indicate descent. Sibs are ordered according to birth from left to right. Double horizontal lines call attention to parents that are closely related (inbreeding); sometimes they are used to join monozygotic twins as well. Filled, colored, or shaded symbols indicate affected individuals in pedigrees that represent the inheritance of certain traits; partly modified symbols indicate unaffected heterozygous carriers. Several sibs can be pooled together by indicating their number within a single symbol. A pedigree may be described in words, but this is usually impractical.
Figure 1. Conventional symbols used in the representation of pedigrees.
In the analysis of pedigrees, attention must be given to inconstant phenotypes due to effects of the environment or the rest of the genotype of the affected individuals on the expression of the allele. Some alleles are unstable and change from one generation to the next (see Trinucleotide Repeats). Because of the small size of human families, linkage has to be deduced from the pooled results of many groups of sibs (see Lod scores).
