A pedigree chart is an orderly presentation of family information, using standardized symbols. Once a pedigree has been constructed, the principles of Mendelian inheritance are used to determine whether the trait is dominant or recessive and whether the gene in question is located on an autosome (human chromosomes 1-22) or a sex chromosome (human chromosomes X and Y).
The actual collection of pedigree information is not always straightforward. Knowledge about distant relatives is often incomplete, and recollections about medical conditions can be blurred by the passage of time. Older family members are sometimes reluctant to discuss relatives who had abnormalities or who were placed in institutions. As a result, collecting accurate pedigree information for a large family over several generation can be a challenge for the geneticist. The collection and storage of pedigree information can now be done using software such as Cyrillic. These programs give on-screen displays of pedigrees and genetic information that can be used to analyze patterns of inheritance.
Once a pedigree has been constructed, the information in the pedigree is analyzed to determine how the trait is inherited. Examples of modes of inheritance include:
Autosomal recessive
Autosomal dominant
X-linked dominant
X-linked recessive
Y-linked
Mitochondrial (non-Mendelian)
As outlined above, pedigrees can be difficult to construct. For several reasons, they can also be difficult to analyze. In analyzing a pedigree, a geneticist first forms a hypothesis about how the trait is inherited (for example, is it autosomal dominant?) Then, the pedigree is examined for evidence that supports or rejects this mode of inheriance. Even if the information in the pedigree supports the hypothesis, the analysis of the pedigree is complete only when all possible modes of inheritance have been considered. If only one mode of inheritance is supported by the information in the pedigree, it is accepted as the mode of inheritance for the trait being examined.
It may turn out that the pedigree does not provide enough information to rule out other possible modes of inheritance. For example, analysis of a pedigree may indicate that a trait can be inherited in an autosomal dominant or an X-linked dominant fashion. If this is so, the pedigree is examined to determine whether one mode of transmission is more likely than another. Then, the most likely mode of inheritance is used as the basis for further work.