Introduction to Pedigrees
Factors to Consider in Pedigrees
In this module, we will go through the factors to consider in working out pedigrees. A pedigree illustrates a family history that shows how a trait is inherited over several generations. In the simplest pedigrees, a trait is considered either present or absent, such as with the expression of a disease.
The first step in working out simple pedigrees is to make sure the trait is autosomal (not on the sex chromosomes). This is done by looking for sex-specific trends in the expression of the trait.
The second step is to identify the pattern of expression. Recall that a dominant allele masks a recessive allele. Dominant alleles are expressed every generation. For recessive alleles, the expression may skip generations.
Sex-specific patterns can be due to linkage to the X chromosome or the Y chromosome. Recall that in humans, two X chromosomes produce a female, while the X-Y combination produces a male. If the trait is Y-linked, any individual who expresses the trait also has to be male. Thus, if only males express the trait, the gene for the trait is on the Y chromosome. In a sense, Y-linked genes are always dominant, since there is no homologous (matching) gene on the X-chromosome that could mask its expression. For X-linked recessive traits, we find a bias for sons to inherit a disease from normal parents.
We will go through the four basic types of pedigrees: Autosomal dominant, autosomal recessive, Y-linked, and X-linked recessive.
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