What does a completely shaded shape in a pedigree indicate?

In a pedigree, a completely shaded shape signifies that an individual expresses the particular trait being studied, which is typically a condition or a phenotype. This usually indicates the presence of a homozygous genotype if the trait is recessive, meaning both alleles for that trait are the same and result in the manifestation of the trait. For dominant traits, a completely shaded individual could be heterozygous or homozygous, but in the context of many pedigree analyses focusing on recessive traits, it is commonly understood that shading indicates homozygosity for the recessive allele.

In contrast, a heterozygous genotype would typically not result in a completely shaded representation unless it pertains to a dominant trait where at least one allele expresses the phenotype. An unknown genotype would not be represented with complete shading but rather would remain unshaded or lightly shaded. Similarly, a non-expressed trait would not be indicated by a completely shaded shape, as that would suggest that the trait is indeed expressed. Thus, the shading in the pedigree is a clear visual representation of expressed traits, particularly in cases where recessive traits are being tracked and analyzed.