ABSTRACT
For looking at patterns of inheritance in this chapter, the authors are concerned with the phenotypic consequences of a person's genotype. For most rare dominant conditions, homozygotes have never been reported. Huntington disease is a rare example of a dominant condition where homozygotes are known and are indistinguishable from the usual heterozygotes. For some X-linked dominant conditions, absence of the normal allele is lethal before birth. The classic Mendelian pedigree patterns are best seen with rare conditions, where there is little chance that somebody who marries into the family might coincidentally also carry the disease mutation that is segregating in the family. However, mosaicism is only noticed and commented on when a person has a relevant phenotypic abnormality or if they have multiple children affected by a dominant condition that occurs de novo in the family. The authors survey the theoretical tools that help the reader understand non-Mendelian inheritance.
