For there even to be a chapter titled "Atypical Inheritance" in a book that focuses on human genetics, we are actually highlighting an important fact. On one hand, in spite of our developmental and functional complexity, the number of genetic rules is surprisingly small. The mechanisms of inheritance are generally so direct that most examples are "typical." But that is not always the case. The path from an inherited DNA sequence to an expressed phenotype can sometimes be a convoluted and complex one. Most of the time this complexity is seen in the way genetic processes and their products interact with each other and with the environment. But not always.
A couple of examples will illustrate how our normal assumptions can lead to surprises. After fertilization, the regular nuclear divisions of mitosis yield a population of genetically identical cells that differentiate into the tissues of the adult. True? Yes, but not necessarily. Somatic mutation or other genetic events can yield genetic mosaics (Figure 12-1) in which subpopulations of cells within the individual differ from each other. But in other situations the genome itself does not change. Only the ability of the genes to function is affected. This is called genetic imprinting and is a normal phenomenon. It is similar to inactivation of one X chromosome in females in that some portions of the chromosome are turned off.
Mosaicism can occur in any multicellular specimen. Specific mutations may cause phenotypic differences that correlate with the distribution the affected cells. (a) Segmental mosaicism in an orange showing hypertrophy (overgrowth) of one section. (b) Clonal mosaicism seen in the feathers of a duck. The small patch of abnormal feathers on the head is actually a collection of abdominal feathers.
One example is a type of dwarfism in mice caused by imprinting of the insulin-like growth factor 2 (Igf2) gene. Let Igf2+ represent the normal allele and Igf2– represent the dwarf allele. Normally imprinting results in the inactivation of the Igf2 allele the offspring inherits from the mother. If a heterozygous offspring () inherits the normal allele from the father, it will be average sized. But, if it inherits the normal allele from the mother, the normal allele will be inactivated (Igf2+/Igf2–) and the offspring will be dwarf. Sometimes the genome is not even involved. Drosophila can inherit carbon dioxide sensitivity from their mother due to a rhabdovirus passed cytoplasmically just like the maternally inherited mitochondria.
The more we learn about the genome, the more we realize how diverse the body's information pathways can be. Influences on development go well beyond the traditional role of genes controlling the synthesis of enzymes and structural proteins. Some special cases have been mentioned in other contexts. Here we will look ...