Nonepistatic Interactions and Pleiotropism Help
Genetic interaction may also occur without epistasis if the end products of different pathways each contribute to the same trait. This is often referred to as complementary gene action.
EXAMPLE 4.2 The dull-red eye color characteristic of wild-type flies is a mixture of two kinds of pigments (B and D) each produced from nonpigmented precursor compounds (A and C) by the action of different enzymes (e1 and e2) specified by different wild-type genes (g+1 and g+2).
The recessive alleles at these two loci (g1 and g2) specify enzymatically inactive proteins. Thus, a genotype without either dominant allele would not produce any pigmented compounds and the eye color would be white.
In the above example, the genes for color B and color D produce phenotypes that are both dominant to white, but when they occur together they produce a novel phenotype (wild type) by interaction. If the two genes are assorting independently, the classical 9 : 3 : 3 : 1 ratio will be seen in the progeny of the dihybrid parents.
EXAMPLE 4.3 A brown ommochrome pigment is produced in Drosophila melanogaster by a dominant gene st+ on chromosome 3. A scarlet pterin pigment is produced by a dominant gene bw+ on chromosome 2. The recessive alleles at these two loci produce no pigment. When pure scarlet flies are mated to pure brown flies, a different phenotype (wild type) appears in the progeny.
Many and perhaps most of the biochemical pathways in the living organism are interconnected and often interdependent. Products and intermediates of one pathway may be used in several other metabolic schemes. It is not surprising, therefore, that the phenotypic expression of a gene usually effects more than one trait. Sometimes one trait will be clearly evident (major effect) and other, perhaps seemingly unrelated ramifications (secondary effects) will be less evident to the casual observer. In other cases, a number of related changes may be considered together as a syndrome. All of the manifold phenotypic expressions of a single gene are spoken of as pleiotropic gene effects.
EXAMPLE 4.4 The syndrome called "sickle-cell anemia" in humans is due to an abnormal hemoglobin. This is the primary effect of the mutant gene. Subsidiary effects of the abnormal hemoglobin include the sickle shape of the cells and their tendency to clump together and clog blood vessels in various organs of the body. As a result, heart, kidney, spleen, and brain damage are common elements of the syndrome. Defective corpuscles are readily destroyed in the body, causing severe anemia.
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