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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School


First Advisor

Nels H. Granholm


How does the information in a given gene translate into a given function? Or simply, how do genes work? How do genes influence other genes? What makes gene interaction so complex? How does one begin to understand the coordinated functions of all the genes in a genome? The answers to these questions are largely unknown, but we have made impressive advances. A functional understanding of gene interaction may be found in the study of melanogenesis. A number of genes regulating melanogenesis have been identified, studied, and functionally characterized. Mutations of genes that control melanogenesis are present in animals (including humans) with different coat colors. Some aberrations are spontaneous while some are induced. The important thing is they are readily identified. Over 50 genetic loci and over 140 different mutations have been identified and characterized (Silvers, 1979). These mutations affecting melanogenesis provide a comprehensive body of knowledge concerning the genes that control melanogenesis. This system of interacting genes provides a productive model to study genetic interaction of two or more genes. This body of knowledge gives one a powerful tool to dissect the relationships of certain genes that are fundamental to melanogenesis. For example, a mutation at a certain locus can cause a total lack of pigment. Mutations at other loci can affect the type of melanin produced. A most intriguing situation exists when an animal can produce one type of melanin and then "switch" to produce another and then "switch" back to the first type. Such is the case in the system we studied, the agouti switch. In our study we used agouti locus mutations to characterize genetic interactions of melanogenesis. The mice in this study were genetically identical except for one allele. The locus and different alleles investigated are found at the agouti locus on chromosome 2. The agouti locus allele (A), the wild type, causes the hairbulb melanocyte to produce first black then yellow then black pigment. The nonagouti black allele (a), causes the mouse to produce only black melanin (eumelanin), while the lethal yellow (Ay) allele (lethal only when homozygous) causes the mouse to produce exclusively yellow melanin (phaeomelanin). These alleles seem to affect tyrosinase, the gene product of the albino locus on chromosome 7. Tyrosinase is a fundamental enzyme during both eu- and phaeomelanogenesis. Mutations at the albino locus can result in the total absence of tyrosinase or the production of a non-functional tyrosinase. In either case no pigment can be produced, and the resultant animal is termed an albino. Alleles at the agouti and albino locui plus other pigmentation loci work together to produce the color combinations or lack of color found in nature. The wild-type agouti allele (A) allows the switching back and forth while a/a and Ay/a mice seem to be stuck in their respective stages of pigment production. Since all the mice used in this study are genetically identical except for alleles at the agouti locus (congenic mice), differences in the type of pigment produced can be attributed to the effect of one different allele at the agouti locus. This system provides a tool to monitor the actions of the agouti locus alleles on the albino locus product-tyrosinase. These effects of the agouti locus on tyrosinase may vary from a post-translational processing difference to a reduction in the synthesis of the protein. It's conceivable that the agouti locus could modulate the following components of tyrosinase: transcription, translation, glycosylation and other posttranslational processes, deployment of tyrosinase through the cell's endomembrane system to the tyrosinase - containing coated Golgi Vesicle, migration of the Golgi vesicle to premelanosomes, incorporation into melanosomes, parameters of tyrosinase - matrix protein interaction, and others. Some of these effects of the agouti locus on tyrosinase activity were studied in this research. It is important to keep in mind that knowledge gained from the study of genes that regulate melanogenesis may help in revealing the function of genes that cause cancer, obesity, infertility, and other abnormalities. Because the mouse is a mammal, the modus operandi of gene expression and regulation may be closely related to that of Homo sapiens. Because genetic regulation is fundamental not only to embryogenesis and development but also to homeostasis, studies which lead to an understanding of the regulation of mammalian genes are highly meritorious.

Library of Congress Subject Headings

Tyrosine in the body
Phenol oxidase
Genetic regulation
Mice as laboratory animals
Melanins -- Synthesis



Number of Pages



South Dakota State University