In dogs, coat colors are controlled by many variations in gene expressions and alleles. In a simplified analysis of these changes, three main genes produce obvious and predictable traits in dog phenotypes for coat colors and patterns. The TYPR1, MC1R, MSH, and ASIP genes all have complex involvment in the pathways to produce color and pattern of color in dogs. The MC1R allele codes for a receptor protein on the membrane of melanocyte cells, which are cells that are responsible for the expression of the color genes in dogs, and therefore control the color pigments produced in the hair follicles. When this allele is precent in the genome in the wildtype, the receptor protein interacts with all the molecules it is meant to and successfully passes the message from the extracellular matrix. Once received the signal causes the transcription of the TYPR1 gene which produces the pigments required to make black colors. If TYPR1 is the only mutated gene and is homozygous recessive (AARRbb) then the color will be brown. ASIP genes code for a molecule that also get received by the MCIR receptor. If ASIP is the only signaling molecule present, the pigment becomes red by a different mechanism. MSH is the other molecule (in the simplified model) that is recepted by MC1R, and if this molecule is present it is dominant in reception to ASIP, and the transcription for black/brown color does occur. If MC1R is mutated (rr) then there is no transcription of the pigment genes and the hair will lack color and appear yellow or white. However, MC1R is only vital for HAIR pigmentation, therefore other parts of the dog will be colored black if the B gene is present even if the R gene is not present, giving the possibility for a yellow dog with black nose. A brown dog cannot have a black nose because the hair color determined that the black gene was already missing in the dog, and the receptor is wildtype.
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