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General Biology II: Yellow fur color: a recessive trait

General Biology II
Yellow fur color: a recessive trait
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table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Reference Information
  6. The Process of Science
  7. 3. Biological Molecules
  8. 4. Structure of DNA
  9. 5. DNA Replication
  10. 6. Protein Synthesis
    1. 6.1 What are proteins and what do they do?
    2. 6.2 What is a gene?
    3. 6.3 How do genes direct the production of proteins?
    4. 6.4 Transcription: from DNA to mRNA
    5. 6.5 Eukaryotic RNA Processing
    6. 6.6 Translation
    7. 6.7 The Genetic Code
    8. Optional Section - Micropigs
  11. 7. Mutations
    1. How Gene Mutations Occur
    2. Intro to Genetic Disorders
    3. Do all gene affect health and development?
    4. Types of Mutations
    5. Changes in Numbers of Genes
    6. Changes in Chromosome Number
    7. Complex Multifactorial Disorders
    8. Genetic Predispositions
    9. Genetics and Statistics
  12. Gene Regulation
    1. 8.1 Prokaryotic versus Eukaryotic Gene Expression
    2. 8.2 What is the epigenome?
    3. 8.3 Alternative RNA splicing
  13. 9. Biotechnology
    1. 9.1 Manipulating Genetic Material
    2. 9.2 Cloning
    3. 9.3 Genetic Engineering
    4. 9.4 Biotechnology in Medicine and Agriculture
    5. 9.5 Genomics and Proteomics
    6. 9.6 Applying Genomics
    7. 9.7 Proteomics
  14. 10. Cell Division - Binary Fission and Mitosis
    1. 10.1 Prokaryotic Cell Division
    2. 10.2 Eukaryotic Cell Division
    3. 10.3 Control of the Cell Cycle
    4. 10.4 Cancer and the Cell Cycle
  15. 11. Meiosis
    1. 11.1 Sexual Reproduction
    2. 11.2 Overview of Meiosis
    3. 11.3 Interphase
    4. 11.4 Meiosis I
    5. 11.5 Meiosis II
    6. 11.6 Comparing Meiosis and Mitosis
    7. 11.7 Errors in Meiosis
  16. 12. Patterns of Inheritance
    1. 12.1 Mendelian Genetics
    2. 12.2 Garden Pea Characteristics Revealed the Basics of Heredity
    3. 12.3 Phenotypes and Genotypes
    4. 12.4 Monohybrid Cross and the Punnett Square
    5. 12.5 Laws of Inheritance
    6. 12.6 Extensions of the Laws of Inheritance
    7. 12.7 Multiple Alleles
    8. 12.8 Sex-Linked Traits
    9. 12.9 Linked Genes Violate the Law of Independent Assortment
    10. 12.10 Epistasis
  17. Genetics: Dog Coat Color
    1. Introduction to Genetics
    2. Pedigrees and Punnett Squares
    3. Black fur color: a dominant trait
    4. Yellow fur color: a recessive trait
    5. Epistasis: the relationship between black, brown, and yellow fur
    6. Brindle color: partial dominance and epistasis
    7. Incomplete dominance: when traits blend
    8. White spotting: When there's more than two alleles
    9. Hemophilia: a sex-linked disorder
    10. Overall phenotypes: putting it all together
    11. Additional complexity
    12. It's not all in the genes

Yellow fur color: a recessive trait

Yellow color in dogs

Labrador retrievers don’t only come in brown and black, they also come in yellow. Yellow color in labs is caused by variations in a different gene: MC1R. This gene controls the production of the melanocortin 1 receptor protein. MC1R is located on chromosome 5 in dogs (Schmutz, 2001).

11f.YellowLabradorLooking_new
Figure 14: This yellow lab is producing light-colored pheomelanin instead of dark-colored eumelanin. (Credit: Djmirko; from Wikimedia)

Melanocytes make two forms of melanin, eumelanin and pheomelanin. The relative amounts of these two pigments help determine the color of an individual’s hair and skin. Individuals who produce mostly eumelanin tend to have brown or black hair and dark skin that tans easily (in humans). Eumelanin also protects skin from damage caused by ultraviolet (UV) radiation in sunlight. Individuals who produce mostly pheomelanin tend to have red or blond hair, freckles, and light-colored skin that tans poorly. Because pheomelanin does not protect skin from UV radiation, people with more pheomelanin have an increased risk of skin damage caused by sun exposure.

The melanocortin 1 receptor controls which type of melanin is produced by melanocytes. When the receptor is activated, it triggers a series of chemical reactions inside melanocytes that stimulate these cells to make eumelanin. If the receptor is not activated or is blocked, melanocytes make pheomelanin instead of eumelanin. This means that if the receptor is working correctly and is turned on, dark pigment will be produced. If the receptor is not functional or is not turned on, light pigment will be produced.

11.3labradorcols
Figure 14: The three recognized colors of labs are due to black eumelanin, brown eumelanin, or pheomelanin. (Credit: Erikeltic, from Wikimedia)

Schmutz et. al. (2002) determined the DNA sequence for the MC1R gene from dogs of various colors. They determined that black and brown dogs all have one allele of MC1R, while yellow and red dogs have a different allele. The allele that leads to yellow or red color has a premature stop codon which results in a shorter-than-normal protein. This protein would be predicted to not function correctly. Remember that when the melanocortin 1 receptor is not functioning correctly, light pheomelanin pigment is produced and not dark eumelanin.

Dogs that are homozygous for the functioning allele of MC1R (which would cause eumelanin to be produced) are dark in color. Dogs that are homozygous for the non-functioning allele (which would cause pheomelanin to be produced) are light in color. Dogs that are heterozygous are dark in color. What does this tell you about which allele is dominant? If you said “the dark allele is dominant because it covers up the light allele”, you’re correct. We will use “E” to represent the genotype at MC1R because the dominant phenotype in this case is the production of eumelanin. Dogs that have the genotype EE or Ee will produce eumelanin and be dark. Dogs that have the genotype “ee” will produce pheomelanin and be light.

11d.pedigree3
Figure 15: In this pedigree, the shaded individual is yellow. She therefore has the genotype ee and produces pheomelanin. We can’t tell the genotype of her mate by looking (he could be Ee or EE), but since all of their puppies were dark in color, we would predict that his genotype was EE. In this cross: EE x ee, 100% of the puppies would have the genotype Ee, so 100% of the puppies would produce eumelanin instead of pheomelanin.

The cross shown in Figure 15 can also be shown as a Punnett square. Since we are unsure whether the male dog has the genotype “EE” or “Ee”, we have to make two Punnett squares. Since all of the puppies resulting from this cross were black, we would predict that the first Punnett square shows the cross. However, it is possible that the second Punnett square is correct. There are only 4 puppies, so it’s not hard to imagine that they could all be black even though the Punnett square predicts only 50% black. It would be comparable to flipping a coin 4 times and getting 4 heads in a row. Getting 4 heads in a row is less likely, but definitely possible.

11e.epunnett
Figure 16: Cross from Figure 15 shown as Punnett squares

It is very important to note here that yellow dogs still have the TYRP1 gene, even though they are not black or brown!

Human Connection

Common variations (polymorphisms) in the MC1R gene are associated with normal differences in skin and hair color. Certain genetic variations are most common in people with red hair, fair skin, freckles, and an increased sensitivity to sun exposure. These MC1R polymorphisms reduce the ability of the melanocortin 1 receptor to stimulate eumelanin production, causing melanocytes to make mostly pheomelanin. Although MC1R is a key gene in normal human pigmentation, researchers believe that the effects of other genes also contribute to a person’s hair and skin coloring.

The melanocortin 1 receptor is also active in cells other than melanocytes, including cells involved in the body’s immune and inflammatory responses. The receptor’s function in these cells is unknown.

13.Woman_redhead_natural_portrait_1

Photo credit: dusdin on flickr; from Wikipedia.

Resources

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:FtsD6vMd@3/Mendels-Experiments

OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:sbdXt0s3@4/Laws-of-Inheritance

OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:zLLYW2hj@5/Extensions-of-the-Laws-of-Inhe

Human Connection – information about MC1R: “MC1R” by Genetics Home Reference: Your Guide to Understanding Genetic Conditions, National Institutes of Health: U.S> National Library of Medicine is in the Public Domain

Schmutz SM, Moker JS, Berryere TG, Christison KM, Dolf G. 2001. An SNP is used to map MC1R to dog chromosome 5. Anim Genet. 32(1):43-4.

Schmutz SM, Berryere TG, Goldfinch AD. 2002. TYRP1 and MC1R genotypes and their effects on coat color in dogs. Mammalian Genome 13, 380-387.

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Epistasis: the relationship between black, brown, and yellow fur
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Copyright © 2016 by Lisa Bartee and Christine Anderson. Mt Hood Community College Biology 102 by Lisa Bartee and Christine Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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