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Chapter 23 : Mendelian Genetics at the Molecular Level: Dominance and Recessivity

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Mendelian Genetics at the Molecular Level: Dominance and Recessivity, Page 1 of 2

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Abstract:

This chapter contains a reading and thinking activity that connects the observed genetics of dominant and recessive traits to molecular biology, using coat color in Labrador retrievers as an example. The chapter focuses on the molecular biology of black/brown pigmentation in Labrador retrievers, both because it is simple and because most students will be familiar with the animals. The molecular biology of black or brown (called chocolate in the breed) pigmentation in Labrador retrievers is based on the presence or absence of a single enzyme. The pathway to black and brown pigments (the eumelanins) can be viewed as beginning with the amino acid tyrosine. In melanocytes (pigment-producing cells), tyrosine is first converted to the chemical dopaquinone, which is then converted by a second enzyme, tyrosinase-related protein 2 (TYRP2), to a brown form of eumelanin pigment. The genetic basis for brown coat color is a nonfunctional allele for TYRP1. Chocolate is recessive to black because a single functional copy of the gene for TYRP1 leads to the production of plenty of black pigment.

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23

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Figures

Image of Figure 23.1
Figure 23.1

Synthesis of black and brown pigments (eumelanins) in melanocytes. First, tyrosinase converts tyrosine to dopaquinone. Next, TYRP2 converts dopaquinone to brown pigment. Finally, TYRP1 converts the brown pigment into black pigment.

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23
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Image of Figure 23.2
Figure 23.2

Pigment synthesis in Cocoa's melanocytes. First, tyrosinase converts tyrosine to dopaquinone. Next, TYRP2 converts dopaquinone to brown pigment. There is no TYRP1 to convert the brown pigment into black pigment. What effect does this lack of TYRP1 have on Cocoa's coloration?

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23
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Image of Figure 23.3
Figure 23.3

Pigment biosynthesis in the geneflower. Enzyme X converts a colorless precursor into a blue pigment. Next, enzyme Y converts the blue pigment into a purple pigment.

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23
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Image of Figure 23.4
Figure 23.4

Pigment biosynthesis in the roundbud. Enzyme Q converts a white precursor into a red pigment.

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23
Permissions and Reprints Request Permissions
Download as Powerpoint

References

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1. Barsh, G. 1996. The genetics of pigmentation: from fancy genes to complex traits. Trends in Genetics 12:299305. This is a fairly complicated review of pigmentation genetics.
2. Ellgren, H. 2005. The dog has its day. Nature 438:745746. A news story/comment about the first publication of a complete dog genome sequence.
3. O'Brien, S.,, and W. Murphy. 2003. A dog's breakfast? Science 301:18541855. An overview of progress in dog genetics and genomics.
4. Pennisi, E. 2004. Genome resources to boost canines' role in gene hunts. Science 304:10931094. A discussion of how progress in understanding the dog genome and developing molecular tools, like microsatellite markers, have made purebred dogs an invaluable resource for gene hunters.

Tables

Generic image for table
Table 1

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23
Generic image for table
Table 2

Coat color and TYRP1

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23
Generic image for table
Table 1

Genetics of roundbuds

Citation: Kreuzer H, Massey A. 2008. Mendelian Genetics at the Molecular Level: Dominance and Recessivity, p 336-342. In Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition. ASM Press, Washington, DC. doi: 10.1128/9781555816100.ch23

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