Introduction to Genetics

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Introduction to Genetics

• AnS 224
• Spring 2009

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Goals

• Review of genetic terminology
• Genotypes and phenotypes for coat colors in
  various breeds of dogs
• Epistasis
• Primary known genes controlling coat color and
  color patterns

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Terminology

• Chromosomes
• Haploid
• Diploid
• Meiosis
• Genes

• Locus
• Genotype
• Phenotype
• Traits
• Epistasis

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Chromosomes

• DNA and associated proteins
• Acrosomic
• 78 chromosomes in domestic dog
• Autosomal
• 38 pairs
• Sex chromosomes
• 1 pair (XX or XY)
• Gametes

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Haploid vs. Diploid

• Somatic cells are diploid
• Two sets of chromosomes
• One maternal, one paternal
• Mature germ cells are haploid
• Meiosis
• Meiosis genetic recombination of parental
  chromosomes
• Crossovers

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Meiosis

• Two divisions
• First division like mitosis
• Second division yields haploid cells - Gametes

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Crossing Over
Process which occurs during the early stages of
cell division in meiosis. Homologous chromosomes
may exchange segments of DNA. This produces
genetic variations in germ cells.
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Genes and Loci

• Essentially the same thing
• Locus (plural loci)
• An older term pre-dating the DNA and gene
  sequence age
• Example
• E locus (extension)
• MC1r gene (melanocortin 1 receptor)
• Genotype versus phenotype

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Genotype versus Phenotype

• Genotype the combination of alleles
• Two copies per loci
• Three possibilities for a bi-allelic gene
• EE (homozygous dominant)
• MC1R /
• C916, C916
• Ee (heterozygous)
• MCIR /-
• C916, C916T
• ee (homozygous recessive)
• MC1R -/-
• C916T, C916T
• Phenotype the physical expression of the
  genotype in combination with environmental factors

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EE Ee ee
(Example of Complete Dominance)
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ede20.jpg
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Traits and Inheritance

• Traits
• Qualitative versus quantitative
• Sex limited versus sex linked
• Simple gene action and inheritance
• Dominant/recessive A/a
• Three genotypes, two phenotypes
• Incomplete dominance
• Three genotypes, three phenotypes

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Traits and Inheritance (Cont)

• Complex gene action and inheritance
• Polygenic versus monogenic/digenic
• Polygenic more than one/two genes involved
• Interactions between genes
• Additive, epistasis, modifier

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Modifier Genes in Golden Retrievers
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Example of Additivity

• Four alleles for spotting may act additively to
  produce gradations of spotting.

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Epistasis

• Suppression of the effect of a gene by a
  non-allelic gene
• A gene located at one locus hides the action of
  another gene at a different locus
• Can occur on the same chromosome or differing
  chromosomes
• Sometimes simply referring to an interaction
• Example
• Dominant black at the K locus is dominant over
  certain alleles at other loci

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Coat Color Genetics

• AnS 224
• Spring 2009

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Coat Color Genetics

• Basis of Color
• Coat color loci
• Inheritance
• Functionality
• Genotypes
• Associated phenotypes

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Basis of Color

• Melanin produced by melanocytes

• Eumelanin
• Dark black or brown
• Phaeomelanin
• Light cream, tan, or red

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Coat Color Genetics in Dogs

• Primary, recognized genes controlling coat color

A Agouti E Extension K Black B Brown M
Merle D Dilution (Blue) G Graying R
Roaning T Ticking S White spotting
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Agouti Color

• Basic color of many wild animals
• Camouflage
• Patterns on individual hairs and over the body
• Stripes or banding on individual hairs
• Dark often near hair tips
• Pattern over the body

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Agouti Agouti Color
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Agouti Locus

• Function of agouti
• Inhibits signaling through MC1r
• Receptor antagonist
• Shuts off eumelanin production
• Default phaeomelanin production
• Expression specific to
• Hair growth cycle
• Body position

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Eumelanin versus PheomelaninInactivation of
MC1r (ee)versusAgouti signaling peptide
antagonism of MC1r (atat)
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Agouti Locus in Dogs
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Agouti Locus in Dogs
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K (Dominant) Black Locus in Dogs

• Dominant allele epistatic to alleles at agouti
• Alleles for dominant black locus
• Description Symbol
• Dominant black K
• Brindle kbr
• Normal k

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K Locus in Dogs
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Brindle Coat Color in Dogs

• Several theories were proposed
• E locus as Ebr
• Tabby (Ta) as in cats
• K, kbr, k
• Experimental evidence in greyhounds corroborates
  the existence of a separate locus (K)
• b-defensin Previously unknown to be involved
  in pigment

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Brindle Coat Color in Dogs
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Extension Locus in Dogs

• Epistatic to A and K loci
• Historically great disagreement over alleles for
  E locus
• Early work lists three or four
• Early work does not agree with results in other
  mammals
• Recent molecular work seems to indicates EM, E,
  and e
• EMEM, or EM_
• Black Mask, Exception epistasis to K and A
• EE, or E_
• Normal MC1r function, normal A locus expression
• Exception epistasis to K
• ee
• A, a, asa, at result in red/yellow

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Extension Locus in Dogs
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Brown Locus in Dogs

• Brown locus in dogs - B, b
• Tyrosinase related protein 1 (TRP1)
• Eumelanin pigment granules in bb dogs are smaller
  and more round
• Influences the perceived color
• Little to no effect on phaeomelanin
• B_ dogs are black
• bb dogs are liver or chocolate colored
• Have brown nose leather

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Brown Locus in Dogs
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Dilution Locus in Dogs

• Influences dispersion of pigment granules
• Eumelanin and phaeomelanin
• Now known to be melanophilin

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Dilution Locus in Dogs
WeimaranerbbddKK
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Dilution Locus in Dogs
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Graying Locus in Dogs

• Not well understood
• Animals are born with normal color and lighten
  with age - prematurely
• Believed to be present in
• Poodles
• Old English Sheepdogs
• Terriers
• Incomplete dominance - additivity?
• GG - extreme lightening
• Gg - slight lightening

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Graying Locus in Dogs
Kerry Blue TerrierK_G_
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Merle Locus in Dogs

• Incomplete dominance
• Causes patchy dilutions in coat and eye colors
• Mutation in silver gene (SILV)
• Additive gene
• Normal dogs - mm
• Homozygous MM dogs often have abnormalities
• Deafness
• Blindness
• Small eyes
• Strongly recommended to avoid mating two merles

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Merle Locus in Dogs
Cardigan Welsh Corgiasa asa Mm
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Harlequin Locus in Dogs

• Predominately white with black torn patches
• A dually heterozygous state
• Harlequin (Hh) and merle (Mm)
• Homozygous HH
• Not compatible with life
• Does not breed true
• Only observed in Great Danes

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Harlequin Locus in Dogs
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Spotting Locus in Dogs

• Percent white probably influenced greatly by
  modifier genes
• Molecular genetics still incompletely understood

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Spotting Locus in Dogs (Cont)

• S solid color, no white
• si Irish allele (from Irish rat), few
  definitely spotted areas
• sp piebald 15-20 of coat solid (pigmented)
• sw extreme piebald, virtually no spotting
• i.e. solid white

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Spotting Locus in Dogs

• S 0
• si 1-3
• sp 3-9
• sw 9-10

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Ticking Locus in Dogs

• White areas have flecks of color
• Most dogs are tt

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Ticking Locus in Dogs
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Roan Locus in Dogs
Australian Cattle DogR_

• Disagreement about whether R is a distinct locus
• May represent extreme ticking

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Take Home Points

• All dogs possess various genes which code for
  coat color, texture, etc.
• When breeds do not express a given pattern, the
  normal allele is probably fixed
• Several alleles are lethal or semi-lethal when
  homozygous
• MM, HH
• Breeders should avoid matings that can produce
  lethal homozygotes

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Take Home Points (Cont)

• Be aware of loci important for individual breeds
  which produce desired coat characteristics
  through mating.
• Off color dogs may indicate parentage is
  questionable other factors may also play a role
• Mutation rate, low frequency of recessive allele
• Molecular genetics can provide great insight

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From The Ultimate Dog Book