Extensions of Mendelian Genetic Principles

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Extensions of Mendelian Genetic Principles

• Chapter 4

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Exceptions and Extensions to Mendels Rules

• Not every gene conforms
• Some are exceptions
• Some are extensions
• Number of genes
• Gene interaction
• Modifications of dominance

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• Multiple Alleles

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ABO Blood Type

• Allele that predominates in wild-type
• Alternate is mutant
• Gene may have several alleles
• Multiple allelic series
• Single individual still only has two copies
• ABO blood groups
• Other blood groups
• 4 group Phenotypes O, A, B, AB

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ABO Blood Type

• Different combinations of IA, IB, i
• IA, IB dominant to i
• Cellular antigens attached to outside of blood
  cells

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ABO Blood Type

• Antibodies attach to foreign antigens
• Clump (agglutinate) any blood cells with anti-b
  antigens
• A have A-antigens and anti-B antibodies
• B have B-antigens and anti-A antibodies
• AB have both antigens and neither Anti-A or
  Anti-B antibodies
• O have neither antigen and both Anti-A and Anti-B
  antibodies

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ABO Blood Type

• AB universal recipient
• O universal donor
• Glucosyltransferases
• Enzymes add specific
• sugar to existing polysaccharide (H-antigen)
• ?-N-acetylgalactosamine
• Galactose
• h/h mutants

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i-Activity

• Was she Charlie Chaplins Child?

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Drosophila Eye Color

• Eye color
• White mutant (w)
• Wild type red (w)
• X-linked
• Found mutants that did not conform to experiments
  done before
• Eosin eyes (we) dominant to white, recessive to
  red
• Several different mutations of the white allele

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Multiple Alleles at a Molecular Level

• Change of one base pair changes the AA and
  changes the protein
• Can change at several places and lead to several
  different alleles

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• Modifications of Dominance Relationships

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Dominance

• Complete Dominance
• Complete Recessiveness
• Extremes of a range
• Many alleles share a different dominance
  relationship
• Incomplete
• Codominance

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Incomplete Dominance

• One allele is not completely dominant
• Partial dominance
• Heterozygote phenotype is intermediate
• F1 cross shows all intermediate color
• 121 ratio re-established in the F2

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Codominance

• Related to complete dominance
• Heterozygote exhibits phenotypes of both
  homozygotes
• NOT an intermediate
• ABO blood series

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Incomplete Dominance and Codominance at a
Molecular Level

• In codominance products of both alleles are
  expressed
• Does not always happen
• In Incomplete dominance only enough product for
  intermediate form is made
• In complete dominance the dominant is said to be
  haplosufficient

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• Gene Interactions and Modified Mendelian Ratios

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Modified Mendelian Ratios

• Phenotype is result of complex gene interactions
• Any deviation from the 9331 ratio is a result
  of interaction between two or more genes
• Interactions from genes for the same phenotypes
• One gene masks or modifies or masks the other
• Environmental interactions

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Gene Interactions that Produce New Phenotypes

• If two allelic pairs affect the same phenotype
• Interaction gives novel phenotypes
• Comb shape in chickens
• Rose dominant to single
• Pea dominant to single
• Rose x pea walnut
• Results from two dominant R and P

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AHHHHHHH!!!
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Molecular Basis of Funky Combs

• Not known
• Single comb phenotype has nothing to do with rose
  or pea comb
• Dominant R and P interacts with single comb gene
  product
• P and R together interact together as well to
  form an entirely new phenotype

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Gene Interactions that Produce New Phenotypes

• Fruit shape in summer squash
• Long fruit and sphere-shaped
• Long fruit is always true breeding
• Interaction between certain varieties of
  sphere-shaped results in disk shaped fruit

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Molecular Basis of Squidgy Squash

• Modification of 9331, so 2 genes involved
• A dominant allele of either results in the
  dominant phenotype
• A double dominant of each allele results in the
  new phenotype
• Doubly homozygous recessive results in LONG
  FRUIT!!???

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• Epistasis

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Epistasis

• Interaction between two or more genes to control
  a single phenotype
• Masks or modifies the phenotype expression
• No new phenotype is produced
• Masker is called epistatic
• Maskee is called hypostatic
• May be caused by double recessive allele or the
  presence of a dominant allele

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Recessive Epistasis

• A/_ b/b and a/a b/b have the same genotype
• Phenotypic ratio is 934
• Coat color in mice
• Wild mice have grey color (alternating black and
  yellow bands) called agouti pattern
• Albinos are totally white without any pigment
• Recessive to everything
• Black rodents do not have the agouti gene and
  have no yellow bands
• Recessive to agouti

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Recessive Epistasis

• 934 agoutiblack
• albino
• Albino shows recessive epistasis over agouti
• White hairs are produced regardless of other gene

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Molecular Basis of Ti-Dyed Mice

• Three alleles involved
• C- allows pigment formation
• cc prevents pigment formation regardless of color
• A- dominant for agouti
• B- codes for black coat color
• bb codes for brown
• All of the mice in figure 4.11 have B otherwise
  some brown mice would be present

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Molecular Basis of Yoohoo Labs

• Two genes involved
• B codes for formation of black pigment
• bb codes for formation of brown pigment
• E allows expression of the B gene
• ee does not allow expression of the B gene
• B/_E/_ produces a black lab
• b/b E/_ produces a chocolate lab
• _/_ e/e produces a yellow lab

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Molecular Basis of Yellow Labs

• Variation of yellow labs
• B/_ e/e genotype have dark noses and lips
• b/b e/e genotype have pale noses and lips

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Dominant Epistasis

• A/_ B/_ and A/_ b/b have the same phenotype
• 1231 phenotypic ratio rather than 9331
• Dominant gene is epistatic to other gene
• Three common colors white, yellow and green
• White x yellow white
• White x green white
• Yellow x green yellow

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Molecular Basis of Garish Squash

• Y is needed to convert white to yellow with a
  green intermediate
• W allele codes for a product that inhibits white
  to green transition
• Plants that have at least one W are white no
  matter the Y
• White is W/_ Y/_ and W/_ y/y
• Yellow is w/w Y/_
• green is w/w y/y

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Dominant Epistasis

• Two genes W/w and Y/y
• W/_ white no matter what
• w/w Y/_ is yellow
• w/w y/y is green
• Progeny of F1 are 12 white 3 yellow 1 green

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Dominant Epistasis

• Greying in horses
• GG or Gg progressive silvering of the coat
• gg do not go gray
• G allele affects the gene that grays the animal
• Lipizzaner
• Lipizzaner Clip

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Epistasis Involving Duplicate Genes

• Genes on different loci that produce identical
  phenotypes
• Can be dominant or recessive epistasis
• Example sweet pea flower color (recessive)
• Crosses between true-breeding whites produce all
  purple
• F1 produces 9/16 purple and 7/16white
• All white progeny breed true when selfed
• 1/9 of purple progeny breed true when selfed

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Epistasis Involving Duplicate Genes

• Pea plants, two genes
• Colored flowers only appear when at least on of
  each of the dominant alleles are present
• White only appears when homozygous recessive for
  either gene
• C/c flower will be colored
• P/p flower will be purple
• Called duplicate because either gene being
  recessive makes the phenotype

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• Essential Genes and Lethal Genes

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Essential Genes and Lethal Genes

• Lethal allele allele that results in the death
  of an organism
• Gene that is involved in the death is called the
  essential gene
• Dominant lethal allele
• Recessive lethal allele
• Recessive Lethal allele
• Yellow body color in mice
• Hetero-cross exhibited only hetero yellow mice

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Mystery of The Mustard Mice

• 21 ratio of hetero yellow and non-yellow
• Yellow (AY) is dominant over non-yellow (A)
• AY/ AY lethal
• AY/ A yellow
• A/ AY non-yellow

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Essential Genes and Lethal Genes

• Recessive lethal genes
• Tay-Sachs enzyme deficiency that prevents nerve
  function
• X-linked lethal genes
• Hemophilia
• Huntingtons Disease

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• Gene Expression and the Environment

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Penetrance and Expressivity

• Not all individuals with a genotype show expected
  phenotype
• Frequency with which it manifests is called
  penetrance
• Depends on genotype of environment
• Complete penetrance
• Incomplete penetrance
• Brachdactyly

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Expressivity

• Degree to which a penetrant gene or genotype is
  phenotypically expressed
• Depends on gene and environment
• Different degrees of function in the gene
• Osteogenesis imperfecta
• Blue sclera
• Brittle bones
• Deafness

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Incomplete Penetrance and Variable Expressivity

• Neurofibromatosis
• Creates fibrous growths on the body
• 50-80 penetrance
• Variable expressivity
• Light conditions express as café-au-lait spots
• Severe cases cause neurofibromas large head,
  short stature, curvature of the spine

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Effects of the Environment

• Age of Onset
• Age creates internal environment change
• Programmed turn on of certain genes
• Male pattern baldness
• Duchenne muscular distrophy

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Effects of the Environment

• Sex- expression may be influenced by sex
• Difference in phenotype because of different
  genes
• Sometimes only in one sex
• Sex-limited traits
• Horns in sheep
• Frequency changes from sex to sex
• Sex-influenced traits
• Pattern baldness

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Effects of the Environment

• Temperature
• Some enzymes are affected or catalyzed by
  temperature
• Function at one temperature but not by the other
• Himalayan rabbits
• Cold temperature causes dark fur to develop
• Siamese cats

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Effects of the Environment

• Chemicals
• Certain chemicals have effects on an organism
• Phenylketouria
• Do not possess metabolites for phenylalanine
• Diet determines severity of the disease
• Phenocopies
• Exposure to chemicals during pregnancy can change
  cause a phenotype similar to that of an other
  gene mutation
• Phocomelia rare recessive allele, mimicked by
  thalidomide

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Effects of the Environment

• Nature vs Nurture
• Human height is influenced by both nature and by
  better diets and improved health care
• Genes set certain limits for the phenotype
  dependant on environment
• Norm of reaction
• Behavioral traits involve nature and nurture
• Genes make people more susceptible
• Person must be exposed to drinking
• May be due to influence of personality traits