Beyond Mendel: Exceptions/Additions to Mendel

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Beyond Mendel Exceptions/Additions to Mendels
Laws

• AP Biology

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Exceptions to Mendelian Genetics

• Mendel chose traits in peas that showed 2
  distinct forms.
• Not all genes exhibit such simple inheritance.
• Alleles interact
• Genes interact
• Segregation of genes on same chromosome
• Mitochondrial DNA

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Multiple Alleles

• A population can have more than 2 alleles for a
  gene.
• Ex. Labrador Retriever coat color
• Determined by 1 gene with 4 alleles.
• Even if more than 2 alleles exist in a
  population, any given individual can only have 2
  of them
• (1 from mother, 1 from father)

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Multiple Alleles

• Black is dominant to chocolate
• B black
• b chocolate
• Yellow is recessive epistatic (when present, it
  blocks the expression of the black and chocolate
  alleles)
• Yellow E or e
• must be ee to produce a yellow lab

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BBEE BbEE BBEe BbEe bbEEbbEe BBeeBbeebbee
Possible Genotypes
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Problem 1

• How do the multiple alleles act together to
  determine the coat color of a mouse?
• There are 3 varieties of coat color black,
  brown, and white
• They are controlled by 4 different alleles
  (similar to the Labradors)
• BbCc x BbCc
• Bblack, bb brown
• cc is epistatic (white)

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Problem 2

• Determine the number of chocolate labs produced
  from a black female and a yellow male
• BbEe x bbee

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Codominance

• Both alleles can be expressed
• For example, red cows crossed with white will
  generate roan cows.
• Roan cows have red coats with white blotches.

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Incomplete Dominance

• In some cases, an intermediate phenotype is
  shown.
• Neither allele is dominant.
• Ex. Snapdragons flower color
• 3 phenotypes red, white, pink
• Heterozygous condition results in pink flowers
  (the intermediate trait).

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Incomplete Dominance - Snapdragons

• A white (rr) snapdragon crossed with a red (RR)
  snapdragon produces all pink (Rr) offspring.
• Two pinks crossed together (Rr x Rr) produce 1/4
  white, 2/4 pink, 1/4 red

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Sex-Linked Genes

• Genes that are located on the X chromosome.
• Females receive 2 alleles males receive one.
• Ex. Color blindness, hemophilia
• Women can be carriers when they carry one gene
  for the disorder and one normal gene.
• Carriers can have sons with the disorder.

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Normal Male and Female Carrier
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Problem 3

• Determine the probability of a woman with
  hemophilia having children with hemophilia
  assuming she marries a normal man.

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Pleiotropy

• Some single alleles have more than one
  distinguishable phenotypic effect.
• This is called pleiotropy.
• Ex. Coloration pattern and crossed eyes of
  Siamese cats
• Both caused by the same allele.
• Unrelated characters caused by the same protein
  produced by the same allele.

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Siamese Cat

• Siamese cats have a gene that codes for darker
  pigments - this gene is more active at low
  temperatures.
• Parts of the body that are colder will develop
  the darker pigmentation - ears, feet tail of the
  siamese cats

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Pleiotropy

• Another example is the gene that causes pigment
  color in rats.
• White rats also have very sensitive eyes and
  often become blind.

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Pleiotropy

• Another example is Marfan Syndrome.
• Marfan Syndrome is a disease of the connective
  tissue.
• Symptoms tall thin, long extremities,
  deficiencies in eyes and skeletal system,
  enlarged heart

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Polygenic Traits

• Individual heritable characters are often
  controlled by groups of several genes.
• These genes are called polygenes.
• Each allele intensifies or diminishes the
  phenotype.
• Variation is continuous or quantitative (adding
  up)
• Also called quantitative inheritance

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Polygenic Traits

• Examples
• Human hair, eye, and skin color
• Height
• Weight
• Intelligence

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Lethal Genes

• Some genes are lethal when both alleles are
  present.
• Lethality (death) can occur before or after
  birth.

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Lethal Genes

• Ex. The creeper allele in chickens, which
  causes the legs to be short and stunted.
• Creeper is a dominant gene.
• Heterozygous chickens display the creeper
  phenotype.
• If 2 creeper chickens are crossed, one would
  expect to have ¾ creeper and ¼ normal
• Instead, the ratio is 2/3 creeper and 1/3 normal.

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Lethal Genes Creeper Chickens
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Lethal Genes

• Mexican hairless dogs result from a mutation in a
  gene that shows lethality.
• hh hairy normal trait
• Hh hairless one mutation present
• HH lethal two mutations lethal

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Manx Cats

• Cats possess a gene for producing a tail.
• The tailless Manx phenotype in cats is produced
  by an allele that is lethal in its homozygous
  state.
• The allele interferes with normal spinal
  development, in heterozygous cats this results in
  lack of a tail.

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Blood Types Multiple Alleles and Codominance

• 4 blood types (humans)
• A, B, AB, and O
• Blood type is controlled by 3 alleles
• A, B, O
• O is recessive (must have two O alleles to have
  blood type O)
• A and B are codominant (if inherit an A and B,
  blood type is AB)
• Crosses involving blood type often use an I to
  represent the alleles

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Blood Types

• The blood type determines what antibodies are
  located within the blood.
• Type A blood has type B antibodies
• If Type B blood is put into their body, their
  immune system reacts and antibodies clump the
  blood can cause death
• Type AB blood has no antibodies, any blood can be
  donated to them they are universal acceptors
• Type O blood has no antigens, antibodies in the
  blood do not react to type O blood, they are
  universal donors

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Blood Type Cross