Extensions of Mendelian Genetics

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Extensions of Mendelian Genetics

• Mendelian genetics seems to be relevant to only a
  small set of heritable features
• For only a few characters there are
• Only 2 versions of an allele (green or yellow)
• 1 gene codes for a single external character
• 1 allele is completely dominant to the other
• The basic patterns of segregation independent
  assortment apply to more complex patterns of
  inheritance

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Different Types of Dominance

• Complete dominance
• Phenotype of heterozygote HomoD are
  indistinguishable
• The pattern with which you are already familiar
• Incomplete dominance
• Phenotype of heterozygote is in between the 2
  Homo phenotypes
• Example pink snapdragons

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Snapdragons Incomplete Dominance
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So incomplete dominance does NOT provide evidence
for blending theories
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Codominance

• Codominance
• Phenotype of heterozygote is separate
  distinguishable from Homozygous Dominant
  Homozygous Recessive
• Example AB blood type or Rhododendron flower

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Dominance Phenotype

• The observed dominance/recessiveness of alleles
  depends on the level of the investigation
• Consider Tay-Sachs disease
• Brain cells of the baby do not metabolize certain
  lipids
• As lipids accumulate, seizures, blindness, and
  mental degeneration
• Death occurs within a few years of conception

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Tay-Sachs Disease

• At the Organismal level, the disease is recessive
• Only children with 2 copies of the recessive
  trait will have the malady
• Heterozygote is not afflicted they produce some
  lipid-metabolizing enzyme, though not as much as
  in HomoD
• So intermediate enzyme production
• This suggests that _at_ the biochemical level, the
  disease is an example of incomplete dominance
• Which is Tay-Sachs dominance or incomplete
  dominance?

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Prevalence Dominance

• Polydactyly
• Extra fingers or toes
• 1 of 400 in the US
• The allele for polydactyly is dominant, but
  rarely present
• Recessive homozygotes (HomoR) are found 399 out
  of 400 instances

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Blood Typing

• Only 2 alleles existed for Mendels peas, but
  this is not typical for most traits
• Consider ABO blood group in humans
• A refers to the A membrane carbohydrate type
  A blood
• B refers to (seriously, Im not writing this
  down)
• O means neither A or B carbohydrate is found
• AB means both A B are found
• BUT the A and B alleles are codominant and are
  both expressed if an individual inherits both
  alleles

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Epistasis

• A gene at one locus alters the phenotypic
  expression of a gene at a second locus
• Example Mouse fur color
• Bb or BB Black bb brown
• If HomoR for (C) gene cc, then no fur color
  (albino or white)
• Regardless of fur color specified by brown-black
  gene
• If NOT HomoR for (c) gene Cc or CC, then can be
  brown (bb) or black (Bb or BB)

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What is the phenotype of

• BBcc?
• BbCc?
• bbCC?
• Bbcc?
• BBCC?
• bbcc?

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Pleiotrophy

• Single gene has multiple effects
• Should be unsurprising given intricate molecular
  and cellular interactions for development of an
  organism
• Phenylketonuria
• Mental Retardation
• Reduced skin and hair pigmentation

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Polygenic Inheritance

• The additive effect of 2 or more genes on 1
  phenotypic character
• Called quantitative characters since there is a
  continuum of gradations
• Normal curve of phenotypes
• Example human skin pigmentation is determined by
  at least 3 separately inherited genes
• AABBCC Dark
• AaBbCc Intermediate
• aabbcc Light

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Pedigree Analysis

• Family tree describing the interrelationships of
  parents children across the generations

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Recessively Inherited Disease

• Requires 2 copies of the recessive allele
  (Homozygous Recessive) to express the mality
• Heterozygotes are called carriers
• Normal phenotype, but may transmit disease to
  offspring
• Examples
• Cystic Fibrosis
• Tay Sachs disease
• Sickle-cell disease

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Cystic Fibrosis

• Recessive autosomal disease
• Common in those of European descent
• 1 of 2,500 affected, but 1 of 25 are carriers
• Affects Chloride ion transport between a cell and
  extracellular fluid
• If untreated, most die before 5th birthday
• Typically, patients live until their 20s or 30s
  with efficacious treatment

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Sickle-Cell Anemia

• Recessive autosomal disease
• African descent
• Affects Hemoglobin protein in RBCs
• Low blood oxygen hemoglobin molecules clump
  together forming sickle shaped RBCs
• Sickle-celled RBCs clump together creating
  chronic vascular occlusion of small vessels
• Example of incomplete dominance
• Heterozygotes are usually normal but will show
  some symptoms during prolonged periods of reduced
  blood oxygenation

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Dominant Alleles

• Most harmful alleles are recessive, but some
  human diseases are due to dominant alleles
• Only require one copy of the allele to be
  expressed
• Hypothesis if there is a lethal disease carried
  on a dominant allele, it would have burned out
  its carriers by now.
• UNLESS, the lethal disease carried by a dominant
  allele is one that affects organisms of advanced
  age
• Like Huntingtons disease

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Examples of Dominant Allele Disease

• Achondroplasia form of dwarfism
• Heterozygous individual dwarf
• 1 in 25,000 have achondroplasia, so 99 of the
  population are HomoR
• Huntingtons disease
• Caused by a lethal dominant allele
• Degenerative disease of nervous system
• Usually only affects those gt 40 yrs old

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Genetic Testing

• Pedigree analysis gives some info about risk to
  offspring
• There are also tests to identify carriers of
  certain genetic diseases
• Amniocentesis amniotic fluid is removed and
  then cells contained in the fluid are cultured to
  identify certain chromosomal defects via
  karyotype
• Chorionic villus sampling (CVS) placental
  tissue is removed for same purpose as amnio, but
  results are available far faster, can be
  performed earlier in pregnancy
• Karyotype can be immediately obtained

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