Mendel and the Gene Idea

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Mendel and the Gene Idea

• Chapter 14

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Father of Modern Genetics

• After failing to qualify as a biology teacher,
  the Austrian monk Gregor Johann Mendel
  (1822-1884) began to research heredity in pea
  plants (1860s).
• He was the first to take a scientific,
  experimental approach and to quantify his data.
• Particulate theory of heredity -- parents
  transmit separate inheritable factors (now called
  genes) to their offspring replaced the blending
  hypothesis.
• Why peas?
• 1. There were many varieties.
• 2. Peas do not cross-fertilize, so he could have
  strict control over mating.
• 3. Easy to grow many generations in a short
  amount of time.

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Mendels Work

• Mendel chose to study seven characters, each of
  which occurred in two contrasting traits
• 1. Flower color purple(d) or white(r)
• 2. Flower position axial(d) or terminal(r)
• 3. Seed color yellow(d) or green(r)
• 4. Seed shape round(d) or wrinkled(r)
• 5. Pod shape inflated(d) or constricted(r)
• 6. Pod color green(d) or yellow(r)
• 7. Stem length tall(d) or dwarf(r)

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Some vocab

• True breeding -- Always producing offspring with
  the same traits as the parents (self-pollination).
  
• P (parental) generation -- true-breeding parent
  plants.
• F1 (first filial) generation -- hybrid
  (cross-pollinated) offspring of the P generation.
• F2 (second filial) generation offspring of
  self-pollinated F1 generation.
• Genotype combination of genes in an organism.
• Phenotype expressed traits in an organism.
• Allele different forms of an gene for the same
  character.

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Give peas a chance

• When Mendel crossed true-breeding plants with
  different characters, the traits did not blend.
• P generation purple (PP) x
  white (pp)
• homozygous dominant
  homozygous recessive
• F1 generation all purple (Pp)
  x (Pp)
• 
  heterozygous
• F2 generation purple(PP) purple(Pp)
  purple(Pp) white(pp)
• Mendel found similar 31 ratios with the other 6
  characters.

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Mendels Principles

• 1. Organisms inherit two alleles for a trait, one
  from each parent.
• 2. (Law of dominance) If the two alleles differ,
  one is expressed (dominant) and the other is
  masked (recessive).
• 3. (Law of segregation) Two alleles for each
  character separate during gamete production
  homologous chromosomes separate during meiosis.
• If different alleles are present in the
  parent, there is a 50 chance that a gamete will
  receive the dominant allele, and a 50 chance
  that it will receive the recessive allele.

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Punnett Square

• A chart used to predict probabilities of possible
  genetic outcomes.
• Rule of multiplication -- probability that
  independent events will occur simultaneously is
  the product of their individual probabilities.
• Monohybrid cross a mating with reference to one
  character.

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Mendels Principles (cont.)

• 4. (Law of independent assortment) -- Each
  allele pair segregates independently of other
  gene pairs during gamete formation one gene does
  not influence the inheritance of a different
  gene.
• Dihybrid cross a mating with reference to two
  characters.

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Test Cross

• Crossing a parent with unknown genotype with a
  homozygous recessive parent.

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Other patterns of inheritance (exceptions to
Mendels rules)

• 1. Incomplete Dominance -- one allele is not
  completely dominant over the other
  heterozygotes phenotype is intermediate.
• Snapdragon red x white pink
• SrSr SwSw SrSw
• Incomplete dominance is not support for the
  blending theory of inheritance, because alleles
  maintain their traits.

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Other patterns of inheritance (cont.)

• 2. Codominance -- both alleles in the
  heterozygote are fully expressed.
• Human ABO blood groups
• Three alleles possible (multiple alleles), but
  you only inherit two (one from each parent).
• IA IB i

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phenotype/genotype rbc antigen/

serum antibody

• Type A IA IA or IAi
• Type B IB IB or IBi
• Type O ii
• Type AB IA IB
• rh RR or Rr
• rh - rr

• A anti- B
• B anti- A
• None anti- A B
• A B none
• rh none
• None anti

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Other Patterns of Inheritance (cont.)

• 3. Pleiotropy -- The ability of a single gene to
  have multiple effects.
• In some cats, a fur pigmentation gene also
  influences connections between cat's eyes and
  brain.
• 4. Epistasis one gene alters the expression of
  a second gene.
• The phenotypic ratio resulting from a dihybrid
  cross will deviate from the 9331 Mendelian
  ratio.
• In mice, the pigment production gene(C) is
  epistatic to the pigment color gene(B).
• BB or Bb black pigment bb brown pigment.
• CC or Cc normal pigment cc no pigment
• Dihybrid cross will result in 9 black mice (CCBB,
  CCBb, CcBB, CcBb), 3 brown mice (CCbb, Ccbb) and
  4 white mice (ccBB, ccBb, ccbb).

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Other Patterns of Inheritance (cont.)

• 5. Polygenic Inheritance more than one gene
  determines a single character.
• Produces quantitative characters that vary on a
  continuum.
• Skin pigmentation is controlled by at least three
  separately inherited genes (A, B, and C).
• AABBCC very dark person aabbcc very light
  person.
• AaBbCc intermediate shade.
• Environmental factors, such as sun exposure,
  could also affect the phenotype (nature vs.
  nuture).

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Pedigrees

• Our understanding of Mendelian inheritance in
  humans is based on the analysis of family
  pedigrees (a family tree that shows the
  inheritance pattern of a particular character
  among parents and children).
• Squares symbolize males and circles represent
  females.
• A horizontal line connecting a male and female
  indicates a mating offspring are listed below in
  birth order, from left to right.
• Shaded symbols indicate individuals showing the
  trait being traced.
• Pedigrees can be used to predict probabilities.

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Dominant Disorder
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Autosomal Recessive Disorders

• Caused by defective recessive alleles (not on sex
  chromosomes) that code for either a malfunctional
  protein or no protein at all.
• Can be non-lethal (albinism) or lethal (cystic
  fibrosis).
• Disorders occur only in homozygotes (aa) who
  inherit one recessive allele from each parent.
• Heterozygotes (Aa) are normal, but carriers (can
  transmit the allele to offspring).
• Most people with recessive disorders are born to
  normal parents, both of whom are carriers.
• Probability is 1/4 that a mating of two carriers
  (Aa x Aa) will produce a homozygous recessive
  zygote.
• 2/3 chance that a normal child will be a carrier.
• Some of these disorders are more common in
  certain ethnic groups cystic fibrosis
  (caucasians),Tay-Sachs disease (central European
  Jews), and sickle-cell disease (African descent).

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Autosomal Dominant Disorders

• Caused by defective dominant alleles only takes
  one copy to cause disorder.
• Lethal dominant alleles are rarer in populations
  than lethal recessives.
• In achondroplasia (dwarfism), AA is lethal in the
  fetus Aa affects 1 in 10,000 people.
• Huntington's disease (degenerative nerve disease)
  is caused by a late-acting lethal dominant
  allele effects do not appear until 35 to 40
  years of age, often after individuals have
  reproduced.
• Children of an afflicted parent have a 50 chance
  of inheriting the dominant allele a test can
  detect the Huntington's allele before disease
  symptoms appear.

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Multifactorial Disorders

• Diseases that have both genetic and environmental
  influences (heart disease, diabetes, cancer,
  alcoholism, and some forms of mental illness).
• Traits are often polygenic and poorly understood.

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Genetic Testing and Counseling

• Carrier testing identify genes in prospective
  parents and in embryos.
• Fetal testing blood tests, ultrasound, CVS
  (8-10 weeks), amniocentesis (14-16 weeks) and
  karyotyping.
• Newborn screening most states do routine blood
  test for phenylketonuria (PKU).

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