The Living World Chapter 8

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Mendel and the Gene Idea Chapter 14
Associate Professor Pamela L. Pannozzo Principle
of Biology I BSC 1010
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Gregor Mendel

• Mendel studied heredity, or the tendency for
  traits to be passed from parent to offspring

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Mendels Experimental System

• Mendel chose the garden pea plant for his
  research on heredity

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Terms

• True-breeding organism that will produce only
  one variety of a trait when self-fertilized over
  many generations
• Self-fertilization fertilization by male and
  female gametes of the same organism
• P (parental) generation two true-breeding
  organisms
• F1 generation Offspring of cross-fertilization
  of P generation
• F2 generation Offspring of self-fertilization
  of F1 generation

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Mendels Experimental Design
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Mendels Results

• All plants of the F1 generation had purple
  flowers (where did the white flowers go? Hmmmm)
• He called purple the dominant trait
• He called white the recessive trait
• F2 generation plants had 3 purple flowers1 white
  flower (How did the white flowers come back?
  Hmmmm)

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Mendel repeated his experiment with several
other traits and got the same results!
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Mendels Hypothesis

• Genes exist in alternative versions, (alleles),
  which account for variations in inherited traits
• Organisms inherit two alleles, one from each
  parent
• Dominant alleles mask recessive alleles, although
  recessive alleles are not lost
• The two alleles separate during gamete formation
  and end up in different gametes

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Terms

• Homozygous two of the same alleles
• Heterozygous two different alleles
• Phenotype the appearance of traits of an
  organism
• Genotype the genetic makeup of an organism a
  combination of alleles

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Analyzing Mendels Results

• Each trait is determined by the inheritance of
  two alleles one maternal and one paternal
• These alleles, present on chromosomes, are
  distributed to gametes during meiosis

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Analyzing Mendels Results

• Consider Mendels cross of purple-flowered with
  white-flowered pea plants
• P (dominant) allele ? Purple flowers
• p (recessive) allele ? White flowers
• Using these conventions, the above cross can be
  symbolized as
• PP X pp

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Punnett Squares

• A Punnett square is a grid structure that enables
  the calculation of the results of simple genetic
  crosses

• Possible gametes are listed along two opposite
  sides

• Genotypes of potential offspring are represented
  by the cells in the square

Pp
PP

• The frequency of these genotypes in the offspring
  is expressed by a probability

pp
Pp
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Fig. 8.8 How Mendel analyzed flower color
Probability is 25
Probability is 100
50
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Testcross PP or Pp?
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Dihybrid Cross

• Does inheritance of one gene affect inheritance
  of another gene?
• Mendel began with true-breeding plants with two
  different traits (pea color and pea shape)
• Y symbol for pea color
• Y yellow pea (dominant)
• y green pea (recessive)
• R symbol for pea shape
• R round pea (dominant)
• r wrinkled pea (recessive)

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• P generation YYRR x yyrr
• F1 generation YyRr
• (all yellow and round)
• (YyRr x YyRr)
• F2 generation ?

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Mendels Laws

• Law of Segregation
• The two alleles of a gene separate when forming
  gametes, and gametes combine randomly in forming
  offspring
• Law of Independent Assortment
• Alleles of genes located on different chromosomes
  are inherited independently of one another

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Today, We Know That

• Genes specify the amino acid sequence of proteins
• Mutations in a gene result in different alleles
• This ultimately leads to a change in the amino
  acid sequence and, hence, activity of the protein
• Natural selection may favor one allele over
  another

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Why Some Traits Dont Show Mendelian Inheritance

• Mendelian segregation of alleles can be disguised
  by a variety of factors
• Polygenic traits
• Pleiotropic effects
• Incomplete dominance
• Environmental effects
• Epistasis
• Codominance
• Multiple Alleles

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Polygenic Traits

• Most traits are polygenic
• They result from the action of more than one gene
• The result is a gradation in phenotypes or
  continuous variation

Extremes are much rarer than the intermediate
values
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Pleiotropic Effects

• Alleles that have more than one phenotypic effect
  are said to be pleiotropic

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Incomplete Dominance

• Not all alternative alleles are fully dominant or
  fully recessive in heterozygotes
• Some pairs of alleles exhibit incomplete
  dominance
• They produce a heterozygote phenotype that is
  intermediate between that of the homozygotes

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Environmental Effects
Color resembles snowy background in winter

• The expression of some genes is influenced by
  environmental factors, such as temperature

Color resembles tundra background in summer

• Some alleles are heat-sensitive
• Arctic foxes make fur pigment only when the
  weather is warm

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Epistasis

• Interaction between two genes where one of them
  modifies the phenotypic expression of the other
• In 1918, the geneticist R. A. Emerson crossed two
  true-breeding corn varieties with white kernels
• To his surprise, all F1 plants had purple kernels
• The plants of the F2 generation showed a ratio of
  9 purple 7 white
• Mendelian genetics predicts a 9331 ratio
• So why is Emersons ratio modified?

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• There are two genes that contribute to kernel
  color
• B ? Production of pigment
• A ? Deposition of pigment
• Either gene can block the others expression
• To produce pigment a plant must possess at least
  one functional copy of each gene

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Codominance
Unlike incomplete dominance, both alleles are
expressed

• Some pairs of alleles exhibit codominance
• They produce a heterozygote phenotype that is a
  combination of that of the two homozygotes
• Example
• Roan color in horses

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Fig. 8.19 Multiple alleles controlling the ABO
blood groups
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Multiple Alleles

• In most cases more than two alleles for a given
  gene exist in a population

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Pedigree Analysis
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Human Traits Following Mendelian Inheritance

• Recessively Inherited Traits
• Cystic Fibrosis
• Sickle-Cell Anemia
• Dominantly Inherited Traits
• Achondroplasia
• Huntingtons Disease

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Sickle-Cell Anemia Recessive Trait

• Sickle-cell anemia is an autosomal recessive
  trait in which the protein hemoglobin is
  defective
• Affected individuals cannot properly transport
  oxygen to their tissues

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Sickle-Cell Anemia Recessive Trait
Smooth shape allows for easy passage through
capillaries
Irregular shape causes blockage of capillaries
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Tay-Sachs Disease Recessive Trait

• Tay-Sachs disease is an autosomal recessive trait
  in which the enzyme hexosaminidase A is defective
• Affected individuals cannot break down specific
  lipids
• These lipids accumulate in brain cells
• Children die by five years of age
• The disease is very rare in human populations
• However, it has high incidence in Ashkenazi Jews

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Tay-Sachs Disease Recessive Trait
Enough enzyme to prevent CNS deterioration
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Huntingtons Disease Dominant Trait

• Huntingtons disease is an autosomal dominant
  trait that causes progressive deterioration of
  brain cells

• It is a fatal disease
• However, it persists in human populations because
  it has a late onset