Mendelian Genetics

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Chapter 14

• Mendelian Genetics

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Mendel and Heredity

• How does a brown haired mom and brown haired dad
  make blonde haired kids?
• How does a purple flower pea plant and purple
  flower pea plant make white flowered offspring?
• These questions were asked by Gregor Mendel in
  1857 and answered by math and scientific method

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Mendels Experiments

• Mendel studied heritable characters of pea plants
• Character flower color
• Traits white flowers and purple flowers
• He also only focused on either or characters
• The plants are either purple or white (not a
  mixture of purple and white)
• He also only used true-breeding plants
• Purple and purple only gave rise to purple

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Figure 14.1
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Mendels Ideas

• Mendels examination of the parents, 1st
  generation, and 2nd generation plants (P, F1 and
  F2) led him to two laws very important in
  understanding heredity
• Mendel did away with the old thinking of blending
  of traits
• Law of Segregation
• Law of Independent Assortment

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Law of Segregation

• Important to understand these terms
• Allele trait for a particular gene (purple is
  and allele for flower color gene)
• Locus space on a chromosome where a gene is
  located
• Character flower color
• Law of segregation implies that two alleles for a
  character are packaged into separate gametes
• Why do we have two alleles for a character?

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Mendels Experiment

• Mendel took a true breeding purple flower and
  mated it with a true breeding white flower
• Result All Purple Flowers
• He then took these flowers and allowed them to
  self pollinate
• Result 31 ratio of Purple to white flowers
• Where did the white flower allele go in the first
  generation?
• Why did it appear again in the 2nd generation?

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Figure 14.2
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Mendels Hypothesis

• 1. Alternative versions of genes (different
  alleles) account for variations in inherited
  characters
• 2. For each character, an organism inherits two
  alleles, one from each parent.

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Mendels Hypothesis

• 3. If the two alleles differ, then one, the
  dominant allele, is fully expressed in the
  organism, the other recessive allele is masked.
• 4. The two alleles for each character segregate
  during gamete production

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Useful Genetic Vocabulary

• Homozygous individual with identical alleles
  for a certain gene
• PP or pp
• Heterozygous individual with different alleles
  for a certain gene
• Pp
• Phenotype organisms outward appearance dictated
  by its genes
• Purple or white flower
• Genotype organisms genetic makeup for a certain
  gene
• PP, Pp, pp

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Figure 14.5
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Testcross

• Suppose we have a pea plant with purple flower,
  what is its genotype?
• It could be PP or Pp
• Testcross will tell us its genotype
• We cross it with a homozygous recessive plant
  (white flowers, pp)
• If the purple plant is PP then all offspring will
  be purple
• If the plant is Pp then we will see a 11 ratio
  of purple to white

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Figure 14.6
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Law of Independent Assortment

• Each pair of alleles segregates in gametes
  independently of other alleles
• Remember the metaphase I stage of meiosis
• Mendel then studied two characters at once
• Seed color (yellow Y and green y) and seed
  shape (round R and wrinkled r)
• Mendels question was will Yellow always be with
  round
• Or do the alleles for yellow seed color assort
  with round seed shape (do they come as a package)

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Figure 14.7
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Extending Mendels Experiments

• Relationship between genotype and phenotype are
  rarely simple
• Mendel got lucky studying flower color and seed
  shape and color
• These traits all are governed by one gene and
  each only has two alleles
• Most traits are governed by more than one gene
  and usually have more than two alleles that do
  not exhibit dominant recessive relationships

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Incomplete Dominance

• F1 hybrids have an appearance somewhere between
  the two parental alleles
• Red X White flowers make pink flowers

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What is a Dominant allele?

• Complete Dominance phenotypes of the
  heterozygote are indistinguishable from the
  homozygote
• That is the dominant allele masks the affects of
  the recessive allele
• Codominance two alleles affect the phenotype in
  separate distinguishable ways
• Example M, N, and MN blood groups in humans
• Note that MN blood groups does mean that the
  individual is an intermediate between M and N
  like in incomplete dominance
• Dominant alleles do not somehow subdue recessive
  alleles, both are expressed but dominant alleles
  mask the affects of the recessive allele

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Multiple Alleles
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Pleiotropy

• So far we have examined how single genes affect a
  single phenotype
• Purple flower allele (P) causes purple flowers
• However most genes have multiple affects on
  phenotype
• Single gene causes more than one phenotypic change

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Epistasis

• A gene at one locus alters the phenotypic
  expression of a gene at a second locus
• For example, fur color in mice Black fur (B) is
  dominant to brown fur (b)
• So BB-black, Bb-black, bb-brown
• However a second gene controls whether or not the
  protein gets deposited into the fur of the mouse
• We will use C for color
• CC-protein gets deposited Cc-deposited cc-not
  deposited

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Polygenic Inheritance

• Some traits are not either or characters (like
  mendels flower color pea plants)
• Some characters vary along a continuum among a
  population
• Like skin color in humans
• Controlled by at least three different genes

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Mendelian Inheritance in Humans

• Pedigree study of traits passed along
  generations in humans

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Human Disorders

• Recessively Inherited Disorders
• Individual must be homozygous recessive to show
  the disease
• Recessive allele codes for malfunctioning protein
  or no protein
• Problem with inbreeding
• Examples Cystic fibrosis
• Tay-Sachs disease
• Sickle-cell anemia

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Dominantly Inherited Disorders

• Lethal if homozygous dominant, phenotypic causes
  if heterozygous, and not shown if homozygous
  recessive