Genetics

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Genetics

• 11/15/2007

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Essential Question

• How do chromosomes get passed from one generation
  to the next?

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Genetics

• Genetics? the scientific study of heredity

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Gregor MendelThe father of genetics

• grew up on a small farm in what is today the
  Czech Republic
• Lived in a monastery
• Studied math and science at the University of
  Vienna.
• bred garden peas to study inheritance
• He made a lucky choice of plants
• many varieties with distinct heritable features
  (characters) with different variants (traits)
• Mendel had strict control over which plants mated
• In nature, pea plants typically self-fertilize

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Must know terms

• Traitspecific characteristic varies from one
  individual to another example height
• Hybridoffspring of crosses between parents with
  different traits
• Dominantexpressed trait
• Recessivenon expressed trait present only in
  the absence of the dominant

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Must know terms

• Homozygousindividuals with two identical alleles
  for a particular trait true breeding
• Heterozygousindividuals with two different
  alleles for a particular traithybrid
• PhenotypePhysical characteristics
• GenotypeGenetic makeup

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Must know terms

• AlleleDifferent forms of a gene
• P generationParental generation
• F1 generationfirst generation after P
• F2 generationsecond generation after P
• Probabilityis the likelihood that a particular
  event will occur

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• Pea plants use parts of their flowers to
  reproduce.
• The male part of each flower (stamen) produces
  pollen.
• The female part (carpel) produces eggs.
• Each flower has both parts and can self-pollinate.

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Stamen
Carpel
Carpel
Stamen
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Probability

• The likelihood that a particular event will occur
  is called probability.
• The principles of probability can be used to
  predict the outcomes of genetic crosses.
• The chances of getting heads or tails

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• If you flip a coin three times in a row, what is
  the probability that it will land heads up?

½
x
x
½
½


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Mendels Experiments

• Mendel would cross-pollinate (hybridize) two
  contrasting, true-breeding pea varieties
• P generation ? true-breeding parents
• F1 generation ? hybrid offspring
• F2 generation ? F1 hybrids self-pollination
  offspring
• analysis of F2 plants lead to the 2 fundamental
  principles of heredity
• the law of segregation
• the law of independent assortment

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Genes and Dominance

• From pea plant set of experiments, Mendel drew
  two conclusions
• 1. Biological inheritance is determined by
• factors that are passed from one
• generation to the next (genes).
• 2. Principle of Dominance
• Some alleles are dominant and others are
  recessive.

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Dominant and Recessive Alleles

• A capital letter (T) represents a dominant
  allele.
• A lowercase letter (t) represents a recessive
  allele.
• If an organism produces two types of gametes, one
  with the dominant allele and one with the
  recessive allele, what trait will the organism
  have?
• If both types of alleles are present, the
  dominant will take over

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Law of Segregation

• 2 alleles for a trait are packaged into separate
  gametes
• Blending ?
• purple x white F1 pale purple flowers
• Instead, the F1 hybrids all have purple flowers

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Monohybrid cross

• Mating of two individuals either homozygous or
  heterozygous
• Capital letters represent dominant traits
• Lower case letters represent recessive traits
• For heterozygous crosses ratio is 31 dominant to
  recessive

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• Testcross ?breeding a homozygous recessive with
  dominant phenotype, but unknown genotype

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Punnett Squares!

• Punnett square ? predicts the results of a
  genetic cross between individuals of known
  genotype
• Gene combinations that might result from a
  genetic cross can be determined by drawing a
  diagram known as a Punnett Square.
• Letters represent alleles.
• Parents are crossed and the offspring are a
  result.

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Practice Problems
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Practice Problems
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Practice Problems
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Practice Problems
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Practice Problems
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Monohybrid Crosses

• In pea plants, spherical seeds (S) are dominant
  to dented seeds (s). In a genetic cross of two
  plants that are heterozygous for the seed shape
  trait, what fraction of the offspring should have
  spherical seeds?

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Monohybrid Crosses

• In a pea plants, tall plants (T) are dominant to
  short plants (t). In a genetic cross between two
  plants that are heterozygous for height, what
  fraction of the offspring should be short? What
  fraction of the offspring should be tall?

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Monohybrid Crosses

• In dogs, wire hair (S) is dominant to smooth (s).
  In a cross of a homozygous wire-haired dog with a
  smooth-haired dog, what will be the phenotype of
  the F1 generation

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Monohybrid Crosses

• In woodrats let's assume that the trait of
  bringing home shiny objects (H) is controlled by
  a single gene and is dominant to the trait of
  carrying home only dull objects (h). Suppose two
  heterozygous individuals are crossed. How many of
  each genotype would be expected if only 4
  offspring were produced?

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Monohybrid Crosses

• The ability to curl your tongue up on the sides
  (T, tongue rolling) is dominant to not being able
  to roll your tongue. A woman who can roll her
  tongue marries a man who cannot. Their first
  child has his father's phenotype. What are the
  genotypes of the mother, father, and child?

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Dihybrid Crosses

• Cross in which two hybrids are mated to test for
  dominant genes and recessive genes in two
  separate characteristics.
• For heterozygous crosses the ratio is 9331

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• In one dihybrid cross experiment, Mendel studied
  the inheritance of seed color and seed shape.
• The allele for yellow seeds (Y) is dominant to
  the allele for green seeds (y).
• The allele for round seeds (R) is dominant to the
  allele for wrinkled seeds (r).
• Mendel crossed true-breeding plants that had
  yellow, round seeds (YYRR) with true-breeding
  plants that has green, wrinkled seeds (yyrr).
• From this determined that traits are not
  inherited together ? independent assortment
• law of independent assortment ? each pair of
  alleles segregates into gametes independently

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9331
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Practice Problems
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Practice Problems
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Practice Problems
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Dihybrid Crosses

• Wolves are sometimes observed to have black coats
  and blue eyes. Assume that normal coat color (N)
  is dominant to black (n) and brown eyes (B) are
  dominant to blue (b). Suppose the alpha male and
  alpha female of a pack (these are the dominant
  individuals who do most of the breeding) are
  black with blue eyes and normal colored with
  brown eyes, respectively. The female is also
  heterozygous for both traits. What will the
  genotypes of the offspring be (assume 16) live in
  the pack.

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Dihybrid Crosses

• Suppose that anoles prefer to mate with lizards
  who bob their heads fast (F) and have red throat
  patches (R) and that these two alleles are
  dominant to their counterparts, slow bobbing and
  yellow throats. A male lizard heterozygous for
  head bobbing and homozygous dominant for the red
  throat patch mates with a female that is also
  heterozygous for head bobbing but is homozygous
  recessive for yellow throat patches. How many of
  the F1 offspring have the preferred fast bobbing
  / red throat phenotype (assume 16 young)?

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Dihybrid Crosses

• Carrion beetles lay their eggs in dead animals
  and then bury them in the ground until they
  hatch. Assume that the preference for fresh meat
  (F) is dominant to the preference for rotted meat
  and that the tendency to bury the meat shallow
  (S) is dominant to the tendency to bury the meat
  deep. Suppose a female carrion beetle homozygous
  dominant for both traits mates with a male
  homozygous recessive for both traits. What will
  be the genotype of the F1 generation?