Chapter 9 Introduction to Genetics

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Chapter 9Introduction to Genetics

• Charles Page High School
• Stephen L. Cotton

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Section 9-1The Work of Gregor Mendel

• OBJECTIVES
• Discuss Mendels experiments.

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Section 9-1The Work of Gregor Mendel

• OBJECTIVES
• Describe dominance, segregation, and independent
  assortment.

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Section 9-1The Work of Gregor Mendel

• Biological inheritance, or HEREDITY, is the key
  to difference between species.
• Cats give birth to kittens, dogs produce puppies,
  etc.
• Genetics - the branch of biology that studies
  heredity

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Section 9-1The Work of Gregor Mendel

• Until the 19th century, a common explanation for
  the resemblance between parent and offspring was
  the theory of blending inheritance
• each parent contributed factors which simply
  blended together
• Gregor Mendel changed peoples views about how
  traits are inherited

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Section 9-1The Work of Gregor Mendel

• Mendel- was born in 1822
• entered a monastery at Brno at 21
• ordained as a priest
• the monastery was a center for scientific
  learning
• went to the University of Vienna to study science
  and mathematics
• returned to teach at the high school in Brno had
  charge of the monastery garden

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Section 9-1The Work of Gregor Mendel

• Self-pollination pollen from the male part of
  the flower fertilizes the female egg cells of the
  very same flower
• produces seeds that inherit characteristics from
  the single plant
• Cross-pollination pollen from one plant onto the
  flowers of the other plant
• seeds are from two different plants

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Section 9-1The Work of Gregor Mendel

• If allowed to self-pollinate, they are called
  purebred
• 7 traits Mendel studied in the pea plants- Fig.
  9-3, page 183
• What happens when peas are crossed that have
  different characters for the same trait?
• Tall x Short Yellow x Green etc.

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Section 9-1The Work of Gregor Mendel

• These combinations produce hybrids
• One trait seems to have disappeared
• Two conclusions
• 1. Individual factors, which do not blend,
  control each trait. Mendel called them
  Merkmals today called genes, and the different
  forms called alleles

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Section 9-1The Work of Gregor Mendel

• 2. Principle of dominance- some factors are
  dominant, some are recessive
• P generation represents parents
• F1 - first filial generation
• F2 - second filial generation
• Segregation-separation of alleles

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Section 9-1The Work of Gregor Mendel

• Punnett Square- a way to represent a genetic
  cross
• Gametes- reproductive cells placed on the
  Punnett square
• Fig. 9-7, page 186
• Phenotype- physical characteristic, such as tall
  or yellow
• Genotype- genetic makeup (letters)

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Section 9-1The Work of Gregor Mendel

• Homozygous- organisms that have two identical
  alleles for a particular trait (TT or tt for
  example)
• Heterozygous- organisms that have two different
  alleles for the same trait (Tt for example)
• Mendel also explored the question of whether the
  alleles segregate independently of each other-
  YES!

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Section 9-1The Work of Gregor Mendel

• Two-factor crosses two different traits are
  being examined
• Fig. 9-9, page 187
• The two-factor cross to produce the F2 generation
• A two-factor cross between two totally
  heterozygous individuals gives the phenotypic
  ratio of 9331

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Section 9-1The Work of Gregor Mendel

• Summary of Mendels work
• 1. Factors that control heredity genes
• 2. Gene may be dominant or recessive
• 3. The two forms of each gene are segregated
  (separated) during formation of reproductive
  cells (gametes)
• 4. Genes for different traits may assort
  independently of one another

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Section 9-1The Work of Gregor Mendel

• To Solve, Genetics problems need
• 1. A Key (tells what the traits are)
• 2. Information (what the parents are)
• 3. Question (usually about offspring)
• Start by converting words to letters
• a. Use Punnett square if parent known
• b. Use homozygous recessive if not

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Section 9.2Applying Mendels Principles

• OBJECTIVES
• Relate probability to genetics.

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Section 9.2Applying Mendels Principles

• OBJECTIVES
• Solve genetics problems using a Punnett square.

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Section 9.2Applying Mendels Principles

• Mendels work was far ahead of his time- his
  ideas remained unappreciated during his lifetime.
• More than 20 years after his death, his
  conclusions were recognized as breakthroughs!
• Mendel applied the laws of probability to biology

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Section 9.2Applying Mendels Principles

• Probability- the likelihood that a particular
  event will occur
• number of times an event occurs
• number of opportunities for the event
• Consider flipping a coin- what is the chance of
  getting heads?
• One out of two, or 1/2

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Section 9.2Applying Mendels Principles

• Important note you will get closer to the
  expected ratio if you do more trials (more data
  gives better results)
• Why did Mendel work with thousands of pea plants?
• Lets say you get heads 10 times in a row. What
  is the chance of getting heads again on the next
  toss?

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Section 9.2Applying Mendels Principles

• Previous events do not affect future outcomes-
  each flip of the coin is a separate, independent
  event
• The rules of probability also apply to genetics,
  just like tossing a coin.
• The Punnett square is a handy device to give us
  the possible outcomes of a genetic cross

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Section 9.2Applying Mendels Principles

• One-Factor Crosses only one characteristic (or
  trait) is examined
• might need to do a test cross if you do not know
  the parents (this means use the homozygous
  recessive) Two-Factor Crosses?
• Usually, we want to find the genotypic and
  phenotypic ratios

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Section 9.3Meiosis

• OBJECTIVES
• Describe the process of meiosis.

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Section 9.3Meiosis

• OBJECTIVES
• Compare meiosis and mitosis.

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Section 9.3 - Meiosis

• The principles of genetics described by Mendel
  require that organisms inherit a single copy of
  each gene from each of their parents
• these make gametes (or, sex cells- called sperms
  and eggs)
• But, how does this occur if we produce cells
  identical to the original during mitosis (cell
  division)?

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Section 9.3 - Meiosis

• The 8 chromosomes in a Drosophila (fruit fly) can
  be divided into 2 sets
• one set of 4 from the male parent
• another set of 4 comes from female parent
• Each chromosome in the male has a corresponding
  one in the female
• These are called homologous pairs

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Section 9.3 - Meiosis

• Each chromosome of a homologous pair is called a
  homolog
• A cell that contains both sets of homologous
  chromosomes (one set from each parent) is said to
  be diploid, and sometimes represented as 2N
  (Fig. 9-16, p.193)
• Thus for Drosophila 2N 8

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Section 9.3 - Meiosis

• This agrees with Mendels idea that all of an
  organisms cells (except the gametes) contain two
  alleles for a given trait
• Cells that contain a single set of chromosomes
  (the gametes) are said to be haploid, represented
  simply as N
• For Drosophila, N 4

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Section 9.3 - Meiosis

• There must be a process that divides the diploid
  number of chromosomes in half
• called Meiosis (or a cell division known as
  reduction-division)
• In most cases, this takes place in two stages
  known as the first and second meiotic divisions

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Section 9.3 - Meiosis

• Many of the names are the same as in mitosis-
  dont confuse them, because different things are
  happening!
• Meiosis I replicates the chromosomes twice, thus
  forming a tetrad (four chromatids)
• Fig. 9-17, page 194

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Section 9.3 - Meiosis

• The tetrad often exchanges portions of their
  chromatids in a process called crossing-over -
  produces new combinations of genes (more in
  Chapter 10 on this) Fig. 9-18, p.195
• Now, to complete Meiosis I, the cell divides, and
  prepares to enter Meiosis II (this stage is the
  difference between mitosis and meiosis)

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Section 9.3 - Meiosis

• Meiosis II unlike a cell undergoing a second
  mitotic division, neither cell goes through a
  process of DNA replication before the second
  division
• The result 4 daughter cells, each having the
  haploid (N) number, and the amount of genetic
  material has been reduced

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Section 9.3 - Meiosis

• In male animals - the haploid gametes are called
  sperm in higher plants, pollen grains contain
  haploid sperm
• All four cells are used in reproduction
• Fig. 9-19, page 196

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Section 9.3 - Meiosis

• In female animals - generally only one of the
  cells produced is used for reproduction - this is
  the egg
• This one cell (the egg) has received a large
  amount of the cytoplasm, and is much larger than
  the other 3
• The other 3 are called polar bodies, and are not
  part of reproduction

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Section 9.3 - Meiosis

• Comparison
• Mitosis- results in the production of two
  genetically identical cells diploid gives rise
  to diploid identical to each other and the
  original parent cell
• Meiosis- one diploid cell produces four haploid
  cells which have one-half the number of
  chromosomes