Fundamentals of Genetics

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Fundamentals of Genetics

• Chapter 9

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Genetics

• The field of biology dedicated to understanding
  how characteristics are transmitted from parent
  to offspring.
• Studied by Gregor Mendel
• Heredity- the transmission of characteristics
  from parents to offspring.

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Gregor Mendel

• Austrian monk science teacher in the mid 1800s
• Worked with pea plants in his garden
• Studied 7 characteristics of peas

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• Each characteristics had 2 different traits
• EX) tall/short, purple/white
• Page 176

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Mendels Experiments

• Started off with 2 pure plant groups for a
  particular characteristic
• P1 generation
• Cross pollinated these plant groups (sexual
  reproduction) to produce the next generation of
  plants
• F1 generation

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• When the seeds matured from the F1 generation, he
  counted recorded the traits of the offspring.
• Then he allowed the F1 generation plants self
  pollinate (sexual reproduction with self) to
  produce the next generation of plants
• F2 generation

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(notice that the predicted ratios the actual
ratios are not exact!)
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Mendels Results

• The F1 generation always showed only 1 of the 2
  traits for the characteristic
• Mendel named this the dominant factor
• The F2 generation always showed a 31 (or a 75
  to a 25) ratio between the 2 traits
• Mendel named the second the recessive factor

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Example Purple is dominant White is recessive
F1 generation all purple
F2 generation 75 purple 25 white
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The Law of Segregation

• Mendel stated that a pair of factors is
  segregated, or separated, during the formation of
  gametes.
• So, What does this mean?
• Each egg and sperm receives only one factor from
  each parent.

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The Law of Independent Assortment

• Mendel also stated that factors for different
  characteristics are distributed to gametes
  independently.
• So, What does this mean?
• The factors for different characteristics are not
  connected.

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Human traits

• link

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Molecular Genetics

• The study of the structure function of
  chromosomes genes
• Allele alternate form of a gene
• Mendel called them factors
• Abbreviations
• Dominant allele capital letter (B)
• Recessive allele lower case letter (b)

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9.2 Vocabulary

• Genotype the genetic makeup of the organism
  (BB, Bb, bb)
• Phenotype the physical appearance of the
  organism (Black, white)

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Genotype Vocabulary

• Homozygous same genes
• When the organism has the same alleles for the
  characteristic
• BB homozygous dominant
• bb homozygous recessive
• Heterozygous different genes
• When the organism has different alleles for the
  characteristic
• Bb

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Genetic Probability

• The likelihood (probability) of offspring of
  known parents can be determined by Punnett squares

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3 Types
Punnett Squares diagrams biologists use to
predict the probability that certain traits will
be inherited by offspring.

• Monohybrid Cross One characteristic
• Dihybrid Cross Two characteristics
• Trihybrid Cross Three characteristics

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Monohybrid Cross

• A cross between individuals that involves one
  pair of contrasting traits.
• 1 characteristic
• 2 contrasting traits

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• Question
• In rabbits, the allele for black coat color (B)
  is dominant over the allele for brown coat color
  (b). Predict the results of a cross between a
  rabbit homozygous for black coat color and a
  rabbit homozygous for brown coat color.
• What is the genotypic ratio?
• What is the phenotypic ratio?

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• Brown hair color is dominant over blond hair, and
  brown eyes are dominant over blue eyes.
• Q- If 2 heterozygous parents for both traits
  mate, what is the phenotypic ratio of their
  offspring?
• __________ Brown hair, Brown eyes
• __________ Brown hair, Blue eyes
• __________ Blond hair, Brown eyes
• __________ Blond hair, Blue eyes

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Dihybrid Cross

• A cross between individuals that involves 2 pairs
  of contrasting traits.
• 2 characteristics
• 4 contrasting traits
• Assumes the genes arent linked

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How do you find an unknown genotype?

• Testcross
• Example) Is the red flower a pure (RR) or hybrid
  (Rr) ?
• Cross the unknown flower with a pure recessive
  flower (rr).
• This will allow the recessives to show up in the
  next generation.

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The results

• If the flower is hybrid, the offspring will be
  50 red and 50 white.
• If the flower is pure, the offspring will be all
  red.

Result if flower is hybrid
Result if flower is pure
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Complications!

• Incomplete Dominance occurs when the F1
  offspring has a phenotype between that of the
  parents.
• neither allele is completely dominant

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Example) Incomplete Dominance

• Four oclock flowers
• Q- What would happen if you crossed a red four
  oclock flower with a white four oclock flower?
• A- a pink flower!

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2) Codominance

• Occurs when both alleles are expressed in a
  heterozygous offspring.
• Neither allele is dominant or recessive
• Q- What would happen if you crossed a horse with
  a white coat with a horse with a red coat?
• A- A horse with a roan coat (both red and white
  hairs) !

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Codominance
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• Ex) ABO blood groups

Dominant
Dominant
Codominant
Recessive
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3) Epistasis

• occurs when the alleles of one gene cover up or
  alter the expression of alleles of another gene
• Ex) Mouse coat color
• B - black coat
• b - brown coat
• C - pigment
• c - no pigment

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4) Polygenic Inheritance

• One trait determined by multiple genes
• Ex) skin color at least 3 genes

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