Mendelian Genetics

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Mendelian Genetics
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Vocabulary

• dominant gene
• F1 generation
• F2 generation
• gene
• genetic trait
• genotype
• monohybrid cross

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• Hybrid
• phenotype
• principle of independent assortment
• principle of segregation
• Punnett square
• Recessive gene

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Study Questions

• 1. List and explain the four principles of
  genetics established by Mendel.
• 2. Complete a monohybrid cross between a pure
  breeding tall plant (TT) and a pure breeding
  dwarf plant (tt). Carry the cross through to the
  second generation (F2 generation) by letting the
  plants of the first cross (Tt) self-fertilize.
  Describe the phenotypes and genotypes of both
  generations.

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• 3. Show how your knowledge of meiosis provides
  tangible evidence concerning Mendels Principles
  of Segregation and Independent Assortment.
• 4. Complete a monohybrid cross between (Tt X
  tt). List all the potential types of gametes
  produced by each parent and then combine them
  into all possible combinations in the offspring.
  List the various phenotypes and ratios from this
  cross.
• 5. List and discuss several of the approaches
  which made Mendels work successful.

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Review Meiosis
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• Mendels Work with Pea Plants
• First Steps to Genetics

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Mendels Studies of the Pea Plant

• A. Characteristics studied by Mendel (7 traits)

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Dominant vs Recessive

• 1.Form of ripe seed 2.Color of seed coat
• Smooth Yellow
• Wrinkled Green
• 3.Flower Color 4.Form of ripe pods
• Purple Inflated
• White Constricted
• 5.Color of unripe pods 6.Position of flowers
• Green Axial
• Yellow Terminal
• 7.Length of stem Tall Dwarf

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Why Pea Plants?

• 1- Many true breeding varieties available
• 2- The flower is self-fertile
• 3- Generation time is very short

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MendelsProblems

• 1- No concept of DNA or chromosomes
• 2- No concept of meiosis

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Some wise (educated guesses) decisions made by
Mendel

• 1- Selection of well-defined, contrasting traits
• 2- Extensive groundwork completed prior to
  establishment of final experimental design
• 3- Extensive replication of crosses

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Mendels Test Evaluating the Inheritance of Seed
Shape in Pea Plants

• Recognized two different traits for the seed
  shape character in pea plants round versus
  wrinkled seeds.

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Testing continued

• Established true-breeding varieties for each of
  these traits.
• RR and rr

• Offspring (F1 generation) are called hybrids.
• Rr

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100 of these hybrids produced round seeds.

• Why?

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The Next Step

• Crossed these F1 generation hybrids among each
  other (individual crosses being selected at
  random).
• Rr X Rr

• Results for their offspring (F2 generation)
• 5474 (74.7) were plants that produced round
  seeds.1850 (25.3) were plants that produced
  wrinkled seeds.Ratio of roundwrinkled approx.
  31

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The Explaination

• For every character (e.g., seed shape) an
  individual possess two instruction sets
  (alleles).
• One of these alleles was originally derived from
  the individuals mother, the other allele being
  originally derived from the individuals father.

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True vs Hybrid

• In true-breeding individuals, both alleles are
  the same.
• Homozygous
• RR or rr

• Hybrids, on the other hand, one of each kind of
  allele.
• Heterozygous
• Rr

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Mendels Further Explanation

• Mendel believed that only two alleles were
  possible for a given genetic character, and that
  one of the alleles (the dominant one) masked the
  expression of the other (the recessive one) in
  the hybrid.
• When the hybrid formed gametes, only one of the
  two possible alleles would end up in a gamete.

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Law of Segregation

• When the hybrid formed gametes, only one of the
  two possible alleles would end up in a gamete.

• However, both alleles possessed an equal chance
  of appearing in a gamete.

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Law of Segregation cont

• In the formation of gametes, two members of a
  gene pair (alleles) segregate into different
  haploid gametes with equal probability.

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Dominant and Recessive

• Defining alleles
• R the round allele (dominant).
• r the wrinkled allele (recessive).

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Phenotype Genotype

• Defining genotypes and their respective
  phenotypes
• RR genotype (homozygous dominant) round
  phenotype.
• rr genotype (homozygous recessive) wrinkled
  phenotype.
• Rr genotype (heterozygous) round phenotype

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Round Round
RR
Rr
rr
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P Generation and F1 Generation

• Parental (P) Cross
• Round X Wrinkled
• RR rr
• Gametes
• R r
• F1 (first felial) Generation Hybrids
• 100 Round phenotype
• 100 Rr genotype

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F1 and F2 Generations

• Mendels Explanation for his Results
• F1 Generation Cross Round
  X Round Rr
  RrGametes R r R
  rF2 Generation Outcome Round
  WrinkledRR Rr Rr rrNote 31
  ratio of roundwrinkled

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Punnett Square Diagram
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• Mendels Genetics Experiments With Pea Plants
  continued

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• Law of Segregation

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Genotype Vs. Phenotype
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Test Crosses

• When you cross an individual whose genetics you
  are not sure about with an individual who is
  homozygous recessive for the trait(s) in
  question.

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Test Crosses

• F1 Hybrids (Rr) xF2 Recessives (rr)

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Test Cross Pea Plants
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Mendels Law of Independent Assortment

• Whenever two or more pairs of contrasting
  characters are brought together in a hybrid, the
  alleles of the different pairs segregate
  independently of one another during gamete
  formation.

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Dihybrid Crosses

• Define Alleles and Associated TraitsSeed Shape
  CharacterR round seed trait (dominant)r
  wrinkle seed trait (recessive)Seed Color
  CharacterY yellow seed trait (dominant)y
  green seed trait (recessive

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First Cross (Dihybrid)

• Parental (P) cross crossed true-breeding plants
  that produce round-yellow (RRYY) seeds with
  true-breeding plants that produce wrinkled-green
  seeds (rryy).
• F1 Genereation hybrids for both characters (Rr
  Yy or RrYy) were100 round-yellow.

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Results

• F1 Generation Individuals (RrYy) crossed among
  each other.F2 Generation Results315 (56.7)
  round-yellow108 (19.4) round-green101 (18.2)
  wrinkled-yellow32 (5.8) wrinkled-green

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• With Independent Assortment

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Calculating Expected FrequenciesRound-Yellow

• Expect 9/16 of the F2 generation offspring to be
  round-yellow.Therefore, of a total of 556
  offspring the expected number (frequency) of
  round-yellow offspring may be calculated as
  followsFrequency (9/16)556 312.75

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• Calculating Expected Frequencies
• Round-Green
• Expect 3/16 of the F2 generation offspring to be
  round-green.Therefore, of a total of 556
  offspring the expected number (frequency) of
  round-yellow offspring may be calculated as
  followsFrequency (3/16)556 104.25

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Calculating Expected FrequenciesWrinkled- Yellow

• Expect 3/16 of the F2 generation offspring to be
  wrinkled-yellow.Therefore, of a total of 556
  offspring the expected number (frequency) of
  round-yellow offspring may be calculated as
  followsFrequency (3/16)556 104.25

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Calculating Expected FrequenciesWrinkled- Green

• Expect 1/16 of the F2 generation offspring to be
  wrinkled-yellow.Therefore, of a total of 556
  offspring the expected number (frequency) of
  round-yellow offspring may be calculated as
  followsFrequency (1/16)556 34.75

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Comparing Observed to Expected Results

• Expected
• Round- Yellow312.75Round-Green104.25Wrinkled-
  Yellow104.25Wrinkled-Green34.75

• ObservedRound-Yellow315Round-Green108Wrinkled
  -Yellow101Wrinkled-Green32

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Results of Mendels Work

• A. The principle of unit characters (elementum
  or genes)
• B. The phenomenon of dominant and recessive
  genes
• C.    The principle of segregation alleles
  separate from one another during
• D. The principle of independent assortment