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The Work of Gregor Mendel

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Title: Chapter 11 Introduction to Genetics Author: Default Last modified by: Kristen Musto Created Date: 7/11/2004 1:49:26 AM Document presentation format – PowerPoint PPT presentation

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Title: The Work of Gregor Mendel


1
The Work of Gregor Mendel
2
What is genetics? Why study it?
  • Genetics study of heredity
  • Heredity passing on of characteristics from
    parents to offspring
  • Geneticists study genes, stretches of DNA, that
    code for proteins

3
What are some features in humans that everyone
has in common?
4
What are some features that are different from
human to human?
5
Why pea plants?
  • Easy to grow and mature quickly
  • Structure method of pollination made them easy
    to use in controlled experiments could
    self-fertilize or cross-fertilize
  • Different plants show contrasting traits
  • Characteristic heritable feature that varies
    among individuals, ex flower color
  • Trait different varieties for a characteristic,
    ex purple or white flowers

6
  • Why did Mendel Choose the Pea?
  • Flower Structure/ allowed controlled mating
  • Many variable traits/ simple traits
  • Fast Generation Time

7
Gregor Mendel
  • Genetics the scientific study of heredity
  • Gregor Mendel (born in 1822) worked with true
    breeding garden peas to study inheritance
  • True breeding produce offspring identical to
    themselves when they self-pollinate

8
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9
Genes and Dominance
  • Mendel crossed plants with seven different pea
    plant traits that each had two contrasting
    characteristics and studied the offspring.
  • P generation original pair of plants
  • F1 generation 1st generation of offspring
  • F2 generation 2nd generation of offspring

10
Mendels Crosses
  • Self-pollination sperm cells fertilize egg
    cells of the same plant (i.e. one parent, but
    still sexual reproduction)
  • True-breeding plants if allowed to
    self-pollinate, they would always produce
    offspring identical to themselves
  • Ex true-breeding short plants always have
    offspring that are short when then self-pollinate

11
The F1 Cross
  • Mendel wondered if the recessive alleles had
    disappeared or if they still existed in the F1
    generation.
  • He let the F1 plants self-pollinate creating the
    F2 generation.
  • Traits controlled by the recessive alleles
    reappeared in 1/4th of the F2 generation.

12
Mendel made two conclusions
  • Inheritance is determined by factors passed on
    from one generation to the next.
  • Genes pieces of DNA that determine a trait
  • Alleles different forms of genes
  • Some alleles are dominant and others are
    recessive. (Principle of Dominance)
  • Dominant allele always expressed as a trait
    when present (represented by a capital letter)
  • Recessive allele only expressed when the
    dominant allele is not present (represented by a
    lower case letter)

13
Current genetic terminology
  • Phenotype is the outward appearance ex tall or
    short height
  • Memory trick
  • Genotype is the genes
  • Phenotype is the physical feature you see

14
General Terms
  • Homozygous having two identical alleles for a
    trait (ex TT homozygous dominant or tt
    homozygous recessive)
  • Heterozygous having two different alleles for a
    trait (ex Tt)
  • Hybrid offspring of crosses with different
    traits
  • Genotype genetic makeup of an individual (ex
    TT, Tt or tt)
  • Phenotype physical appearance of an individual
    (ex brown, tall)
  • Probability the likelihood that an event will
    occur

15
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16
The Albino Trait
17
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18
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19
Genetics and Probability
  • Mendel realized that probability could be used to
    explain the results of genetic crosses.
  • Punnett Squares
  • T tall 25 TT tall
  • 50 Tt tall
  • t short
    25 tt short
  • Possible offspring of a Tt x Tt cross
    75 chance - tall and 25 - short
    (31 ratio)

20
Mendels Law of Segregation As seen in P and F1
generations
Dad
Mom
T tall (dominant) TT x tt t
short (recessive)
Parental!
meiosis


Gametes?


21
Mendels Law of Segregation As seen in P and F1
generations
Dad
Mom
T tall (dominant) TT x tt t
short (recessive)
Parental!
meiosis
Parent 1 Gametes
Gametes


T t tall T t tall
T t tall T t tall
Parent 2 Gametes
22
Mendels Law of Segregation As seen in P and F1
generations
Dad
Mom
T tall (dominant) Tt x Tt t short
(recessive)



T T tall T t tall
T t tall t t short
T
Probabilities?
23
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24
Practicing with Punnett Squares
  • Parents TT and Tt
  • T T
  • T TT TT
  • t Tt Tt
  • Offspring
  • 100 TT or Tt tall
  • ratio - (40)
  • Parents Tt and tt
  • T t
  • t Tt tt
  • t Tt tt
  • Offspring
  • 50 Tt tall
  • 50 tt short
  • ratio - (22) or (11)

25
Beyond Dominant Recessive
  • Some genes are neither dominant nor recessive
    and many traits are controlled by multiple
    alleles or multiple genes.
  • Incomplete Dominance neither allele is
    completely dominant so the heterozygous phenotype
    is a blending of traits
  • Ex four oclocks - red flower crossed
  • with white flower produces pink flower

26
Codominance
  • Q What does cooperate mean?
  • A Operate together
  • Codominance when both alleles are expressed
    neither is dominant nor recessive
  • Q If the two homozygotes are red and white,
    whats the phenotype of the heterozygote?
  • A The heterozygote shows red and white hairs
  • Roan coat in cows and horses
  • Both red and white hairs are present

27
  • Codominance both alleles are dominant so both
    traits show in the heterozygous phenotype
  • Ex chickens white chicken crossed with black
    chicken produces black and white speckled chicken
  • Multiple Alleles many genes have three or more
    alleles of the same gene
  • Ex blood type
  • Polygenic traits traits that are controlled by
    two or more genes show a wide range of
    phenotypes.
  • Ex skin color is controlled by at least 4 genes

28
Sex Linked Traits
  • Q What are the two types of chromosomes?
  • A Sex chromosomes and autosomes
  • Q How many sex chromosomes are there? What are
    they? What do the different combinations mean?
  • A Two sex chromosomes X Y
  • XX female
  • XY male

29
Color-blindness test!
30
Color-blindness test!
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