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Chapter 14 Mendel and the Gene Idea

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Title: Chapter 14 Mendel and the Gene Idea


1
Chapter 14 Mendel and the Gene Idea
2
Inheritance
  • The passing of traits from parents to offspring.
  • Humans have known about inheritance for thousands
    of years.

3
Genetics
  • The scientific study of the inheritance.
  • Genetics is a relatively new science (about 150
    years).

4
Genetic Theories
  • 1. Blending Theory -
  • traits were like paints and mixed evenly from
    both parents.
  • 2. Incubation Theory -
  • only one parent controlled the traits of the
    children.
  • Ex Spermists and Ovists

5
  • 3. Particulate Model -
  • parents pass on traits as discrete units that
    retain their identities in the offspring.

6
Gregor Mendel
  • Father of Modern Genetics.

7
  • Mendels paper published in 1866, but was not
    recognized by Science until the early 1900s.

8
Reasons for Mendel's Success
  • Used an experimental approach.
  • Applied mathematics to the study of natural
    phenomena.
  • Kept good records.

9
  • Mendel was a pea picker.
  • He used peas as his study organism.

10
Why Use Peas?
  • Short life span.
  • Bisexual.
  • Many traits known.
  • Cross- and self-pollinating.
  • (You can eat the failures).

11
Cross-pollination
  • Two parents.
  • Results in hybrid offspring where the offspring
    may be different than the parents.

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Self-pollination
  • One flower as both parents.
  • Natural event in peas.
  • Results in pure-bred offspring where the
    offspring are identical to the parents.

14
Mendel's Work
  • Used seven characters, each with two expressions
    or traits.
  • Example
  • Character - height
  • Traits - tall or short.

15
Monohybrid or Mendelian Crosses
  • Crosses that work with a single character at a
    time.
  • Example - Tall X short

16
P Generation
  • The Parental generation or the first two
    individuals used in a cross.
  • Example - Tall X short
  • Mendel used reciprocal crosses, where the parents
    alternated for the trait.

17
Offspring
  • F1 - first filial generation.
  • F2 - second filial generation, bred by crossing
    two F1 plants together or allowing a F1 to
    self-pollinate.

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21
Another Sample Cross
  • P1 Tall X short (TT x tt)
  • F1 all Tall (Tt)
  • F2 3 tall to 1 short
  • (1 TT 2 Tt 1 tt)

22
Results - Summary
  • In all crosses, the F1 generation showed only one
    of the traits regardless of which was male or
    female.
  • The other trait reappeared in the F2 at 25 (31
    ratio).

23
Mendel's Hypothesis
  • 1. Genes can have alternate versions called
    alleles.
  • 2. Each offspring inherits two alleles, one from
    each parent.

24
Mendel's Hypothesis
  • 3. If the two alleles differ, the dominant
    allele is expressed. The recessive allele
    remains hidden unless the dominant allele is
    absent.
  • Comment - do not use the terms strongest to
    describe the dominant allele.

25
Mendel's Hypothesis
  • 4. The two alleles for each trait separate during
    gamete formation. This now called Mendel's Law
    of Segregation

26
Law of Segregation
27
Mendels Experiments
  • Showed that the Particulate Model best fit the
    results.

28
Vocabulary
  • Phenotype - the physical appearance of the
    organism.
  • Genotype - the genetic makeup of the organism,
    usually shown in a code.
  • T tall
  • t short

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Helpful Vocabulary
  • Homozygous - When the two alleles are the same
    (TT/tt).
  • Heterozygous- When the two alleles are different
    (Tt).

31
6 Mendelian Crosses are Possible
  • Cross Genotype Phenotype
  • TT X tt all Tt all Dom
  • Tt X Tt 1TT2Tt1tt 3 Dom 1 Res
  • TT X TT all TT all Dom
  • tt X tt all tt all Res
  • TT X Tt 1TT1Tt all Dom
  • Tt X tt 1Tt1tt 1 Dom 1 Res

32
Test Cross
  • Cross of a suspected heterozygote with a
    homozygous recessive.
  • Ex T_ X tt
  • If TT - all dominant
  • If Tt - 1 Dominant 1 Recessive

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Dihybrid Cross
  • Cross with two genetic traits.
  • Need 4 letters to code for the cross.
  • Ex TtRr
  • Each Gamete - Must get 1 letter for each trait.
  • Ex. TR, Tr, etc.

35
Number of Kinds of Gametes
  • Critical to calculating the results of higher
    level crosses.
  • Look for the number of heterozygous traits.

36
Equation
  • The formula 2n can be used, where n the
    number of heterozygous traits.
  • Ex TtRr, n2
  • 22 or 4 different kinds of gametes are
    possible.
  • TR, tR, Tr, tr

37
Dihybrid Cross
  • TtRr X TtRr
  • Each parent can produce 4 types of gametes.
  • TR, Tr, tR, tr
  • Cross is a 4 X 4 with 16 possible offspring.

38
Results
  • 9 Tall, Red flowered
  • 3 Tall, white flowered
  • 3 short, Red flowered
  • 1 short, white flowered
  • Or 9331

39
Law of Independent Assortment
  • The inheritance of 1st genetic trait is NOT
    dependent on the inheritance of the 2nd trait.
  • Inheritance of height is independent of the
    inheritance of flower color.

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Comment
  • Ratio of Tall to short is 31
  • Ratio of Red to white is 31
  • The cross is really a product of the ratio of
    each trait multiplied together. (31)
    X (31)

42
Probability
  • Genetics is a specific application of the rules
    of probability.
  • Probability - the chance that an event will occur
    out of the total number of possible events.

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Genetic Ratios
  • The monohybrid ratios are actually the
    probabilities of the results of random
    fertilization.
  • Ex 31
  • 75 chance of the dominant
  • 25 chance of the recessive

45
Rule of Multiplication
  • The probability that two alleles will come
    together at fertilization, is equal to the
    product of their separate probabilities.

46
Example TtRr X TtRr
  • The probability of getting a tall offspring is ¾.
  • The probability of getting a red offspring is ¾.
  • The probability of getting a tall red offspring
    is ¾ x ¾ 9/16

47
Comment
  • Use the Product Rule to calculate the results of
    complex crosses rather than work out the Punnett
    Squares.
  • Ex TtrrGG X TtRrgg

48
Solution
  • Ts Tt X Tt 31
  • Rs rr X Rr 11
  • Gs GG x gg 10
  • Product is
  • (31) X (11) X (10 ) 3311

49
Variations on Mendel
  • 1. Incomplete Dominance
  • 2. Codominance
  • 3. Multiple Alleles
  • 4. Epistasis
  • 5. Polygenic Inheritance

50
Incomplete Dominance
  • When the F1 hybrids show a phenotype somewhere
    between the phenotypes of the two parents.
  • Ex. Red X White snapdragons
  • F1 all pink
  • F2 1 red 2 pink 1 white

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Result
  • No hidden Recessive.
  • 3 phenotypes and 3
    genotypes (Hint!
    often a dose effect)
  • Red CR CR
  • Pink CRCW
  • White CWCW

53
Another example
54
Codominance
  • Both alleles are expressed equally in the
    phenotype.
  • Ex. MN blood group
  • MM
  • MN
  • NN

55
Result
  • No hidden Recessive.
  • 3 phenotypes and 3 genotypes
    (but not a dose effect)

56
Multiple Alleles
  • When there are more than 2 alleles for a trait.
  • Ex. ABO blood group
  • IA - A type antigen
  • IB - B type antigen
  • i - no antigen

57
Result
  • Multiple genotypes and phenotypes.
  • Very common event in many traits.

58
Alleles and Blood Types
  • Type Genotypes
  • A IA IA or IAi
  • B IB IB or IBi
  • AB IAIB
  • O ii

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Comment
  • Rh blood factor is a separate factor from the ABO
    blood group.
  • Rh dominant
  • Rh- recessive
  • A blood dihybrid trait

62
Epistasis
  • When 1 gene locus alters the expression of a
    second locus.
  • Ex
  • 1st gene C color, c albino
  • 2nd gene B Brown, b black

63
Gerbils
64
In Gerbils
  • CcBb X CcBb
  • Brown X Brown
  • F1 9 brown (C_B_)
  • 3 black (C_bb)
  • 4 albino (cc__)

65
Result
  • Ratios often altered from the expected.
  • One trait may act as a recessive because it is
    hidden by the second trait.

66
Epistasis in Mice
67
Problem
  • Wife is type A
  • Husband is type AB
  • Child is type O
  • Question - Is this possible?
  • Comment - Wifes boss is type O

68
Bombay Effect
  • Epistatic Gene on ABO group.
  • Alters the expected ABO outcome.
  • H dominant, normal ABO
  • h recessive, no A,B, reads as type O blood.

69
Genotypes
  • Wife type A (IA IA , Hh)
  • Husband type AB (IAIB, Hh)
  • Child type O (IA IA , hh)
  • Therefore, the child is the offspring of the wife
    and her husband (and not the boss).

70
Bombay - Detection
  • When ABO blood type inheritance patterns are
    altered from expected.

71
Polygenic Inheritance
  • Factors that are expressed as continuous
    variation.
  • Lack clear boundaries between the phenotype
    classes.
  • Ex skin color, height

72
Genetic Basis
  • Several genes govern the inheritance of the
    trait.
  • Ex Skin color is likely controlled by at least
    4 genes. Each dominant gives a darker skin.

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Result
  • Mendelian ratios fail.
  • Traits tend to "run" in families.
  • Offspring often intermediate between the parental
    types.
  • Trait shows a bell-curve or continuous
    variation.

75
Genetic Studies in Humans
  • Often done by Pedigree charts.
  • Why?
  • Cant do controlled breeding studies in humans.
  • Small number of offspring.
  • Long life span.

76
Pedigree Chart Symbols
  • Male
  • Female
  • Person with trait

77
Sample Pedigree
78
Recessive Trait
Dominant Trait
79
Human Recessive Disorders
  • Several thousand known
  • Albinism
  • Sickle Cell Anemia
  • Tay-Sachs Disease
  • Cystic Fibrosis
  • PKU
  • Galactosemia

80
Sickle-cell Disease
  • Most common inherited disease among
    African-Americans.
  • Single amino acid substitution results in
    malformed hemoglobin.
  • Reduced O2 carrying capacity.
  • Codominant inheritance.

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82
Tay-Sachs
  • Eastern European Jews.
  • Brain cells unable to metabolize type of lipid,
    accumulation of causes brain damage.
  • Death in infancy or early childhood.

83
Cystic Fibrosis
  • Most common lethal genetic disease in the U.S.
  • Most frequent in Caucasian populations (1/20 a
    carrier).
  • Produces defective chloride channels in membranes.

84
Recessive Pattern
  • Usually rare.
  • Skips generations.
  • Occurrence increases with consaguineous matings.
  • Often an enzyme defect.

85
Human Dominant Disorders
  • Less common then recessives.
  • Ex
  • Huntingtons disease
  • Achondroplasia
  • Familial Hypercholsterolemia

86
Inheritance Pattern
  • Each affected individual had one affected parent.
  • Doesnt skip generations.
  • Homozygous cases show worse phenotype symptoms.
  • May have post-maturity onset of symptoms.

87
Genetic Screening
  • Risk assessment for an individual inheriting a
    trait.
  • Uses probability to calculate the risk.

88
General Formal
  • R F X M X D
  • R risk
  • F probability that the female carries the gene.
  • M probability that the male carries the gene.
  • D Disease risk under best conditions.

89
Example
  • Wife has an albino parent.
  • Husband has no albinism in his pedigree.
  • Risk for an albino child?

90
Risk Calculation
  • Wife probability is 1.0 that she has the
    allele.
  • Husband with no family record, probability is
    near 0.
  • Disease this is a recessive trait, so risk is
    Aa X Aa .25
  • R 1 X 0 X .25
  • R 0

91
Risk Calculation
  • Assume husband is a carrier, then the risk is
  • R 1 X 1 X .25
  • R .25
  • There is a .25 chance that every child will be
    albino.

92
Common Mistake
  • If risk is .25, then as long as we dont have 4
    kids, we wont get any with the trait.
  • Risk is .25 for each child. It is not
    dependent on what happens to other children.

93
Carrier Recognition
  • Fetal Testing
  • Amniocentesis
  • Chorionic villi sampling
  • Newborn Screening

94
Fetal Testing
  • Biochemical Tests
  • Chromosome Analysis

95
Amniocentesis
  • Administered between 11 - 14 weeks.
  • Extract amnionic fluid cells and fluid.
  • Biochemical tests and karyotype.
  • Requires culture time for cells.

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Chorionic Villi Sampling
  • Administered between 8 - 10 weeks.
  • Extract tissue from chorion (placenta).
  • Slightly greater risk but no culture time
    required.

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99
Newborn Screening
  • Blood tests for recessive conditions that can
    have the phenotypes treated to avoid damage.
    Genotypes are NOT changed.
  • Ex. PKU

100
Newborn Screening
  • Required by law in all states.
  • Tests 1- 6 conditions.
  • Required of home births too.

101
Multifactorial Diseases
  • Where Genetic and Environment Factors interact to
    cause the Disease.

102
Ex. Heart Disease
  • Genetic
  • Diet
  • Exercise
  • Bacterial Infection

103
Summary
  • Know the Mendelian crosses and their patterns.
  • Be able to work simple genetic problems
    (practice).
  • Watch genetic vocabulary.
  • Be able to read pedigree charts.

104
Summary
  • Be able to recognize and work with some of the
    common human trait examples.
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