Mendel and the Gene Idea

1
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.

12
(No Transcript)
13
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.

18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
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

29
(No Transcript)
30
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

33
(No Transcript)
34
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.

40
(No Transcript)
41
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.

43
(No Transcript)
44
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
Tips for Dihybrid Problems

• Identify all of the alleles that can be
  identified from the phenotypes of the parents or
  kids.
• Work from the monohybrid ratios to solve for the
  missing alleles.

50
Variations on Mendel

• 1. Incomplete Dominance
• 2. Codominance
• 3. Multiple Alleles
• 4. Epistasis
• 5. Polygenic Inheritance

51
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

52
(No Transcript)
53
Result

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

54
Another example
55
Codominance

• Both alleles are expressed equally in the
  phenotype.
• Ex. MN blood group
• MM
• MN
• NN

56
Result

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

57
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

58
Result

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

59
Alleles and Blood Types

• Type Genotypes
• A IA IA or IAi
• B IB IB or IBi
• AB IAIB
• O ii

60
(No Transcript)
61
(No Transcript)
62
Comment

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

63
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

64
Gerbils
65
In Gerbils

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

66
Result

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

67
Epistasis in Mice
68
Problem

• Wife is type A
• Husband is type AB
• Child is type O
• Question - Is this possible?
• Comment - Wifes boss is type O

69
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.

70
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).

71
Bombay - Detection

• When ABO blood type inheritance patterns are
  altered from expected.

72
AP Biology
Best Looking Bio Teacher EVER.
Whos Hungry?
It Came From Space!
He Made New Friends.
FEED ME!
This Isnt a Human at All!

• Zachary - IASMH

73
Homework

• Readings Chapters 14, 47
• Lab changed to Chi Square and other genetics.
• Chapter 47 Wed. 12/1
• Chapter 14 Fri. 12/3

74
New 1-800

• You can now reach my office directly 24/7 by
  calling
• 1-800-316-3163 ext. 31

75
Polygenic Inheritance

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

76
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.

77
(No Transcript)
78
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.

79
Genetic Studies in Humans

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

80
Pedigree Chart Symbols

• Male
• Female
• Person with trait

81
Sample Pedigree
82
Recessive Trait
Dominant Trait
83
Human Recessive Disorders

• Several thousand known
• Albinism
• Sickle Cell Anemia
• Tay-Sachs Disease
• Cystic Fibrosis
• PKU
• Galactosemia

84
Sickle-cell Disease

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

85
(No Transcript)
86
Tay-Sachs

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

87
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.

88
Recessive Pattern

• Usually rare.
• Skips generations.
• Occurrence increases with consaguineous matings.
• Often an enzyme defect.
• Affects males and females equally.

89
Human Dominant Disorders

• Less common then recessives.
• Affects males and females equally.
• Ex
• Huntingtons disease
• Achondroplasia
• Familial Hypercholesterolemia

90
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.

91
Genetic Screening

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

92
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.

93
Example

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

94
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

95
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.

96
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.

97
Carrier Recognition

• Fetal Testing
• Amniocentesis
• Chorionic villi sampling
• Newborn Screening

98
Fetal Testing

• Biochemical Tests
• Chromosome Analysis

99
Amniocentesis

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

100
(No Transcript)
101
Chorionic Villi Sampling

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

102
(No Transcript)
103
Newborn Screening

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

104
Newborn Screening

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

105
Multifactorial Diseases

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

106
Ex. Heart Disease

• Genetic
• Diet
• Exercise
• Bacterial Infection

107
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.

108
Summary

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