Chapter 14 Mendel and the Gene Idea

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Chapter 14 Mendel and the Gene Idea
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Inheritance

• The passing of traits from parents to offspring.
• Humans have known about inheritance for thousands
  of years.

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Genetics

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

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Gregor Mendel

• Father of Modern Genetics.

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

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Why Use Peas?

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

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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.

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Mendel's Work

• Used seven characters, each with two expressions
  or traits.
• Example
• Character - height
• Traits - tall or short.

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Monohybrid or Mendelian Crosses

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

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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.

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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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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)

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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).

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Mendel's Hypothesis

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

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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.

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Mendel's Hypothesis

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

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Law of Segregation
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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).

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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

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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.

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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.

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Results

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

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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)

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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

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Rule of Multiplication

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

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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

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Comment

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

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Variations on Mendel

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

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Incomplete Dominance

• When the F1 hybrids show a phenotype somewhere
  between the phenotypes of the two
  parents.(blending)
• 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
• Red CR CR
• Pink CRCW
• White CWCW

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Another example
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Codominance

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

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Result

• No hidden Recessive.
• 3 phenotypes and 3 genotypes
  

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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

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Result

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

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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

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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

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Gerbils
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In Gerbils

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

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Result

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

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Epistasis in Mice
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Polygenic Inheritance

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

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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.

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Genetic Studies in Humans

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

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Pedigree Chart Symbols

• Male
• Female
• Person with trait

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Sample Pedigree
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Recessive Trait
Dominant Trait
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Human Recessive Disorders

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

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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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Tay-Sachs

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

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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.

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Recessive Pattern

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

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Human Dominant Disorders

• Less common then recessives.
• Ex
• Huntingtons disease
• Achondroplasia
• Familial Hypercholsterolemia

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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.

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Genetic Screening

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

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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.

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Example

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

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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

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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.

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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.

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Carrier Recognition

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

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Fetal Testing

• Biochemical Tests
• Chromosome Analysis

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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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Newborn Screening

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

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Newborn Screening

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

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Multifactorial Diseases

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

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Ex. Heart Disease

• Genetic
• Diet
• Exercise
• Bacterial Infection