chapter 14 mendel and the gene idea. inheritance the passing of traits from parents to offspring....
TRANSCRIPT
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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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Genetic Theories
1. Blending Theory -
traits were like paints and mixed evenly from both parents.
What happen over time if this were true?
This would lead to a homogenous population over time.
Can’t explain traits that skip a generation
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2. Particulate Model -
parents pass on traits as discrete units that retain their identities in the offspring.
-like a deck of cards
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Gregor Mendel
• Father of Modern Genetics.
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Reasons for Mendel's Success
• Used an experimental approach.
• Applied mathematics to the study of natural phenomena.
• Kept good records.
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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 traits or expressions.
• Example:
• Character - height• Traits - tall or short.
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Monohybrid Crosses
• Crosses that work with a single character at a time.
Example - Purple X White
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P Generation
• The Parental generation or the first two individuals used in a cross.
Example - Purple X White
• 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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Results - Summary
• In all crosses, the F1 generation showed only one of the traits regardless of which color was male or female.
• The other trait reappeared in the F2 at ~25% (3:1 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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Allele for purple flowers
Homologouspair ofchromosomes
Locus for flower-color gene
Allele for white flowers
Allele
Purple
White
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Mendel's Hypothesis
4. The two alleles for each trait separate during gamete formation and end up in different games.
This now called:
Mendel's Law of Segregation
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Law of Segregation
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Mendel’s Experiments
• Showed that the Particulate Model best fit the results.
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Vocabulary
• Phenotype - the physical appearance of the organism.
• Genotype - the genetic makeup of the organism, usually shown in a code.• P = purple• p = white
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Helpful Vocabulary
• Homozygous - When the two alleles are the same (PP or pp).
• Heterozygous- When the two alleles are different (Pp).
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6 Mendelian Crosses are Possible
Cross Genotype Phenotype
TT X tt all Tt all Dom
Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res
TT X TT all TT all Dom
tt X tt all tt all Res
TT X Tt 1TT:1Tt all Dom
Tt X tt 1Tt:1tt 1 Dom: 1 Res
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Test Cross
• Cross of a dominant phenotype of unknown genotype with a recessive phenotype.
• Ex: P? X ppIf PP - all dominant phenotype
If Pp - 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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Number of Kinds of Gametes
• Critical to calculating the results of higher level crosses.
• Look for the number of heterozygous traits.
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Equation
The formula 2n can be used, where “n” = the number of heterozygous traits.
Ex: TtRr, n=2
22 or 4 different kinds of gametes are possible.
TR, tR, Tr, tr
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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: 9:3:3:1
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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 3:1
• Ratio of Red to white is 3:1
• The cross is really a product of the ratio of each trait multiplied together. (3:1) X (3:1)
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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: 3:175% chance of the dominant25% chance of the recessive
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Rule of Multiplication or Product Rule
• 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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Solution
“T’s” = Tt X Tt = 3:1
“R’s” = rr X Rr = 1:1
“G’s” = GG x gg = 1:0
Product is:
(3:1) X (1:1) X (1:0 ) = 3:3:1:1
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Dominance vs Phenotype• A dominant allele does not
subdue a recessive allele; alleles don’t interact.
• Alleles are simply variations in a gene’s nucleotide sequence.
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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.
• Often a “dose” effectEx. 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
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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 (but not a “dose” effect)
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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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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?• Can’t 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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Dominant Trait Recessive 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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Dominance vs Phenotype
• For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype.
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Example -Tay-Sachs
• Disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain.
• At the organismal level, the allele is recessive.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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Tay-Sachs
• At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant.
• At the molecular level, the alleles are codominant.
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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: • Huntington’s disease• Achondroplasia• Familial Hypercholsterolemia
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Inheritance Pattern• Each affected individual had
one affected parent.• Doesn’t skip generations.• Homozygous cases show
worse phenotype symptoms.• May have post-maturity onset
of symptoms.
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Homework
• Read Chapter 14 (Hillis – 8)
• Chapter 14 – Mon. 1/28
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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 FormulaR = F X M X D
R = riskF = 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 any child will be albino.
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Common Mistake
• If risk is .25, then as long as we don’t have 4 kids, we won’t 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.
• Becoming more widely recognized in medicine.
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Ex. Heart Disease
• Genetic
• Diet
• Exercise
• Bacterial Infection
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Genes & Environment
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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.
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Summary
• Be able to recognize and work with some of the “common” human trait examples.