Mendel and the Gene Idea - Biolympiads...•Mendel used reciprocal crosses, where the parents...
Transcript of Mendel and the Gene Idea - Biolympiads...•Mendel used reciprocal crosses, where the parents...
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Chapter 14 Mendel and the Gene Idea
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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 3. Particulate Model - parents pass on traits
as discrete units that retain their identities
in the offspring.
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Gregor Mendel
• Father of Modern Genetics.
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• Mendel’s paper was published in 1866, but was not recognized by Science until the early 1900’s.
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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 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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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% (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 term “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
When does this occur?
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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. – 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 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 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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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: 3:1
75% chance of the dominant 25% 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.
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Rule of Addition
• Rule of Addition—the probability of an event that can occur in two or more different ways
– Yy x Yy
• What is the chance of a heterozygous offspring?
–¼ + ¼ = 1/2
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PpYyRr x Ppyyrr (trihybrid cross)
• What is the chance that an offspring will have at least 2 recessive traits? – ppyyRr =
• ¼ x ½ x ½ = 1/16 (Rule of Multiplication) – ppYyrr =
• ¼ x ½ x ½ = 1/16 – Ppyyrr =
• ½ x ½ x ½ = 2/16 – PPyyrr =
• ¼ x ½ x ½ = 1/16 – ppyyrr =
• ¼ x ½ x ½ = 1/16 Total = 6/16 = 3/8 (Rule of Addition)
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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” effect
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
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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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Epistasis in Mice
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Problem
• Wife is type A
• Husband is type AB
• Child is type O
Question - Is this possible?
Comment - Wife’s boss is type O
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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.
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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).
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Bombay - Detection
• When ABO blood type inheritance patterns are altered from expected.
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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
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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 Diseases
• 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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Genetic Screening
• Risk assessment for an individual inheriting a trait.
• Uses probability to calculate the risk.
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General Formula
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 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. Phenylketonuria (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 genetic problems (practice!).
• Watch genetic vocabulary.
• Be able to read pedigree charts.
• Be able to recognize and work with some of the “common” human trait examples.