Download - Genetics chapter 2 part 1 (1)
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State of genetics in early State of genetics in early 1800’s1800’s
What is inherited?
How is it inherited?
What is the role of chance in heredity?
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Johann Gregor Mendel(1822-1884)
Born to simple farmers in the Czeh Republic
1843: Augustinian monastery (Brno)
1851-53: University of Vienna; Physics Institute
Mathmatics, chemistry, entomology, paleotology, botany, plant physiology
1856-1863: Pea plant breeding experiments
1866: Published his findings
1900: Three botanists (De Vries, von Tschermak & Correns) independently conduct the same experiments, come across Mendel’s paper and draw attention to his work.
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Mendelian genetics
• Mendel’s work unnoticed until 1900’s
• Introduced concept of “units of inheritance”
• When correlated with cytological data → Transmission genetics was born
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Mendel’s workplace
Fig. 2.5
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Chapter 2 Opener
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Why pea plants?
• Reproduce well.• Each seed is a new individual, can
measure the characteristics of a large number of offspring after one breeding season
• Grow to maturity in single season
Easy to grow and hybridize artificially
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Mendel’s Approach
• Mendel obtained 34 different varieties of peas from local suppliers and examined the characteristics of each
• He identified 14 strains representing seven specific traits each with two forms that could be easily distinguished. He spent two years making sure these varities bred true.
• He worked with these strains for 5 years, determining how each character was inherited
Jos A. Smith
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1900 - Carl Correns, Hugo deVries, and Erich von
Tschermak rediscover and confirm Mendel’s laws.
Mendel published in 1866, was not
appreciated in his lifetime.
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Mendel’s Approach Followed the Modern Scientific Method
1. Make initial observations about a phenomenon or process
2. Formulate a testable hypothesis
3. Design a controlled experiment to test the hypothesis
4. Collect data from the experiment
5. Interpret the experimental results, comparing them to those expected under the hypothesis
6. Draw a conclusion and reformulate the hypothesis if necessary
One of Mendel’s strengths was his
careful experimental
design
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Five Critical Experimental Innovations
• There were five features of Mendel’s breeding experiments that were critical to his success
• Controlled crosses
• Use of pure breeding strains
• Selection of dichotomous traits
• Quantification of results
• Use of replicate (repeated), reciprocal, and test crosses
• Luck?
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Controlled Crosses Between Plants
• Pea plants are capable of self-fertilization and artificial cross-fertilization
• Self-fertilization occurs naturally
• Cross-fertilization involves removing the anthers from a flower and introducing pollen of the desired type with a small brush
From Peirce Genetics
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Pure-Breeding Strains to Begin Experimental Crosses
• Mendel took 2 years prior to beginning his experiments to establish pure-breeding (or true-breeding) strains
• Each experiment began with crosses between two pure-breeding parental generation plants (P generation) that produced offspring called F1 (first filial generation)
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Monohybrid Crosses
Smooth Seeds
Wrinkled Seeds
Female Male
Monohybrid Cross: a cross-pollination involving two true-breeding lines that differ for only one trait
“P”
“F1”Progeny:
All progeny had smooth seed!“First Filial generation”
“Parental generation”
Parents:
All progeny had same PHENOTYPE: “the form that is shown”
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Monohybrid Crosses
Smooth Seeds
Wrinkled Seeds
Female Male
Two possible Hypotheses
Hypothesis 2: The child’s phenotype is determined by the mother’s phenotype
Hypothesis 1: The smooth phenotype is “dominant” to the wrinkled phenotype
“P”
“F1”How could you
differentiate between these possibilities?
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Mendel Made Reciprocal CrossesReciprocal Cross: Repeating a particular genetic cross
but with the sexes of the two parents switched
All F1 had smooth seed.
Smooth Seeds
Wrinkled Seeds
Female Male
- The smooth trait is “dominant” to the wrinkled trait
Conclusion
- Phenotype is not determined by the mother’s phenotype
“P”
“F1”
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• The trait shown by the F1 offspring was called the dominant phenotype (round peas, e.g.)
• The other trait not apparent in the F1 was called the recessive phenotype (wrinkled)
• When F1 were crossed, 75% of the resulting F2 had the dominant trait, but the recessive trait reappeared in the other 25%
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Alleles
• Mendel’s results rejected the blending theory of heredity
• Theorized that plants carry two discrete hereditary units for each trait, alleles; a plant receives one of these in the egg and the second in pollen
• Together the two alleles for each trait determine the phenotype of the individual
Alleles
Phenotype
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Homozygous and Heterozygous Individuals
• Pure-breeding individuals, like Mendel’s parent plants, have identical copies of the two alleles for a trait (homozygous individual)
• The F1 plants had different alleles from each parent and were heterozygous
Homozygous (TT & tt)
Heterozygous (Tt)
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• A 3:1 phenotypic ratio is predicted for the F2 produced by a monohybrid cross
• A 1:2:1 genotypic ratio is also predicted (¼ G/G, ½ G/g, ¼ g/g)
Now that we have a theory, we can do
some real predicting!
Now that we have a theory, we can do
some real predicting!
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Punnett Square
• The alleles (in gametes) carried by one parent are arranged along the top of the square and those of the other parent, down the side
• The results expected from random fusion of the gametes are placed within the square
R r
R
r
RR Rr
Rr rr
Punnett Square
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Mendel’s Results Revisited: F1
Smooth Seeds
Wrinkled Seeds
“AA”
“aa”
Gametes possible: “A” or “A”
Gametes possible: “a” or “a”
“Aa”
Smooth
Aa Aa
AaAa
a a
A
A
♂ Gametes
♀ G
ame
tes
Use a “Punnett Square” to determine all possible progeny genotypes
♂♀
Explains why all progeny were smooth
Genotype?Genotype?
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Br rr
A
B
C
D
E
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What is the predicted cross of homozygous recessive red and heterozygous dominant brown?
A.All brownB.3 brown, 1 redC.2 brown, 2 redD.1 brown, 3 red
E.All red
All bro
wn
3 brown, 1
red
2 brown, 2
red
1 brown, 3
red
All red
20% 20%20%20%20%
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Br rr
A
B
C
D
E
Br
Br
rr
rr
B r
r
r
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Red + Brown = Blond! (sometimes?)Not all traits are dominant/reccessive!
More in upcoming lectures!
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Punnett Square Practice Problems!
Chapter 2: Problems 2 & 3
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Mendel’s First Law
• Mendel used his theory of particulate inheritance to formulate the law of segregation (Mendel’s first law)
• Alleles are separated into gametes. Gametes randomly combine to create progeny in predictable proportions.
• Hypothesis!: Mendel expected that half of the gametes of heterozygous F1 individuals would carry the dominant allele and half the recessive
• How can we test this?
Genotype?Genotype?
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Conclusion: all F1 plants are heterozygous!
Conclusion: all F1 plants are heterozygous!
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The Test Cross
• Allows to distinguish genotype of individual expressing dominant phenotype by crossing it with homozygous recessive individual
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What other predictions can we test?
• Mendel’s hypothesis predicts that F2 plants with the dominant phenotype can be homozygous or heterozygous
• The heterozygous state (2/3) is twice as likely as the homozygous state (1/3)
• HOW? Mendel used a self-fertilization experiment to test the predictions of the hypothesis
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F3 generation
• Hypothesis confirmed:
• 1/3 of plants were homozygous and breed true
• 2/3 of heterozygous F2 plants generated a 3:1 ratio of dominant:recessive phenotype among their progeny
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WHAT HAPPENS IF WE STUDY TWO TRAITS?
Will the presence of one charastics affect the prescent of another?
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Dihybrid-Cross Analysis of Two Genes
• To study the simultaneous transmission of two traits, Mendel made dihybrid crosses between organisms that differed for two traits
• He began each cross with pure-breeding lines (e.g., RRGG and rrgg) and produced F1 that were heterozygous for both traits (e.g., RrGg).
• If assortment is random, four gametes should be equally likely in the F1 (e.g., RG, Rg, rG, rg)
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2.3 Dihybrid and Trihybrid Crosses
• How can we calculate the crosses of two or more traits at the same time?
• Dihybrid Punnet Square
• Forked Diagram
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An Aid to Prediction of Gamete Frequency
• The forked-line diagram is used to determine gamete genotypes and frequencies
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Let’s give it a try!
• Self Fertilization of a heterozygous yellow, round pea?
• Round (R) is dominant to wrinkled (r)
• Yellow (G) is dominant to green (g) • What does the dihybrid
Punnet square look like?
• What does the forked diagram look like?
F2 ?
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Independent Assortment of Alleles from the RrGg × RrGg Cross
• Mendel predicted that alleles of each locus unite at random to produce the F2, generating
• round, yellow R-G- (¾)(¾) = 9/16
• round, green R-gg (¾)(¼) = 3/16
• wrinkled, yellow rrG- (¾)(¼) = 3/16
• wrinkled, green rrgg (¼)(¼) = 1/16
9:3:3:1 ratio!9:3:3:1 ratio!The dihybrid ratio: 9/16 both dominant traits, 3/16 each for two combinations
of one dominant and one recessive, and 1/16 both recessives
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Mendel’s Second Law
• The 9:3:3:1 ratios generated in Mendel’s dihybrid crosses illustrate Mendel’s second law, also known as Mendel’s law of independent assortment
• The law states that during gamete formation the segregation of alleles at one locus is independent of the segregation of alleles at another locus.
• Within the 9:3:3:1 ratio, Mendel recognized two 3:1 ratios for each trait
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Testing Independent Assortment by Test-Cross Analysis
• Mendel wants to test his hypothesis about independent assortment. HOW?
• He predicted that the F1 seeds were dihybrid, of genotype RrGr, and that crossing them to a plant of genotype rrgg would yield four offspring phenotypes with equal frequency
Test Cross!
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Testing Independent Assortment by Trihybrid-Cross Analysis
• To test his hypothesis about independent assortment further, Mendel performed trihybrid-cross analysis
• The trihybrid cross involved three traits: round vs. wrinkled peas, yellow vs. green peas, and purple vs. white flowers
• The cross was: RRGGPP × rrggpp; the F1 were RrGgPp
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How many possible combinations?
• Double check yourself! Do you see all the possible combinations of phenotypes in your answer?
• The number of possibilities can be expressed as 2n, where n = number of genes
• In a trihybrid cross, there are 8 possibilties 23 = 8!
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Go try some problems!
• Chapter 2, problem 6
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Questions?