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7/11/2014
Lecture 5
What Darwin didn’t know: Mendel and basic genetics
What Darwin didn’t know:
Mendel and basic genetics
1.
2.
3.
4.
5.
6.
7.
Family resemblance
Gregor Mendel and the collapse of the blending model
Mendel’s basic process
Monohybrid crosses
Mendel’s five element model and the principle of segregation
Punnett squares
Dihybrid crosses
Principle of independent assortment
Extending Mendel
1.
2.
3.
4.
5.
6.
7.
Do peas make it too easy?
Gene linkage
Polygenic inheritance
Epistasis
Pleiotropy
Incomplete dominance and codominance
Environmental effects on gene expression
What Darwin didn’t know: Mendel and basic genetics
Another college drop out revolutionizes science!
Gregor Mendel (1822-1884)
and sweet peas
What Darwin didn’t know: Mendel and basic genetics
True breeding plants
X
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What Darwin didn’t know: Mendel and basic genetics
Non-true breeding plants
What Darwin didn’t know: Mendel and basic genetics
The basic cross
X
X
Hybrids
What Darwin didn’t know: Mendel and basic genetics
The monohybrid cross
What Darwin didn’t know: Mendel and basic genetics
Mendel’s seven traits
Terms to note:
1.
2.
3.
4.
5.
P generation
Cross fertilize
F1 generation
Self cross
F2 generation
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Mendel and basic genetics
The monohybrid cross
Focus on the F3:
1/3 purple breed true
2/3 purple don’t
What Darwin didn’t know: Mendel and basic genetics
Blending inheritance
does not happen!
White ALWAYS
breeds true!
What Darwin didn’t know: Mendel and basic genetics
Mendel’s five element model:
1. Parents transmit information about traits to
their offspring.
2. Each individual receives two copies of each factor
to encode each trait.
3. Not all factors are the same and different
combinations lead to different traits.
4. The two factors do not blend.
5. The presence of a factor does not guarantee it
will be expressed, it can be latent.
Mendel and basic genetics
Focus on the F2:
Who’s a homozygote?
Who’s a heterozygote?
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What Darwin didn’t know: Mendel and basic genetics
Mendel’s five element model:
What Darwin didn’t know: Mendel and basic genetics
Which trait is dominant?
1. Parents transmit information about traits to
their offspring.
2. Each individual receives two copies of each factor
to encode each trait.
3. Not all factors are the same and different
combinations lead to different traits.
4. The two factors do not blend.
5. The presence of a factor does not guarantee it
will be expressed, it can be latent.
What Darwin didn’t know: Mendel and basic genetics
In this case purple is the dominant trait and white is recessive
X
X
What Darwin didn’t know: Mendel and basic genetics
Genotype
AA = Aa
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What Darwin didn’t know: Mendel and basic genetics
Mendel and basic genetics
Phenotype
Phenotype ratio = 3:1
Genotype ratio = 1:2:1
AA = Aa
What Darwin didn’t know: Mendel and basic genetics
What Darwin didn’t know: Mendel and basic genetics
The Punnett Square
Mendel’s
1st
A
law of heredity
The Principle of Segregation –
Two alleles segregate during gamete formation to be
rejoined at random during fertilization.
Dad
a
A
Mom
a
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What Darwin didn’t know: Mendel and basic genetics
What Darwin didn’t know: Mendel and basic genetics
The Punnett Square
The Punnett Square
In a
heterozygous
mom some
gametes are
dominant
A
Dad
a
A
A
Dad
a
A
Mom
Mom
a
a
… and some
are recessive
What Darwin didn’t know: Mendel and basic genetics
What Darwin didn’t know: Mendel and basic genetics
The Punnett Square
The Punnett Square
A
A
AA
Dad
a
A
Aa
Mom
Dad
a
A
AA
Aa
a
aA
aa
Mom
a
aA
aa
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What Darwin didn’t know: Mendel and basic genetics
Let’s complicate things: Dihybrid crosses
What Darwin didn’t know: Mendel and basic genetics
Let’s complicate things: Dihybrid crosses
What Darwin didn’t know: Mendel and basic genetics
Let’s complicate things: Dihybrid crosses
What Darwin didn’t know: Mendel and basic genetics
Let’s complicate things: Dihybrid crosses
Dihybrid ratios:
9/16 round yellow
3/16 round green
3/16 wrinkled yellow
1/16 wrinkled green
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What Darwin didn’t know: Mendel and basic genetics
What Darwin didn’t know: Mendel and basic genetics
Dihybrid crosses – 556 seeds
Dihybrid crosses – 556 seeds
Traits
Genotype
Ratios
Real data
Perfect data
Traits
Genotype
Ratios
Real data
Perfect data
Round, yellow
R_, Y_
9/16
315
313
Round, yellow
R_, Y_
9/16
315
313
Round, green
R-, yy
3/16
108
104
Round, green
R-, yy
3/16
108
104
Wrinkled, yellow
rr, Y_
3/16
101
104
Wrinkled, yellow
rr, Y_
3/16
101
104
Wrinkled, green
rr,yy
1/16
32
35
Wrinkled, green
rr,yy
1/16
32
35
Dihybrid crosses lead to 9:3:3:1 ratios
Mendel had AMZINGLY good data
Even still biology is just random enough that there’s some
skew from perfect data
What Darwin didn’t know: Mendel and basic genetics
What Darwin didn’t know: Mendel and basic genetics
VERY GOOD PRACTICE!
Mendel’s
2nd
law of heredity
The Principle of Independent Assortment –
In a dihybird cross the alleles of each gene assort
independently.
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Extending Mendel
Extending Mendel
Do peas make it too easy?
Do peas make it too easy?
1
Factors assort independently
1
Factors assort independently
Gene linkage
2
Each trait is controlled by a single factor
2
Each trait is controlled by a single factor
Polygenic inheritance
3
Factors do not interact
3
Factors do not interact
Epistasis
4
Each factor only controls one trait
4
Each factor only controls one trait
Pleiotropy
5
Factors only have two alternative outcomes
5
Factors only have two alternative outcomes
6
Factors are unaffected by the environment they are
expressed in
Incomplete dominance
and codominance
6
Factors are unaffected by the environment they are
expressed in
Environmental effects
on expression
Extending Mendel
Factors don’t always assort independently
Genes are sometimes linked
Extending Mendel
Factors don’t always assort independently
Genes are sometimes linked
Mendel’s 2nd law of heredity
The Principle of Independent Assortment –
In a dihybird cross the alleles of each gene assort
independently.
Dihybrid ratios:
9/16 round yellow
3/16 round green
3/16 wrinkled yellow
1/16 wrinkled green
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Extending Mendel
Traits can be controlled by multiple
factors
Polygenic inheritance
Extending Mendel
Traits can be controlled by multiple factors
Polygenic inheritance
Mendel’s traits were all
two state cases
Extending Mendel
Traits can be controlled by multiple factors
Polygenic inheritance
Extending Mendel
Traits can be controlled by multiple factors
Polygenic inheritance
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Extending Mendel
Factors can interact
Extending Mendel
Factors can interact
Traits can be affected by more than one gene:
Epistasis
Traits can be affected by more than one gene:
Epistasis
The E gene determines presence or absence of melanin
ee
Extending Mendel
Factors can interact
EE or Ee(E_ )
Extending Mendel
Factors can interact
Traits can be affected by more than one gene:
Epistasis
Traits can be affected by more than one gene:
Epistasis
The B gene determines the distribution of melanin
ee
EE or Ee(E_ )
E_bb
The B gene determines the distribution of melanin
E_B_
eebb
E_bb
E_B_
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Extending Mendel
Factors can have multiple effects
Extending Mendel
Factors can have multiple effects
Pleiotropy
Pleiotropy
A sickled blood cell
Malaria entering a red blood cell
Extending Mendel
Factors can have more than two outcomes
Extending Mendel
Factors can have more than two outcomes
Dominance in not
always complete
Dominance in not
always complete
Incomplete dominance:
Parental phenotypes
blend in the heterozygote
Incomplete dominance:
Parental phenotypes
blend in the heterozygote
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Extending Mendel
Factors can have more than two outcomes
Extending Mendel
Factors can have more than two outcomes
Dominance in not always complete
Dominance in not always complete
Codominance: Heterozygotes show both parental
phenotypes UNBLENDED
Codominance
A
A A AA
Type A
Extending Mendel
Factors can have more than two outcomes
A A AA
B A BA
B
B
Type AB
B B BB
B
Type B
Example: temperature sensitive expression
Codominance
A
A
Extending Mendel
The environment can affect factor expression
Dominance in not always complete
B A BA
A
A
B
A
Type AB
B
B B BB
B
Type O
Type A
Type B
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Lecture 6
Chromosomes and recombination
What Mendel didn’t know:
Chromosomes and recombination
1.
2.
3.
4.
5.
Chromosomes are discovered and come in pairs
A brief introduction to mitosis and meiosis
Haploidy, diploidy, polyploidy
Sex chromosomes: an unusual pair
Recombination via crossing over
Mendel -1866
No knowledge of chromosomes,
genes, DNA
What does DNA do?
1.
2.
3.
4.
5.
6.
Understanding how we’ve gone from factors to DNA
Nucleic acids, the double helix, and a quick tour of DNA replication
The central dogma (DNA =>RNA=>aa=>Protein)
How central is it?
Codons and translation: A universal code
Closely related species have similar proteins and DNA
Chromosomes and recombination
Chromosomes
Discovered in 1879
Chromosomes and recombination
Chromosomes
Chromosome number differs across taxa
Linked to inheritance in
1900-1902
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Chromosomes and recombination
Chromosomes
Chromosomes and recombination
Karyotypes
Chromosome number differs across taxa
Humans have 46 chromosomes total
23 from mom
23 from dad
Chromosomes and recombination
Terminology
Chromatid
Sister chromatids
Centromere
Homologous pair
Chromosomes and recombination
Karyotypes
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Chromosomes and recombination
Mitosis in brief
Chromosomes and recombination
Meiosis in brief: Meiosis 1 (phase 1)
Check out figure 11.10 in your text!
Chromosomes and recombination
Meiosis in brief: Meiosis 2 (phase 2)
Chromosomes and recombination
Meiosis in brief
Start with 2x DNA
Divide once
normal amount of DNA
Divide again
half as much
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Chromosomes and recombination
Ploidy = number of copies of chromosomes
A cell with one copy is HAPLOID (1N)
Chromosomes and recombination
Ploidy
Mitosis
2N
4N
2N
Meiosis
2N
4N
2N
A cell with two copies is DIPLOID (2N)
More than two copies is polyploid (xN)
1N
Check out figure 11.19 in your text!
Chromosomes and recombination
Sex chromosomes
Chromosomes and recombination
Crossing over
One of these pairs is
not like the others
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Chromosomes and recombination
Crossing over – Another approach
Chromosomes and recombination
Back to gene linkage:
Mom
AABB
Dad
aabb
Chromosomes and recombination
Recombination leads to
genetic variation
Chromosomes and recombination
Back to gene linkage:
Mom
AABB
Dad
aabb
F1
AaBb
When genes are unlinked,
the F1 generation can
produce four gamete
combinations:
F1
AaBb
a
A
B
b
When traits are linked, A will not appear
with b and B will not appear with a
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What does DNA do?
What does DNA do?
James Watson and Francis Crick, 1953,
the structure of DNA
Oswald Avery, Maclyn McCarty, Colin MacLeod, 1944,
genes are made of DNA
What does DNA do?
What does DNA do?
Four nucleotides:
The double
helix
Adanine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
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What does DNA do?
CTAGAGC
What does DNA do?
How does DNA
replicate?
What does DNA do?
How does DNA
replicate?
What does DNA do?
How does DNA replicate?
Semi-conservatively
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What does DNA do?
DNA
What does DNA do?
The central dogma
DNA
RNA
RNA
Protein
What does DNA do?
DNA
The central dogma
Protein
What does DNA do?
The central dogma
The different stages of the central dogma occur in
different parts of the cell
Transcription
RNA
Translation
Protein
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What does DNA do?
What does DNA do?
Does one gene make one protein?
What is a gene?
RNA
Other molecules control transcription?
* Always single stranded
How central is the
central dogma?
* Uracil
What role does RNA really play in the cell?
Epigenetic markers? What do they do?
What does DNA do?
What does DNA do?
What is a protein?
All proteins are different
combinations of 20
amino acids!
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What does DNA do?
What does DNA do?
All proteins are different
combinations of 20
amino acids!
What does DNA do?
How does that work?
What does DNA do?
Some codons are special
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What does DNA do?
What does DNA do?
The universality of the genetic code is one of the
strongest pieces of evidence that all living
organisms share a common evolutionary heritage
5 nucleotides
20 amino acids
What does DNA do?
What does DNA do?
Cytochrome C sequence comparisons
Comparison species
Number of amino acids changed
Chimpanzee
0
Rhesus monkey
1
Dog
11
Rattlesnake
14
Bullfrog
18
Tuna fish
21
Fruit fly
29
Pumpkin
36
Bacteria
56
Cytochrome C
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What does DNA do?
What does DNA do?
DNA sequence comparisons with humans
Comparison species
% of genes with similar
function
Chimpanzee
98
Mouse
90
Fruit fly
67
Plants
33
It’s a lot to ponder
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