Modern Synthesis concepts from Laboratory Genetics

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Modern Synthesis concepts from Laboratory Genetics
P G E Phenotype Genotype Environment
1.
2. Environmental effects on phenotype are not
inherited 3. Heredity is based on particles
(genes). They retain identity and do not
blend. They give rise to continuous and
discrete variation. 4. Genes mutate at
relatively low rates. Recombination is an
important aspect of evolution. 5. Environmental
factors can influence mutation rates but do
not yield specific mutations that are seemingly
the best for surviving in a particular
environment.
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Chromosomes, Genes, and Alleles
Traits are determined by genes (primarily)
3
Genetically simple, single locus traits
Mendels peas
Trait or Charcteristic
Seed Shape
Seed Color
Flower Color
Pod Shape
Pod Color
Flower Position
Plant Stature
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Mode of Gene Action
Dominance / Recessivity
Additivity
Phenotype
Phenotype
aa Aa AA
aa Aa AA
Discrete Phenotypes
Continuous Phenotypes
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Quantitative traits depend on multiple underlying
loci
one locus environment
two loci environment
one locus
four loci environment
many loci environment
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Diploid Adult
C
E
Allele for Brown Coat
Allele for Black Eyes
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What if Mate these ?
What will the offsprings genotype be?
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Hybrid Mouse Genotype
Cc
Ee
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Genetic recombination chromosomal segments are
exchanged between homologues during Meiosis I
E
C
E
c
C
e
c
e
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Genetic recombination New combination of alleles
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Separation of DNA
E
C
G A M E T E S
c
E
C
e
c
e
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WHAT GENOTYPES IF MATE TWO HYBRID MICE (Cc / Ee)?
Cc/EE
CC/Ee
Cc/Ee
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I. Law of Segregation
Cc / Ee x Cc / Ee
II. Law of Independent Assortment
CCEE
CCEe
CcEe
CcEE
CCEe
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Mendels 1st law Characters are controlled by
pairs of genes (alleles) which separate during
the formation of the reproductive cells (meiosis)
Mendels 2nd law When two or more pairs of genes
(alleles) segregate simultaneously, they do so
independently.
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Exceptions to Mendels Second Law
From Thomas Hunt Morgan (1909) 2,839 flies Eye
color A red a purple Wing length B normal b
vestigial
AABB x aabb
AaBb x aabb
AaBb Aabb aaBb
aabb Exp 710 710 710
710 Obs 1,339 151 154
1,195
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Morgans explanation
F1
F2
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Parental types AaBb, aabb Recombinants Aabb,
aaBb
The proportion of recombinants between the two
genes (or characters) is called the
recombination fraction between these two genes.
It is usually denoted by r or ?. For
Morgans traits r (151
154)/2839 0.107 If r lt 1/2 two
genes are said to be linked. If r
1/2 independent segregation
(Mendels second law).
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Linked Loci
Probability of recombination 0.3
0.35
ab
A
a
Meiosis
0.15
aB
All allele combinations in gametes NOT equally
probable
B
b
Ab
0.15
AB
0.35
Probability of recombination 0.1
0.45
ab
A
a
b
B
Meiosis
All allele combinations in gametes NOT equally
probable
0.05
aB
Ab
0.05
AB
0.45
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Concept The closer two loci are on a
chromosome, the lower the probability of
recombination.
Why important? (1) Allows one to determine the
linear order of genes on a chromosome (make a
genome map). (2) Maps allow for the localization
of genes, mutant phenotypes, and QTL in the
genome.