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Modern Synthesis concepts from Laboratory Genetics
1.
P=G+E
Phenotype = Genotype + Environment
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.
Chromosomes, Genes, and Alleles
Locus A
Traits are determined
by genes (primarily)
Many different genes (loci)
per chromosome
Locus B
Locus C
Different forms of a gene
are called alleles
Genetically “simple,” single locus traits:
Mendel’s peas
Trait or Charcteristic
Seed Shape
Seed Color
Flower Color
Pod Shape
Pod Color
Flower Position
Plant Stature
Mode of Gene Action
Dominance / Recessivity
Phenotype
Additivity
Phenotype
aa
Aa
AA
Discrete Phenotypes
aa
Aa
AA
Continuous Phenotypes
Quantitative traits depend on multiple
underlying loci
one locus +
environment
one locus
four loci +
environment
two loci +
environment
many loci +
environment
Diploid
Adult
C
Allele for
Brown Coat
E
Allele for
Black Eyes
What if Mate
these ?
C
E
What will the offspring’s genotype be?
c
E
C
Cc
e
Ee
c
e
C
E
Hybrid Mouse
e
C
Genotype
Homologous
Pair
{
Cc
Ee
c
e
C
E
DNA
Replication
c
E
Homologous
Pair
{
CC
c
c
Ee
e
e
Sister
chromatids
Sister
chromatids
Genetic recombination:
chromosomal segments are exchanged
between homologues during Meiosis I
E
C
c
c
E
C
e
e
Genetic recombination:
New combination of alleles
C
E
c
E
C
e
c
e
Separation of DNA
E
C
C
E
Meiosis II
c
E
c
E
e
C
e
Meiosis I
C
Meiosis II
c
e
c
e
G
A
M
E
T
E
S
WHAT GENOTYPES IF MATE TWO
HYBRID MICE (Cc / Ee)?
Egg Gametes
Sperm Gametes
Genotype
A
L
L
C
E
C
E
CC/EE
P
O
S
S
I
B
L
E
c
E
c
E
Cc/EE
C
e
C
e
CC/Ee
c
e
c
e
Cc/Ee
cE
Cc / Ee
CE
I. Law of
Segregation
ce
x
Cc / Ee
II. Law of
Independent
Assortment
Ce
All Possible Egg Gametes
CE
Ce
cE
ce
CE
CCEE
CCEe
CcEE
CcEe
All
Ce
Possible
Sperm
cE
Gametes
CCEe
CCee
CcEe
Ccee
CcEE
CcEe
ccEE
ccEe
ce
CcEe
Ccee
ccEe
ccee
Mendel’s 1st law: Characters are controlled
by pairs of genes which separate during the
formation of the reproductive cells (meiosis)
Mendel’s 2nd law: When two or more pairs
of genes segregate simultaneously, they do
so independently.
“Exceptions” to Mendel’s Second Law
From Thomas Hunt Morgan (1909): 2,839 flies
Eye color
A: red
Wing length B: normal
a: purple
b: vestigial
AABB
aabb
x
AaBb
Exp
Obs
AaBb
710
1,339
x
Aabb
710
151
aabb
aaBb
710
154
aabb
710
1,195
Morgan’s explanation
A
A
B
a
a

B
F1:
b
b
A
a
B
a
a

b
b
b
F2:
A
a
B
b
b
a
A
a
a
b
Crossover has taken place
b
a
a
b
B
b
Parental types:
Recombinants:
AaBb, aabb
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 Morgan’s traits:
r = (151 + 154)/2839 = 0.107
If r < 1/2: two genes are said to be linked.
If r = 1/2: independent segregation
(Mendel’s second law).
Linked Loci
Probability of recombination = 0.3
A
a
B
b
= 0.35
Meiosis
= 0.15
= 0.15
All allele combinations
in gametes NOT
equally probable
= 0.35
Probability of recombination = 0.1
A
B
a
b
= 0.45
Meiosis
= 0.05
= 0.05
= 0.45
All allele combinations
in gametes NOT
equally probable
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.