modern synthesis concepts from laboratory genetics p = g + e phenotype = genotype + environment 1....
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Modern Synthesis concepts from Laboratory Genetics
P = G + EPhenotype = 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.
Chromosomes, Genes, and Alleles
Traits are determinedby genes (primarily)
Different forms of a gene
are called alleles
Many different genes (loci)per chromosome
Locus A
Locus B
Locus C
Genetically “simple,” single locus traits: Mendel’s peas
Seed Shape
Seed Color
Flower Color
Pod Shape
Pod Color
Flower Position
Plant Stature
Trait or Charcteristic
Dominance / Recessivity
aa Aa AA aa Aa AA
Additivity
PhenotypePhenotype
Mode of Gene Action
Discrete Phenotypes Continuous Phenotypes
Quantitative traits depend on multiple underlying loci
one locusone locus + environment
two loci + environment
four loci + environment
many loci + environment
DiploidAdult
C E
Allele for Brown Coat
Allele for Black Eyes
C
C
c
c
E
E
e
e
C E
What if Mate these ?
What will the offspring’s genotype be?
Cc Ee
C
C
c
c
E
E
e
e
C
c
e
e
DNAReplication
Hybrid Mouse Genotype
Cc Ee
C
c
E
eSister
chromatids
Sister chromatids{Homologous
Pair
{HomologousPair
Genetic recombination:chromosomal segments are exchangedbetween homologues during Meiosis I
C
C
c
c
E
E
e
e
Genetic recombination: New combination of alleles
C
C
c
c
E
E
e
e
C
C
c
c
E
E
e
e
Separation of DNA
Meiosis II
Meiosis II
C
C
c
c
E
E
e
e
Meiosis I
GAMETES
C
C
c
c
E
E
e
e
Sperm Gametes
C
C
c
c
E
E
e
e
ALL
POSSIBLE
Egg Gametes Genotype
CC/EE
Cc/EE
CC/Ee
Cc/Ee
WHAT GENOTYPES IF MATE TWOHYBRID MICE (Cc / Ee)?
CCEE CCEe
CCEe
CcEeCcEE
CcEE
CcEe
CCee
CcEe
CcEe
Ccee
Ccee
ccEE
ccEe
ccEe
ccee
CE
CE Ce
Ce
ce
ce
cE
cE
AllPossibleSpermGametes
All Possible Egg Gametes
Cc / Ee x Cc / Ee
I. Law of Segregation
c
C C
c
E e
E e
II. Law ofIndependentAssortment
Mendel’s 1st law: Characters are controlled by pairs of genes (alleles) which separate during the formation of the reproductive cells (meiosis)
Mendel’s 2nd law: When two or more pairsof genes (alleles) segregate simultaneously, they doso independently.
“Exceptions” to Mendel’s Second Law
From Thomas Hunt Morgan (1909): 2,839 flies
Eye color A: red a: purpleWing length B: normal b: vestigial
AABB x aabb
AaBb x aabb
AaBb Aabb aaBb aabbExp 710 710 710 710Obs 1,339 151 154 1,195
Morgan’s explanation
A A
B B
a a
b b
F1: A a
B b
a a
b b
F2:A a
B b
a a
b b
A a
b b
a a
B b
Crossover has taken place
Parental types: AaBb, aabbRecombinants: 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).
A a
B b Meiosis
Probability of recombination = 0.3
A aB b Meiosis
Probability of recombination = 0.1
AB
Ab
aB
ab
All allele combinations in gametes NOTequally probable
= 0.15
= 0.15
= 0.35
= 0.35
AB
Ab
aB
ab
= 0.05
= 0.05
= 0.45
= 0.45
Linked Loci
All allele combinations in gametes NOTequally 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.