Transcript Purple is dominant to white A

Genetics can be used to characterize
biological pathways
Complementation tells us if variation
is due to mutations in one gene
or several genes.
Epistasis tells which gene products
are involved in common pathways
and
which act earlier or later in a process.
What are the relationships
between color types?
X
purple
RR
purple
Rr
white A
rr
Relationship between
2 chosen color variants
Purple is dominant to white A
Purple is dominant to White1
purple
RR
F1
F2
1 RR,
X
white A
rr
X
purple
Rr
2Rr
and
1rr
Punnet square
Male
gametes
R
r
Female gametes
r
R
RR
Rr
Rr
rr
What are the relationships
between color types?
X
purple
RRPP
Purple
RrPP or
RRPp
Purple is dominant to red
Red
rrPP or
RRpp
Complementation test
Cross two recessive mutants to determine if
the mutations are in one gene or more than one.
X
white A
rrPP
Purple
RrPp
red
rrPP or
RRpp
Red and white A
are caused by
mutations in
different genes
Epistasis
Two genes for flower color
Are they two steps in the same pathway
to make pigment?
Where are the two genes in the pathway?
1. Purple is either a mixture of
blue and red pigments each made in a separate
biochemical pathway.
or
2. Purple results from modification of
the same precursor from a white precursor to a
red intermediate and finally a purple pigment.
We can use genetics to distinguish the two possibilities.
The effect of variant alleles in multiple genes that
affect pigment in combination will answer the question.
Pathway 2
Pathway 1
Precursor 1
Precursor 2
R
R
P
Blue
Precursor 1
Red
Red
P
Coexpression of
blue and red pigment
Purple
derived from different precursors
makes purple.
Modification of the same
precursor leads to first
a red pigment and then
a purple pigment
Epistasis test
Start with complementation test:
Cross two recessive mutants to determine if
the mutations are in one gene or more than one.
X
White A
rr
Purple
Rr Pp
Red
pp
Epistasis test part 2
Cross F1 plants from the complementation test
And follow how the different alleles segregate in the
F2 generation.
X
Purple F1
Rr Pp
Purple F1
Rr Pp
?
Punnet Square:
two genes with randomly segregating alleles
Male
gametes
Female gametes
RP
Rp
rP
rp
RP RRPP RRPp RrPP RrPp
Rp RRPp RRpp RrPp Rrpp
rP RrPP RrPp rrPP rrPp
rp RrPp Rrpp rrPp rrpp
RrPp
X
RrPp
9R_P_
3R_pp
3rrP_
1rrpp
If Pathway 1
Precursor 1
Precursor 2
R
P
Red
Blue
Coexpression of
Blue and red pigment
derived from different precursors
Makes purple
9R_P_
3R_pp
Recessive alleles
Lead to lack of either
Red or blue pigment
3rrP_ 1rrpp
Phenotypes:
purple
red
blue
white
Relationship between white a and red
X
white A
rrPP
red
RRpp
X
F1 is all purple
RrPp
F2
9
3
4
Pathway 2
Modification of the same
precursor leads to first
a red pigment and then
a purple pigment
Precursor 1
R
rr - get no red precursor
neither purple nor
red pigment can be made
Red
P
pp – can get red pigment
if correct R alleles are present
but not purple
Purple
F2: 9R_P_
Phenotypes:
purple
3R_pp
3rrP_
red
white
1rrpp
white
R is epistatic to P
Mutations in the R gene cover the effect
of mutations in the P gene.
This is because R is upstream of P in a biological pathway
The P protein requires the wild type function of the
R protein.
R can be a regulator required to activate expression of P
or
R can be an enzyme upstream in a biochemical pathway
Using multiple allelism tests with
diverse recessive mutants,
We can identify all the genes specifically
involved in making the purple pigment
Genetics can be used to determine the
order of steps in a biological pathway
Epistasis tells which gene products
are involved in common pathways
and
which act earlier or later in a process.
Mouse as a model for mammalian genetics
Origins of Mouse Genetics
Early domestication by Greeks and Romans
Chinese and Japanese fondness for unusual-looking mice
Early 19th century-popular objects of fancy in Europe
Early 20th century-English and American mouse fanciers
Early pioneers included LC Dunn, Clarence Little,
Sewall Wright, and George Snell
Why Mice As an Experimental Organism?
Hardy
Requires little space
Short life cycle
Easily bred
High fecundity
Mammalian species
Large amount of phenotypic variation
Easy to genetically engineer
Evolutionary Relationships
Humans
Mice
Xenopus
D. melanogaster
C. elegans
1000
900
800
700
600
500
400
300
200
100
0 myr bp
A mouse is not a mouse is not a mouse
Hundreds of strains
Great phenotypic diversity
Variation exceeds that in the
human population
Why is there biological concordance
for human and mouse
Evolutionary conservation!!
genome (gene content,
arrangement and
sequence)
structure (gross and
molecular anatomy)
function (physiology and
molecular circuits)
regulatory systems
Why is there biological concordance
for human and mouse
Evolutionary conservation!!
Important loci
represent a finite set of
key regulatory genes
“Key” means location in
the regulatory network
(nodes)
Engineered Models
Allows controlled experimental testing of
• specific genes
• specific environmental conditions
or exposures
Ideally suited to test specific hypothesis
generated from human population
studies or other laboratory findings
Engineered Models
Transgenics
• usually used to over-express genes
• can be global or tissue-specific
• can be temporally regulated
Knockouts/knockins
• usually used in inactivate genes
• can be global or tissue-specific
• can be temporally regulated
• can introduce genes into a foreign locus
• can make amino acid modifications
UV Mutagenesis in Yeast
Geneticists need variation to study the function of gene products.
We create variation in the laboratory by mutagenesis
Fig. 7.2
Fig. 7.6
Fig. 7.12b1
Fig. 7.12b2
By choosing the correct mutagens,
we can control the type of mutations we make
Fig. 7.7
Photoreactivation
requires photolyase enzyme
Mutagenesis of yeast
haploid
Irradiate with UV. Calculate survival curve
Select optimal dose for isolation of mutations.
Select on appropriate selective media:
Replica plating to identify nutrient deficiencies.