Transcript Purple is dominant to Red

Biol 423L Laboratories in Genetics
Rules: Cell phones off
Computers only for class related work
No food or drink in lab room
Text Book: Hartwell et al., 2nd Edition 2004
Genetics from Genes to Genomes Mc Graw-Hill, Boston.
Web page: www.bio.unc.edu/courses/2006Fall/Biol423L
Goals for course:
Reinforce basic genetic principles
Introduce model organisms commonly
used by geneticists
Learn how genetics is used to understand
Disease
Biochemical pathways
Development
Lab reports:
Abstract
Introduction
Results
Discussion
Course information page has instructions
about preparing your lab reports.
Grading:
Lab Reports: 50% of grade
5% of that is participation
1 day late, 50% off
more than that will only be
graded under special circumstances.
Research Paper: 10% of grade
Topics due Oct. 9.
Outline due Oct. 23.
Paper due Nov. 29
2 quizzes: 10% each of final grade.
Oct. 2 and Nov. 13.
Final exam: 20% of final grade
comprehensive
Dec. 11.
Genes Alleles and Epistasis
Genetics starts with observation
Observe variability
Use genetics to understand the cause
of the variability.
What proteins or RNAs are
responsible for the variability you can see?
Easy example, flower color
How many genes affect flower color?
How variable are the proteins
encoded by those genes?
What is the pathway to make flower color?
List of terms:
Trait: some aspect of an organism that can be observed, measured
Phenotype: the way a trait appears in an individual, the combination
of genotype and environment.
Genotype: the constitution of alleles at any gene in an individual.
Gene: continuous stretch of DNA sufficient to encode a messenger
RNA or a functional RNA.
Locus: A region of a chromosome, usually for a single gene.
Messenger RNA: the RNA message for a single protein.
Allele: a variant of the sequence of a given gene.
Diploid: an individual with two copies of each chromosome.
Haploid: an individual with one copy of each chromosome.
How many genes affect flower color?
How variable are the proteins
encoded by those genes?
What is the pathway to make flower color?
First make sure the types are heritable
and true breeding
(homozygous for flower color alleles)
purple by purple (self)
All uniform
X
Homozygous: a diploid individual with two copies of the same
allele for a given gene.
Heterozygous: a diploid individual with two different alleles for
a given gene.
What are the relationships
between color types?
X
Purple is dominant to Red
What are the relationships
between color types?
X
Purple is dominant to white
1. How many genes are required to make
purple pigment in flowers?
Complementation tests can be made
between recessive alleles.
A dominant allele cannot be used.
Why?
Allelism test:
Cross different white flowered plants
If the mutations are in the same gene,
The progeny will be white
X
White 1
White 1
Allelism test:
Cross different white flowered plants
If the mutations are in different genes,
The progeny will be pigmented
X
White 1
White 2
Allelism or complementation test
If plants with recessive alleles are
crossed and the progeny also have
the recessive trait,
The alleles are variants of the same gene
If plants with recessive alleles are
crossed and the progeny have
the dominant trait,
The alleles are variants of different genes
Pathway to purple
Precursor 1
White1
Intermediate
White2
Purple
White 2
Purple
X
White 1
rrAA
RRaa
RrAa
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
act earlier or later in a process.
What are the relationships
between color types?
X
Purple is dominant to White 1
Purple is dominant to White1
Purple
RR
F1
F2
1 RR, 2Rr
X
White1
rr
X
Purple
Rr
and
1rr
Punnet square
Male
gametes
R
r
Female gametes
r
R
RR
Rr
Rr
rr
What are the relationships
between color types?
X
PP
Pp
Purple is dominant to Red
pp
Epistasis
Two genes for flower color
Two steps in a pathway to make pigment
Where are the two genes in the pathway?
Purple is either a mixture of
blue and red pigments
or
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
Relationship between White1 and Red
X
White1
rrPP
Red
RRpp
X
F1 is all Purple
RrPp
F2
9
3
4
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
9R_P_
3R_pp
3rrP_ 1rrpp
RrPp
X
RrPp
9R_P_
3R_pp
3rrP_ 1rrpp
Phenotypes:
Purple
Red
White
Precursor 1
White
White - no pigment
R
No R - get no red precursor
Neither purple nor
red pigment can be made
P
No P – get red pigment
but not purple
Red
Purple
If Pathway 1
Precursor 1
Precursor 2
R
P
Blue
Red
Coexpression of
Blue and red pigment
derived from different precursors
Makes purple
9R_P_
3R_pp
3rrP_ 1rrpp
Phenotypes:
Purple
Red
Blue
White
R no P would make red pigment only
No R but P would make blue pigment,
Fact that loss of R changes phenotype to rr even if
Functional P is expressed indicates that P and R affect
a common pathway and
R is before P in the pathway to make pigment.
Yeast complementation test for next week:
Brewers Yeast
Saccharomyces cerevisiae:
16 chromosomes
12,052 kb DNA
6183 ORFs
About 5800 expected to encode proteins
Yeast is a very useful model for genetics
because of its life cycle
Haploid life cycle
Yeast is a very useful model for genetics
because of its life cycle
Mating cycle
Diploid
We can isolate mutants as haploids
We can test the mutations for allelism by
a complementation test
Two haploids are mated. The resulting
Diploid has both mutations.
Either the mutations are allelic
and do not complement,
or they are mutations in two different genes
and they do complement.
a2
a1
a1
Select mutants that are defective in
Adenine synthesiscannot grow without adenine in medium.
Turn red on media with adenine
because an adenine precursor accumulates.
X
a1
a2
X
a1
a1
a2
a1
a1
Which mating results in complementation?
Lab experiment: Corn kernels
R and P
RR or Rr is full color
rr is no color
PP or Pp are full color
pp is weak color
Question is pprr weak color?
P epistatic to R or
Is pprr no color?
R epistatic to P
End