Rules: Cell phones off

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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/2007Fa
ll/Biol423L
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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
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Lab reports Abstract Introduction Results D
iscussion Course information page has
instructions about preparing your lab reports.
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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. 15. Outline due Oct.
29. Paper due Nov. 26. 2 quizzes 10 each of
final grade. Oct. 8 and Nov. 5. Final exam 20
of final grade comprehensive Dec. 8.
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Genes, Alleles and Epistasis
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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?
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Easy example, flower color
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How many genes affect flower color? How variable
are the proteins encoded by those genes? What
is the pathway to make flower color?
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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.
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How many genes affect flower color?
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First make sure the types are heritable and true
breeding (homozygous for flower color alleles)
All uniform
purple by purple (self)
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.
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What are the relationships between color types?
X
purple is dominant to white
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P could encode an enzyme needed to convert a
precursor to purple pigment
Precursor
P protein
Pigment
Hypothesis purple plants have an enzyme P that
is needed to convert a precursor to
purple pigment. White plants have a mutant
allele that makes a defective P protein. They
cannot convert the precursor to purple pigment
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Alleles are distributed as Discrete units (quanta)
X
F1 P/pa
White A pa/pa
Purple P/P
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Punnet square helps to predict genotypes and
phenotypes of the next generation
Two distinct alleles at the same locus
X
F1 P/pa
F1 P/pa
1 P/P 2 P/pa 1 pa/pa 3 purple 1 white
Female gametes
pa
P
Male gametes
P/P
P/pa
P
pa
pa/pa
P/pa
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How many genes are required to make purple
pigment in flowers?
Complementation tests can be made between
recessive alleles.
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
A dominant allele cannot be used. Why?
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Allelism test 1 Cross different white flowered
plants
If the mutations are in the same gene, The
progeny will be white
X
F1 pa/pb
white A pa/pa
white B pb/pb
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Complementation test double check
F2 generation Cross white F1 to another white
F1 If the mutations are alleles of the same gene,
what is the next generation?
pa/pb
X
1 pa/pa, 2 pa/pb, 1 pb/pb
pa/pb
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Allelism test 2 Cross different white flowered
plants
If the mutations are in different genes, the
progeny will be pigmented
X
white A pa/pa
white C pc/pc
F1 P/pa P/pc
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Conclusions
P and pa are alleles of the same gene
pa and pc are alleles of different genes. The
dominant allele of pa and the dominant allele of
pc are needed for purple color to be
produced. Therefore, at least 2 gene products
are needed to produce purple pigment.
To avoid confusion, lets call P and pa R and r
and pc p with a dominant allele P.
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Allelism test Cross different white flowered
plants
If the mutations are in different genes, The
progeny will be pigmented
X
white A r/r P/P
white C R/R p/p
F1 R/r P/p
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white C
purple
X
white A
rrPP
RRpp
RrPp
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Pathway to purple
Precursor 1
R or P
Intermediate
R or P
Purple
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Complementation test double check
The discrete alleles of two different genes Will
assort randomly in future generations
X
white A r/r P/P
white B R/R p/p
F1 R/r P/p
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Punnet Square Predict the genotypes and
phenotypes in the F2 generation when the trait
is controlled by two genes with randomly
segregating alleles
F2 after RrPp X RrPp
Female gametes
Male gametes
rp
Rp
rP
RP
RRPP
RRPp
RrPP
RrPp
RP
9R_P_ 3R_pp 3rrP_ 1rrpp Phenotypes if both R
and P needed for purple color
RRPp
RRpp
RrPp
Rrpp
Rp
rP
RrPP
RrPp
rrPP
rrPp
9 purple and 7 white
rp
rrpp
RrPp
Rrpp
rrPp
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Punnet Square Predict the genotypes and
phenotypes in the F2 generation when the trait
is controlled by two genes with randomly
segregating alleles
RrPp X RrPp
Female gametes
Male gametes
rp
9R_P_ 3R_pp 3rrP_ 1rrpp Phenotypes if R and P
affect independent traits Eg. petal color and
leaf size R- is purple, rr is white P- is long
leaf and pp is short leaf
Rp
rP
RP
RrPp
RRPP
RRPp
RrPP
RP
RRPp
RRpp
RrPp
Rrpp
Rp
rP
RrPP
RrPp
rrPP
rrPp
9 purple, long 3 white, long 3 purple, short 1
white, short
rp
rrpp
RrPp
Rrpp
rrPp
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Using multiple allelism tests with diverse
recessive mutants, We can identify all the
genes specifically involved in making the purple
pigment
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Yeast complementation test for next week
Brewers Yeast Saccharomyces cerevisiae 16
chromosomes 12,052 kb DNA 6183 ORFs About 5800
expected to encode proteins
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Yeast is a very useful model for genetics
because of its life cycle
Haploid life cycle
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Yeast is a very useful model for genetics
because of its life cycle
Mating cycle Diploid
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Advantages of yeast for identification of genes
in a biosynthetic pathway
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.
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a2
a1
a1
Select mutants that are defective in Adenine
synthesis- cannot grow without adenine in
medium. Turn red on media with adenine because
an adenine precursor accumulates.
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X
a1
a1
a1
a2
a2
X
a1
a1
Which mating results in complementation?
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End