The Benefits of Sex

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The Benefits of Sex
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• The Benefits of Sex
• Meiosis generates genetic variation by
  independent
• assortment and crossing over
• Two copies of each gene buffers the individual
  from
• deleterious (bad) genes
• Two copies of each gene releases one copy to vary
  more
• than it could in a haploid organism. This
  provides a source
• of variation for the population, ready to respond
  to
• environmental change/challenge.
• sex allows beneficial combinations of alleles to
  evolve
• more quickly than in a haploid species

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• Meiosis generates genetic variation by
  independent assortment and crossing over

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• sex allows beneficial combinations of alleles to
  evolve more quickly than in a haploid species

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Benefits of no sex -rapid reproduction rate.
Rapid adaptation to and colonization of a newly
created niche, for example, a milk spill.
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From Chromosomes and Meiosis to Transmission
Genetics
Genes lie on Chromosomes, at locations called Loci
There are different versions of the same gene,
called alleles A and a Let A the yellow seeded
pea allele Let a green seeded pea allele
Ch. 2 in your book
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Mendels pea crosses share a basic strategy

• begin with 2 plants that differ in their
  phenotype with
• respect to some character, like pea color. Each
  plant must
• come from a pure line, all ancestors have the
  same phenotype

• cross these plants, this is the Parental
  generation (P)

• record the distribution of traits in the
  progeny, called the
• First Filial Generation (F1).

• for Mendels traits, F1 plants share the same
  phenotype
• as one of the parents, this is the dominant
  phenotype.

• Cross F1 plants. The recessive phenotype
  reappears in their
• progeny (F2), in a 13 ratio with the dominant
  phenotype.

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Mendel could explain these results in a simple
way Let A denote the allele of the gene coding
for the dominant Phenotype, and a the recessive
phenotype.

• Genes must come in pairs, so the P1 dominant
  parent is AA,
• and the recessive parent is aa.
• Pairs separate when parents make eggs and sperm,
  and
• recombine at random when sperm fertilize eggs
• One copy of the dominant allele is sufficient to
  make the
• dominant phenotype AA has the same phenotype as
  Aa.
• Two copies of the recessive allele are needed to
  make the
• recessive phenotype.

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• This separation and recombination can be
  performed using a Punnet Square

A yellow seed allele a green seed
allele P AA x aa
Moms Gametes
F1 A A a
a
Aa Aa Aa Aa
Dads gametes
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F2
A a
A a
Align moms gametes On horizontal Align dads
gametes On vertical Combine in square
AA Aa Aa aa
A is dominant to a, thus there is a 31 ratio of
phenotypes 3 yellow seeded offspring to 1
green seeded offspring
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Mendel examined 7 different pea plant
characters Seed Shape, Seed Color, Flower Color,
Pod Shape, Pod color, Flower Position, and Stem
length. Each showed 2 alternate
phenotypes Using his crossing strategy, each
produced 31 F2 phenotypic ratios
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F2 phenotypic ratios provide essential
information about the genetic basis of a
phenotype.
P Cross 2 pure lines AA and aa, Homozygotes
same allele on both homologs
F1 All F1s are Aa, Heterozygotes different
allele on each homolog
F2 phenotypes are in a 31 ratio, which provides
critical information about the genetic basis of
the phenotype A single gene with two alleles
in a dominance relationship is responsible for
the phenotype
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Molecular basis of Mendels phenotypes Alleles
(A, a) are alternate forms of a Gene, Gene
A Different alleles have different DNA and
amino acid sequences ATG CGC CCC GGG ATG
CGC CCC TGG M R P G
M R P W Resulting in slightly
different proteins
AA aa
And different pea phenotypes
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Cloning and characterization of the R gene, the
gene that determines pea seed shape in Mendels
experiments

• Transform 10,000 wild-type, homozygous, pure line
  pea plants (ie. AA BB CC DD, etc.) with a
  transposon of known sequence

Transposons genes that are flanked by bits of
DNA called P-elements, that can be inserted into
the genome using an enzyme called transposase
P
P
transposase
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The transposon will insert into the genome at
random, interrupting one of the 10,000 genes,
eliminating its function (ie. A turns into a, and
a is a non-functional allele) Pea 1
transposon interrupts gene A, resulting in Aa
genotype Pea 2 transposon interrupts gene Q,
resulting in Qq genotype Pea 3 transposon
interrupts gene J, resulting in Jj genotype
Etc. 2) Self-fertilize each plant, obtaining
plants with two copies of the interrupted gene
homozygotes Aa x Aa -gt 3A- to 1 aa 3) Keep
plants that have wrinkled seeds, for example, let
Pea18rr 4) Identify the interrupted gene by DNA
sequencing, what protein does it make?
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I know the site of the interrupted gene that
resulted in my wrinkled pea phenotype, it is
located at the site of transposon insertion. I
make a primer, a 20bp single stranded DNA that
matches, is complementary, to the transposon
sequence. I heat my pea DNA to 95oC, which
breaks the hydrogen bonds holding the double
helix together, generating two single stranded
DNA molecules. My primer finds and binds its
match, resulting in a short stretch of double
stranded DNA. DNA polymerase recognizes and
binds dsDNA. Nucleotides are added 5 to 3
during DNA synthesis. There are two
kinds Deoxyribonucleotides normal bases, -OH
on Carbon 3 Dideoxyribonucleotides
color-labeled bases with an H on Carbon 3.
Without the OH, the chain cannot extend.
5
3
5
1
4
2
3
DNA synthesis
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5

3
atgccgcccttgatcgccgtttcgggcttaatcatgcattcgta
Transposon red Pea r gene - black

tacggcgggaactagcggcaaagcccgaattagtacgtaagcat
cccttgatcgccgtt - primer

tacggcgggaactagcggcaaagcccgaattagtacgtaagcat
cccttgatcgccgtttcggcttaatcatgcat

1 2 Etc.
tacggcgggaactagcggcaaagcccgaattagtacgtaagcat
cccttgatcgccgtttcggcttaa

tacggcgggaactagcggcaaagcccgaattagtacgtaagcat
a a a a a a a a a a a a a a a a a a a a a a g g
g g g g g g g g g g g g g g g g g g c c c
c cc c cc c c c c c c c c c cc c c c c c cc c c
c c t t t t t t t t t t t t t t t t t t t t
Nucleotide pool
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cccttgatcgccgtttcg
cccttgatcgccgtttcgg
cccttgatcgccgtttcggc
cccttgatcgccgtttcggct
cccttgatcgccgtttcggctt
cccttgatcgccgtttcggctta
Separate products on agarose gel by their length,
this gives the sequence Of the R gene GGCTTA..
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R SBEI starch branching enzyme. Peas missing
this gene produce a form of starch that lacks
amylopectin, a branched form of amylose (a
polymer of glucose). Less starch alters osmotic
pressure in the seed such that water leaves the
embryo (seeds are embryos) and the seed is
wrinkled.
Amylose -gt
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The screen (search for genes involved in a
particular process) identified other genes that
affected seed shape via effects on seed starch
content. PGM
Glucose-6-phosphate ADP glucose SBE1

pyrophorylase
Starch biosynthesis pathway
Pea plant
Seed
Seed coat wall
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Dominance is not an intrinsic property of a gene,
it results from how genes make phenotypes
A active enzyme (assume one dose of A turns ½
precursor into product). a inactive
enzyme Genotype Pathway
Phenotype AA AA Colorless
precursor ? Purple pigment Purple
Aa Aa
Colorless precursor ? Half as much
pigment Purple
aa aa
Colorless precursor ? Purple pigment
White
X
A is dominant to a
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But, what if the pigment precursor was not
colorless? an example of incomplete dominance,
the heterozygote phenotype is different from
either homozygote
A active enzyme (assume one A gene dose ½
precursor into product) a inactive enzyme
Genotype Biochemical Pathway
Phenotype AA AA Green precursor
? Purple pigment Purple
Aa Aa
Green precursor ? Purple and Green pigment
Black
aa aa Green precursor ?
Purple pigment Green
X
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The type of dominance is determined by the how
alleles function in making phenotypes, and by
the investigative level of phenotypic analysis
For example, are we measuring color, or rate of
an enzymatic reaction?
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Summary of allelic relationships 1) Dominance
A/A and A/a genotypes have the same phenotype,
a/a has a different phenotype 2) Partial
dominance the heterozygote is different in
phenotype -Incomplete dominance heterozygote is
intermediate in a meristic (measurable) trait
(height, color) 3) Co-dominance the
heterozygote fully embodies the phenotype of each
homozygote (A, B, AB bloodgroups)