Welcome to IB 201!

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Welcome to IB 201!

• Genetics and Evolution

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Extra Credit Question

• A phenotype ratio of 9331 in the offspring of
  a mating between two individuals that are
  heterozygous for two traits occurs when
• A. the genes reside on the same chromosome
• B. each gene contains two mutations
• C. the gene pairs assort independently during
  meiosis
• D. only recessive traits are scored
• E. none of the above

• For extra credit question, please use the index
  cards provided.
• Print your name, TA, and section at top of
  card.
• Thanks!

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Strategies for Learning Genetics

• Strategy
• Take notes during lecture and review before exam
• Summarize lecture in your own words after class
• Self-questioning during lecture

• Exam Grades
• Worst
• Intermediate
• Best

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Why Genetics and Evolution?

• Evolution is unifying principle of biology.
• Nothing in biology makes sense except in the
  light of evolution
• Theodosius Dobzhansky (geneticist)

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Darwins Postulates

• Individuals within species are variable.
• Some of these variations are passed on to
  offspring.
• In every generation, more offspring are produced
  than can survive.
• Survival reproduction of individuals are not
  random. Those that survive and reproduce are
  those with the most favorable variations. They
  are naturally selected.

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Darwins Success

• He compiled massive amounts of evidence that all
  these postulates are true.
• Showed conclusively that individuals within
  species are variable, and that some of these
  variations are passed on to the offspring.

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Darwins Problems

• Because mechanisms of inheritance were not known,
  two objections were raised
• 1. Inheritance acts like pigment in paint
  (blending inheritance). Any new variant would
  mix with existing traits and be swamped by them.
  Thus, even if new variation could be created, it
  would not persist.
• 2. Variation within species is limited. Once
  existing variation is exhausted, evolution by
  natural selection will grind to a halt.

Discoveries in genetics solved both these
problems.
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Mendel solves one of Darwins Problems

• When Origin was published, an unknown monk was
  cultivating peas in central Europe. Gregor Mendel
  would demonstrate that objection 1 is invalid by
  showing that inheritance is particulate, not
  blending.
• Darwin had Mendels paper but did not read it,
  he also did crosses but did not go beyond the F1!

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Morgan solves the other

• In 1900 DeVries, Von Tschermak, and Correns
  published experiments similar to Mendels, and
  attributed priority of discover to him.
• Within a few years, T.H. Morgan colleagues
  showed that new hereditary variation (mutation)
  occurs in every generation.
• By beginning of the 20th century, both objections
  to Darwins principles had been conclusively
  eliminated by discoveries in genetics.

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Modern Synthesis

• Evolution by natural selection was not fully
  accepted until the hereditary mechanism was
  understood.
• The Modern Synthesis is the synthesis of
  Mendelism and Darwinism that occurred in the
  early 20th century.
• The Modern Synthesis is the foundation of modern
  evolutionary biology.

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Modern genetics is also based on evolutionary
biology and the Modern Synthesis

• Medical population genetics patterns of genetic
  disease and disease resistance
• Genetic epidemiologyidentification and spread
  of new disease vectors Hanta virus, AIDS, SARS
• Genome biology Sequencing projects for non-human
  organisms can tell us where did human genes come
  from, which genes are conserved and which have
  diverged,and how did eukaryotic genomes come to
  be organized the way they are (introns, exons,
  repetitive DNA).

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Course Outline

• Deviations from Mendelism
• Epistasis Unusual Modes of Inheritance
• Genetic Data Analysis
• Probability Statistics
• Chromosomal Inheritance
• Chromosomal Abnormalities Sex Determination
• Mapping
• Gene and Genome Mapping
• Traits Affected by Genes Environment
• Quantitative Traits
• Genes in Populations
• Genetic Mechanisms of Evolution Population
  Genetics of Disease and Disease resistance
• Genomes and Genome Evolution

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Additional Deviations from Mendelism

• Lethal Alleles
• Epistasis
• Unusual sex linkage
• Sex influenced inheritance
• Genetic Anticipation

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Manx Cats
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Lethal alleles
F1 Mm x Mm F1 1 MM 2 Mm
1 mm F2 1 Lethal 2 Manx 1
Normal
F2 phenotypic ratio 21 instead of 31
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Other lethal mutations

• Achondroplasia (humans)
• Yellow body color (domestic mice)
• Curly wings (Drosophila)

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Agouti wild type
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BB CC x bb cc
P agouti albino
Bb Cc
F1 agouti
Simple dominant phenotype?
F2 9/16 B- C- 3/16 bb C- 3/16 B- cc 1/16
bb cc
albino
agouti
albino
black
F2 Phen. ratio 9 agouti 3 black 4
albino
novel phenotype
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Epistasis
Locus 1 Locus 2 BB
Bb bb CC Cc cc agouti
agouti black no effect no effect
albino
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Biochemical model

• CC or Cc tyrosinase is produced (involved in
  production of melanin)
• BB or Bb controls distribution of the pigment

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Epistasis Normal dihybrid ratio is altered from
9331 to 934
C and B gene have an epistatic interaction
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Figure 10.18b
Crosses between pure lines produce novel colors.
Parental generation
rrYY RRyy
X
Yellow
Brown
Codominance?
F1 generation
R-Y-
Red
Self-fertilization
F2 generation
R-Y- rrY- R-yy rryy
Red
Yellow
Brown
Green
9/16
3/16
3/16
1/16
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Figure 10.18c
Model to explain 9 3 3 1 pattern observed
above Two genes interact to produce pepper color.
Genotype
Color
Explanation of color
R-Y-
Red
Red pigment no chlorophyll
rrY-
Yellow
Yellow pigment no chlorophyll
R-yy
Brown
Red pigment chlorophyll
rryy
Green
Yellow pigment chlorophyll
Gene 1
Gene 2
Y Absence of green (no chlorophyll)
R Red
y Presence of green ( chlorophyll)
r Yellow
(-) Y or y
(-) R or r
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Practice Problem

• In Labrador retrievers, coat color is controlled
  by two loci each with two alleles B,b and E,e
  respectively. When pure breeding Black labs with
  genotype BB EE are crossed with pure breeding
  yellow labs of genotype bb ee the resulting F1
  offspring are black. F1 offspring are crossed (Bb
  Ee x Bb Ee). Puppies appear in the ratio
• 9/16 black 3/16 chocolate
  4/161/4 yellow.
• What genotypes correspond to these three
  phenotypes?

9/16 B- E- 3/16 B- ee 3/16 bb E- 1/16
eebb B- E- B- ee bb E- and bb ee
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Other kinds of epistasis
9/16 A-B-
1/16 aabb
3/16 A-bb
3/16 aaB-
Hint usually given numbers, not fractions 27
agouti 12 albino 9 black 28 agoute 11
albino 4 black
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Practice Problem

• In the summer squash (Cucurbita pepo) fruit shape
  is determined by two genes. Two different
  true-breeding spherical types were crossed. The
  F1's were all disk, and the F2's segregated
  35 disk, 25 spherical and 4 long. Explain these
  results.

Whats the first step? Notice novel phenotype
disk, long.
Whats the next step? Notice there are three F2
phenotypes. What kind of inheritance will give
three F2 phenotypes?
Expected F2 ratio? 121 Variation on 9331
Incomplete, codominance Epistasis
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Practice Problem, cont.

• In the summer squash (Cucurbita pepo) spherical
  fruit is recessive to disk, True-breeding
  spherical types from different geographic regions
  were crossed. The F1's were disk, and the F2's
  segregated 35 disk, 25 spherical and 4 long.
  Explain these results.

Are the phenotypic ratios closer 121 or to a
variant of 9331 ?
If phenotypic ratios closer to a variant of
9331, then what variant is it? Total of
individuals 35 25 4 64 64/16 4 94
36 64 24 14 4 Phenotypic ratio close to
961
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Practice Problem, cont.

• In the summer squash (Cucurbita pepo) spherical
  fruit is recessive to disk, True-breeding
  spherical types from different geographic regions
  were crossed. The F1's were disk, and the F2's
  segregated 35 disk, 25 spherical and 4 long.
  Explain these results.

If phenotypic ratios are close to 961, then
what are the genotypes associated with each
phenotype? 35 disk 25 spherical
4 long 9/16 A- B- 3/16 A- bb 3/16 aa B-
1/16 aa bb
What were the genotypes of the original spherical
parents? AA bb aaBB