Genetics

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Genetics Biology 130 Lecture 7
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The story of Gregor Mendel

• Monk who bred peas and discovered important
  information about how information is passed from
  one generation to the next
• Highly successful
• His work was ignored and rediscovered only after
  his findings had been rediscovered
• Not very successful

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The story of Gregor Mendel

• Selected peas as his model system
• Self pollinated, highly inbred
• Selected seven traits to follow
• Obtained true breeding lines, that is, one line
  of peas that would always make round peas,
  another that would always make wrinkled peas
• Cross-pollinated true breeding lines
• Examined the first filial (F1) and second
  generation (F2)

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Terminology diversion

• Inbred, plants that have been bred to close
  relatives for so long that all plants have the
  same gene mix
• Outcrossing, plants have mechanisms to make sure
  they do not pollinate themselves
• Hybrid, offspring of genetically dissimilar
  parents
• Monohybrid cross, dihybrid cross
• Hybrid vigor/heterosis, inbreeding depression

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Terminology

• A further diversion
• Outcrossing is forced in many plants. Different
  plants for each sex encourages outcrossing
• Sex determination is highly variable. Some plants
  have the X/Y system humans do. Sex determination
  works imperfectly.
• Flower parts can mature at different locations
• The tassels of corn are the male flower, the ear
  is the female.

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A further diversion

• Flower parts can mature at different times
• The Titan Aram in Madison had to be pollinated by
  pollen from a flower in Florida that had matured
  earlier.

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Flower parts can mature at different times
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A further diversion

• Outcrossing can be ensured by pollen
  incompatibility
• S system of self incompatibility
• Pollen from a plant with S gene versions 1 and 2
  will not germinate on stigmas of plants with
  either S1 or S2
• Can be dozens of versions of S genes.

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Back to the story of Gregor Mendel

• Crossed true-breeding lines that differed in
  traits such as flower color, stem height, smooth
  versus wrinkled seeds
• Consider a monohybrid cross example in the text
  is flower color, purple versus white
• White-flowered peas crossed with purple-flowered
  peas

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Back to the story of Gregor Mendel

• White-flowered peas crossed with purple-flowered
  peas
• WW X ww (dominant crossed with recessive)
• All progeny are Ww (and purple)
• Use Punnett square to keep things straight

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Back to the story of Gregor Mendel

• White-flowered peas crossed with purple-flowered
  peas

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Back to the story of Gregor Mendel

• Filial refers to offspring, the first filial
  generation (F1) of a cross between true-breeding
  parents are shown
• The parents had been homozygous, the offspring
  are heterozygous
• The genotype is the information in the genes, the
  phenotype is the expression of that information
• The phenotype of the F1 in this case is purple
  flowers

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Mendelian genetics

• The information for white flowers is not lost in
  the F1
• Mendel discovered this by crossing the offspring
  (F1) to make the F2
• In the F2, white flowers show up again, though
  only 25 (31 ratio)
• Purple color is dominant, white is recessive

ww
P
X
WW
F1
F2
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Mendels traits
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Mendelian genetics
ww
P
X
WW

• What does it mean to be dominant or recessive?
• Normally, genes code for an enzyme that allows
  certain reactions. If the enzymes to make purple
  color are present, the flower will be purple, if
  absent, it will be white

F1
F2
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The test cross

• How can you determine if a purple flower is
  homozygous or heterozygous?

Test plant
ww
W ?
X
OR
Cross a plant with a dominant phenotype (genotype
uncertain) with a plant with a recessive
phenotype.
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Incomplete dominance

• Consider a gene coding for color
• One allele (version of the gene) codes for an
  enzyme that makes a red pigment
• Another allele does nothing
• With one copy of the gene for red, enough pigment
  is made to look pink
• With two copies enough pigment is made to look red

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Incomplete dominance

• With one copy of the gene for red, enough pigment
  is made to look pink
• With two copies, enough pigment is made to look
  red

Phenotype ratio 121
F2
F1
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The principle of segregation

• The analysis of the past few slides is very
  simple if you assume that each parent has two
  pieces of information for flower color and that
  these pieces of information segregate at meiosis
• Mendels first law is the principle of
  segregation every individual carries pairs of
  factors for each trait and these separate from
  each other during reproduction

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The principle of independent assortment

• Mendels second law is the principle of
  independent assortment and deals with the
  behavior of two traits.
• Factors for one trait segregate independently
  from factors for other traits. That is, factors
  (genes) are not linked to one another.

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The principle of independent assortment

• The dihybrid cross

P
RRYY X rryy
RrYy F1
F2
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The principle of independent assortment

• The dihybrid cross

P
RRYY X rryy
RrYy F1
F2
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The principle of independent assortment

• The dihybrid cross

P
RRYY X rryy
RrYy F1
F2
9 Round yellow
3 Round green
3 Wrinkled yellow
1 Wrinkled green
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Testing the hypothesis

• The monohybrid cross should result in a 31 ratio
  of phenotypes in the F2 generation
• The dihybrid cross should result in a 9331
  pattern in the F2 generation
• The Chi squared (?2) test is used to determine if
  the difference in the observed distribution from
  31 or 9331 is just noise or is so unusual
  that the underlying assumptions might be wrong
• 95 confidence level is normally used

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The chromosomal basis of heredity

• The principles of segregation and independent
  assortment make intuitive sense in light of the
  theory that chromosomes carry genetic information
  (and assuming one gene per chromosome)
• Sturtevant and Morgan are given credit for
  establishing that chromosomes carry the genes
• What happens if genes are on the same chromosome?

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The chromosomal basis of heredity

• What happens if genes are on the same chromosome?
• The genes are linked and do not assort
  independently
• In a dihybrid cross the resulting ratio is
  9331 for unlinked genes and 31 for perfectly
  linked genes
• Consider a dihybrid cross where two genes are
  next to each other and never are split apart

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Linkage

• Consider the gene pair Peanut Butter
• P peanuts, B butter, p no peanuts, b no
  butter
• The linkage of P to B makes the F1 gametes PB or
  pb

P PPBB X ppbb
12 peanut butter
4 nothing
31 ratio
F1 PB pb
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Linkage

• Gene pair Peanut Butter
• P peanuts, B butter, p no peanuts, b no
  butter
• Assuming no linkage

P PPBB X ppbb
9 peanut butter
3 peanuts
3 butter
F1 PpBb
1 nothing
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Linkage

• Gene pair Peanut Butter
• P peanuts, B butter, p no peanuts, b no
  butter
• What if 1 of the time P got separated from B?

Rare
P PPBB X ppbb
74 peanut butter
1 peanuts
1 butter
F1 PpBb
25 nothing
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Linkage

• What if they were separated 50 of the time?

Not quite so rare
P PPBB X ppbb
2368 peanut butter
789 peanuts
765 butter
F1 PpBb
1001 nothing
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?2 analysis
X2 1663 Chances are extremely small that the
observed distribution is a 9331 distribution
X2 ?(O-E)2/E
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Linkage

• What if they were separated 90 of the time?

Not rare
P PPBB X ppbb
2607 peanut butter
809 peanuts
809 butter
F1 PpBb
329 nothing
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?2 analysis
X2 12.28 d.f. 3 (from the table) 0.01 0.02 Chances are that the observed
distribution is not a 9331 distribution
Total observations 4556
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Linkage maps

• Typically, there are 500 to 2000 genes per
  chromosome
• For the genes that show linkage (i.e., are on the
  same chromosome) they can be arranged by how
  often a cross-over event separates them
• Consider 3 linked genes most of the time they
  are together (ABC)
• 10 of the time Ab 10 of the time Bc, 1 AbC

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Linkage maps

• 10 of the time Ab 10 of the time Bc, never
  AbC
• Cross-over occurs 10 of the time between A and B
  and 10 of the time between B and C

A
C
B
Cross over 10 of the time
Cross over 10 of the time
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Linkage maps
The frequency of crossing over allows the order
of the genes to be determined.
Cross over 20 of the time
A
C
B
Cross over 10 of the time
Cross over 10 of the time
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Linkage maps
The distance is measured in centiMorgans (cM) and
is how often (divided by 100) a crossover event
occurs between two genes. A is 10 cM from B but
20 cM from C
Cross over 20 of the time
A
C
B
Cross over 10 of the time
Cross over 10 of the time
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A few more terms

• Gene Basic unit of inheritance, encodes one
  protein
• Locus Location on a chromosome
• Allele Version of a gene

In some cases, there may be more than one gene
for a specific protein. These genes will be found
at different loci on the chromosome. If there
were two loci with a gene for a particular
protein, an individual could have four alleles
for that protein.
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Duplication of parts of chromosomes
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Two important problems

• What if the genes interact to give incomplete
  dominance?
• What if the genes are next to each other on a
  chromosome? linked