Transcript Prentice Hall Biology

CP Biology
Chapter 10 - Genetics
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11-1: The Work of Mendel
•“Father of Genetics”
•Austrian monk, spent time teaching high
school age students and tended to the
monastery garden.
•YouTube - Mendel - From the Garden to the
Genome
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•“self-pollinating” peas – Mendel used to produce future
offspring
– Knew that each flower produces both male (sperm
–found in pollen) and female (egg) gametes
– Fertilized flower’s egg with its own sperm
• ASEXUAL REPRODUCTION!
• Produced what he called a “true breed”
• Offspring would be identical genetically
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•Then experimented with cross-pollination
– Cut the pollen bearing anther off of the flowers
and applied pollen from different flowers to the
stigma (part that catches pollen)
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So with cross-pollination what did Mendel expect?
A: Probably a blend of both parents.
What did he get?
A: Not really what he expected.
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Genes and Dominance
• Trait – specific characteristic that varies from one individual to the next
– Important that Mendel used traits that were unambiguous
•
1.
2.
3.
4.
5.
6.
7.
Mendel’s seven traits:
Seed coat color
Seed color
Seed shape
Pod color
Pod shape
Plant height
Flower position
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•Crossed true breeds with
each other (P generation –
“parental”)
•Offspring known as F1
•Plants that were products
of cross-pollination known
as “hybrids.”
•F1 generation was not a
blend of parental char.
•Instead….
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Mendel’s Monohybrid
Cross – P to F1
A Punnett square,
something we’ll
cover in a moment.
Huh?!?....all yellow seeds!!!
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In fact………here’s what
happened with the rest of
the traits.
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Figure 11-3 Mendel’s Seven F1 Crosses
on Pea Plants
Section 11-1
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
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Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
•Mendel concluded that heredity is dictated by chemical
factors called genes (it would be almost 100 years later
before Watson & Crick and the whole DNA thing)
•Genes have alternate forms depending on the plant
– These alternate versions of the same gene are
called alleles
Plant height = gene
Short and tall = alleles
•Both alleles are present.
•One type (recessive) can only be expressed if the other
(dominant) is not present
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•All
these are found on chromosomes!!
Section:
Genes, Alleles, and Chromosomes
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Segregation
•What happened to the other traits?
•Self-pollinated the F1 plants
•The “other” (recessive) traits reappeared in the F2
– He was floored, I’m sure…
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Principles of Dominance
Section 11-1
P Generation
Tall
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Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Principles of Dominance
Section 11-1
P Generation
Tall
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Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Principles of Dominance
Section 11-1
P Generation
Tall
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Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
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Segregation (cont.)
•How did the alleles get separated (segregated)?
– Mendel suggested the alleles segregated when
the gametes formed.
– So every sperm or egg cell has one version
(allele) for that gene
• Some (50%) have allele for tall, others (50%)
have allele for short
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Genes, Alleles, and Chromosomes
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Interest Grabber
Section 11-2
Tossing Coins
If you toss a coin, what is the probability of getting heads? Tails? If you
toss a coin 10 times, how many heads and how many tails would you
expect to get? Working with a partner, have one person toss a coin
ten times while the other person tallies the results on a sheet of paper.
Then, switch tasks to produce a separate tally of the second set of 10
tosses.
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Interest Grabber continued
Section 11-2
1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the
results of your tosses compare? How about the results of your partner’s
tosses? How close was each set of results to what was expected?
2. Add your results to those of your partner to produce a total of 20 tosses.
Assuming that you expect 10 heads and 10 tails in 20 tosses, how close
are these results to what was expected?
3. If you compiled the results for the whole class, what results would you
expect?
4. How do the expected results differ from the observed results?
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Section Outline
Section 11-2
11–2 Probability and Punnett Squares
A. Genetics and Probability
B. Punnett Squares
C. Probability and Segregation
D. Probabilities Predict Averages
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11-2: Probability and the Punnett Square
•Probability – likelihood of an even to take place
– What are the chances for a coin to be flipped
heads three times in a row?
½X½X½=⅛
•Probability explains everything in predicting outcomes
of genetic crosses.
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Punnett Squares
•Possible gene combinations are represented showing a Punnett
Square.
•Homozygous – same allele (SS or ss)
•Heterozygous - different allele (Ss)
•Genotype – genetic makeup (SS, Ss, ss)
•Phenotype – physical feature observed (Smooth, wrinkled)
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Probability and
Segregation
Consistency is
Good
No matter what the
characteristic, Mendel
observed a 3:1 ratio of
characteristics in the F2.
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Characters
investigated by
Mendel
Monohybrid Crosses Yielded Consistent Results
Therefore,
the Principle of Segregation indeed is a general principle of genetics.
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Probabilities Predict Averages
•More the times you flip the coin, the closer it will be to
the expected result
•Consequently the more offspring that an organism has,
the closer the results will be to the averages.
•This is why scientists perform MANY experiments.
– Avoid the small percentage event and assume it is
the trend.
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Interest Grabber
Section 11-3
Height in Humans
Height in pea plants is controlled by one of two alleles; the allele for a tall
plant is the dominant allele, while the allele for a short plant is the
recessive one. What about people? Are the factors that determine height
more complicated in humans?
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Interest Grabber continued
Section 11-3
1. Make a list of 10 adults whom you know. Next to the name of each
adult, write his or her approximate height in feet and inches.
2. What can you observe about the heights of the ten people?
3. Do you think height in humans is controlled by 2 alleles, as it is in pea
plants? Explain your answer.
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Section Outline
Section 11-3
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11–3 Exploring Mendelian Genetics
A. Independent Assortment
1. The Two-Factor Cross: F1
2. The Two-Factor Cross: F2
B. A Summary of Mendel’s Principles
C. Beyond Dominant and Recessive
Alleles
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits
D. Applying Mendel’s Principles
E. Genetics and the Environment
Independent Assortment
•Mendel wondered if alleles for separate genes
influenced each other when they segregate.
– Ex: Does a round seed always have to be yellow,
or can it be green?
•Two-factor cross
– Followed two separate genes from generation to
generation
Independent Assortment of Alleles
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Two Factor Cross
•Crossed true bred round/yellow (RRYY) pea plants
with wrinkled/green (rryy) plants
•F1 – 100% round/yellow (RrYy)
– No surprise to Mendel
•Mendel hypothesized there was dependent
assortment, therefore predicted 3:1 ratio in the F2 (just
like in the first experiment)
– 3 round/yellow : 1 wrinkled/green
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Fig. 10.12a Dihybrid cross:
F1 generation
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Fig. 10.12b Dihybrid cross:
F2 generation
Ratio:
9:3:3:1
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Beyond dominance and recessiveness
•Incomplete dominance
– Hybrids (heterozygous) exhibit blend of traits
– 4 o’clock plants
• Red and white produce heterozygous pink
plants
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Figure 11-11 Incomplete Dominance in
Four O’Clock Flowers
Section 11-3
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Figure 11-11 Incomplete Dominance in
Four O’Clock Flowers
Section 11-3
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Codominance
•Similar to incomplete
dominance however both traits
are actually visual instead of
blended
– Chickens with speckled
black spots on white
feathers
– Humans have gene for
protein controlling
cholesterol – if
heterozygous two forms
are made.
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MOO
Multiple Alleles
•When there are more than two alleles for a particular
gene
– Blood type (also demonstrates some codominance)
Polygenic traits
•Traits controlled by many genes
–
–
–
–
–
–
–
Height
SLE (Lupus)
Weight
Eye Color
Intelligence
Skin Color
Many forms of behavior
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Applying Mendel’s Principles
•Good animal to test was fruit fly
– Drosophilia melanogaster
•Fast reproductive rate/life cycle and produced many
offspring
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Environmental impact on gene expression
• Environmental factors/conditions may alter gene
expression.
Example: Soil pH and flower color.
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Interest Grabber
Section 11-4
How Many Chromosomes?
Normal human body cells each contain 46 chromosomes. The cell division
process that body cells undergo is called mitosis and produces daughter
cells that are virtually identical to the parent cell. Working with a partner,
discuss and answer the questions that follow.
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Interest Grabber continued
Section 11-4
1. How many chromosomes would a sperm or an egg contain if either one
resulted from the process of mitosis?
2. If a sperm containing 46 chromosomes fused with an egg containing 46
chromosomes, how many chromosomes would the resulting fertilized
egg contain? Do you think this would create any problems in the
developing embryo?
3. In order to produce a fertilized egg with the appropriate number of
chromosomes (46), how many chromosomes should each sperm and
egg have?
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Section Outline
Section 11-4
11–4 Meiosis
A. Chromosome Number
B. Phases of Meiosis
1. Meiosis I
2. Meiosis II
C. Gamete Formation
D. Comparing Mitosis
and Meiosis
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11-4 Meiosis
•Process of turning diploid somatic cells into haploid
gametes
•Must reduce chromosome number so when fertilization
takes place we re-establish the regular chromosome
number
– Humans
• Hapolid – sex cells
N = 23
• Diploid – somatic cells
2N = 46
N = chromosome
number
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Phases of meiosis
Meiosis I and II
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Figure 11-15 Meiosis
Section 11-4
Meiosis I
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Figure 11-15 Meiosis
Section 11-4
Meiosis I
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Figure 11-15 Meiosis
Section 11-4
Meiosis I
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Figure 11-15 Meiosis
Section 11-4
Meiosis I
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Figure 11-15 Meiosis
Section 11-4
Meiosis I
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Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
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Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
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Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
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Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
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Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up in a The sister chromatids
haploid (N) daughter cells,
similar way to the metaphase separate and move toward
each with half the number of stage of mitosis.
opposite ends of the cell.
chromosomes as the original.
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Telophase II
Meiosis II results in four
haploid (N) daughter cells.
Crossing over
•Parts of chromosomes
“switch places”
•Meiosis
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Crossing-Over
Section 11-4
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Crossing-Over
Section 11-4
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Crossing-Over
Section 11-4
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Mitosis vs. meiosis
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Section Outline
Section 11-5
11–5 Linkage and Gene
Maps
A. Gene Linkage
B. Gene Maps
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11-5: Linkage and Gene Maps
•Found that some genes were linked to other ones
– Seems to violate the rule of independent
assortment
– Ex: fruit flies – red eyes and minature wings
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•Mendel missed this….
– He thought the genes independently assorted
• Actually it was the chromosomes that do!
• Why did he miss it?
– 6 of the 7 genes he studied in peas were on
separate chromosomes
– The two that were on the same chromosome
were so far apart they independently
assorted
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Gene Map
•Do two genes on the same
chromosome mean they are
forever linked?
– No! Crossing over may
put them on separate
chromosomes
•The further apart genes are
the more likely they could be
separated during meiosis
•Low rate of separation and
then recombination means
genes are close to one
another.
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Figure 11-19 Gene Map of the Fruit Fly
Section 11-5
Exact location on chromosomes
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Chromosome 2