Download Introduction to Genetics The Work of Gregor Mendel

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Introduction to Genetics
Chapter 11
Chapter 11 Section 1
THE WORK OF GREGOR MENDEL
The Work of Gregor Mendel
• Some Definitions:
– Genetics – the study of biological inheritance and
variation
– Chromosomes – hereditary units of an organism
– Gene – segment of a chromosome that determines
traits
– Alleles – different forms of a gene associated with
a specific trait
– Hybrids – offspring that result from crosses
between 2 parents with different traits
– Dominant & Recessive – some alleles (dominant)
will completely prevent the expression of others
(recessive)
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The Work of Gregor Mendel
• Gregor Mendel’s Peas
– Used ordinary garden peas from the monastery
garden
– Each type used was self-pollinating (each flower
has both male and female parts)
– Used several types of pea plants that were each
true-breeding for particular characteristics
• If allowed to self-pollinate, true-breeding plants will
produce offspring identical to themselves
– Tall plants produce only tall plants; short plants produce
only short plants
– Wanted to produce crosses of different types
• Therefore had to prevent self-pollination
The Work of Gregor Mendel
• Genes and Dominance
– Mendel studied seven different pea plant traits
– Each of the seven traits had two contrasting
characters
• Green seed color or yellow seed color
• Tall plant or short plant
– Each original plant was called the P generation
(parental generation)
– The offspring were called the F1 generation (first
filial generation)
– The offspring of parents with different traits are
called hybrids (in other words, the F1 generation)
The Work of Gregor Mendel
• Genes and Dominance (continued)
– Mendel expected the F1 offspring to have a
“blend” of characteristics of both parental
generation plants
• But ALL the offspring had the characters of just ONE of
the parental types!
• In each cross, the character of the other parent seemed
to have disappeared!
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The Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Tall
Short
The Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
The Work of Gregor Mendel
• Genes and Dominance (continued)
– Mendel expected the F1 offspring to have a
“blend” of characteristics of both parental
generation plants
• But ALL the offspring had the characters of just ONE of
the parental types!
• In each cross, the character of the other parent seemed
to have disappeared!
– Mendel’s conclusion was that biological factors
are passed from one generation to the next
• Today those factors are called genes
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The Work of Gregor Mendel
• Genes and Dominance (continued)
– Each of the traits Mendel studied was produced
by one gene that had two versions
• Each “version” of a gene is called an allele
– Mendel’s second conclusion is called the
“Principle of Dominance”
• Some alleles are dominant and some are recessive
• An organism with the dominant allele will always exhibit
that trait
– A plant that has an allele for a tall plant will be tall; if does
not have the allele for a tall plant, the plant will be short
– The following table shows the seven traits Mendel studied
and the dominant allele for each
The Work of Gregor Mendel
• Mendel’s Seven F1 Crosses on Pea Plants
Seed Coat
Color
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Round
Yellow
Gray
Pod
Shape
Pod
Color
Flower
Position
Smooth
Green
Axial
Tall
Constricted
Yellow
Terminal
Short
Smooth
Green
Axial
Plant
Height
Tall
The Work of Gregor Mendel
• Segregation
– The F1 Cross
• Had the recessive alleles disappeared or were they just
masked?
• Mendel let the F1 plants produce offspring of their own,
an F2 generation
• The results were surprising
– The recessive allele returned!
– Roughly 25% of the F2 offspring showed the “missing” trait
from the P generation
– Explaining the F1 Cross
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The Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Tall
Short
Tall
Short
The Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
The Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
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The Work of Gregor Mendel
• Segregation (continued)
– Explaining the F1 Cross
• The F1 plants had to have a characteristic from each of
the P generation plants
• The recessive allele in the F1 plants was masked by the
dominant allele for each specific trait
• During the F1 plant’s own reproduction the alleles for
tallness had to be separated from each other, or
segregated
– He concluded that during the formation of the sex cells, or
gametes, there had to be segregation of the alleles
The Work of Gregor Mendel
• Each gamete carries only
a single copy of each
gene
• Each F1 plant produces
two kinds of gametes
– Half have the allele for tall
plants and have the allele
for short plants
– In the example (left), Red
is dominant to White so F1
is all red and F2 has both
red and white (only 25%
white)
Real World Example
• In horses, the black gene
uses the designation “E”
and a black coat is
dominant to a chestnut coat.
– A black horse may be “EE” or
“Ee”
– A chestnut horse is “ee”
– An “EE” black horse crossed
with an “ee” chestnut horse
will always produce an “Ee”
black horse.
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Real World Example
• Definitions (again)
– Genotype – the genetic makeup of
an organism (which alleles it has)
– Phenotype – the physical
characteristic of an organism (what
it looks like)
• Examples:
– Top & Bottom
• Genotype: EE or Ee
• Phenotype: “black”
– Middle
• Genotype: ee
• Phenotype: “chestnut”
Another Real World Example
• The white color pattern Tobiano in
horses (left) is dominant to nonpatterned horses.
• The Tobiano gene uses the
designation “T” if the gene is
present (and “n” if not).
• A tobiano horse may be “TT” or
“Tn”
– A “TT” horse (top left) crossed with a
solid color horse will always have a
tobiano patterned offspring
– The adult horses pictured are the
actual parents of the baby horse
(called a foal).
Real World Example
• Many diseases and medical conditions in
humans follow the principles of Mendelian
genetics
– They are caused by a single gene and are
recessive
• Sickle-cell anemia
• Tay-Sachs disease
– Babies appear normal at birth
then nerves begin to be
damaged. The baby becomes
blind, then deaf, then
paralyzed and usually dies
before age 4.
• Cystic fibrosis
• Albinism
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The Work of Gregor Mendel
• Analyzing Inheritance
– Offspring resemble their parents. Offspring inherit
genes for characteristics from their parents. To
learn about inheritance, scientists have
experimented with breeding various plants and
animals.
– In each experiment shown in the table on the next
slide, two pea plants with different characteristics
were bred. Then, the offspring produced were
bred to produce a second generation of offspring.
Consider the data and answer the questions that
follow.
The Work of Gregor Mendel
Parents
First Generation Second Generation
Long stems  short stems
All long
787 long: 277 short
Red flowers  white flowers
All red
705 red: 224 white
Green pods  yellow pods
All green
428 green: 152 yellow
Round seeds  wrinkled seeds All round
5474 round: 1850 wrinkled
Yellow seeds  green seeds
6022 yellow: 2001 green
All yellow
1.
In the first generation of each experiment, how do the
characteristics of the offspring compare to the parents’
characteristics?
2.
How do the characteristics of the second generation
compare to the characteristics of the first generation?
Chapter 11 Section 2
PROBABILITY AND PUNNETT
SQUARES
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Probability and Punnett Squares
• Genetics and Probability
– Since Mendel’s results were repeated regardless
of the trait he was looking at (and he grew some
29,000 pea plants over the course of his studies),
he concluded that probability could be used to
explain the results
– The likelihood that event will occur is called
probability
• 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
Probability and Punnett Squares
• Genetics and Probability – Activity
– 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.
– Now answer the questions on the following slide.
Probability and Punnett Squares
• Genetics and Probability – Activity
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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Probability and Punnett Squares
• Genetics and Probability
– Flip a coin, you have a 1 out of 2 chance of it
coming up heads
– Each flip is an independent event so the chance
of flipping heads 3 times in a row is:
• ½ x ½ x ½ = 1/8
– How does this relate to genetics?
• The way in which alleles segregate is completely
random so probability can be used to predict the
outcome of genetic crosses
Probability and Punnett Squares
• Punnett Squares
– A diagram that shows gene combinations that
might result from a particular cross
Quick Lab - Making Connections
• Find a partner. Each pair of students will
receive a paper bag with 4 paper clips inside.
They are identical except for color. Three are
blue and one is red.
– What is the probability of picking a red item?
– Of picking a red item two times in a row?
• Without looking, pick an item from the bag
(replacing it each time) 20 times, then 60
times. Keep track of your results.
– How many red did you expect for 20? For 60? Did
your results equal your calculated probabilities for
20? For 60?
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Probability and Punnett Squares
• Punnett Squares
– A diagram that shows gene combinations that might
result from a particular cross
– Organisms that have two identical alleles for a trait
are said to be homozygous for that trait
– Homozygous organisms are
“true-breeding”
• BB flowers always are purple
• bb flowers are always white
– Each combination of genes
is the organisms genotype
• This square shows 3 genotypes:
BB, Bb, and bb
Probability and Punnett Squares
• Punnett Squares
– Organisms that have two different alleles for a trait
are heterozygous
– For a trait that exhibits complete dominance, a
homozygous dominant and a
heterozygous organism will
physically appear the same –
they will have the same
phenotype
– You can tell which is which
only by studying their
offspring
Probability and Punnett Squares
• Probability and Segregation
– Since there is an allele for “tall” in 3 of the
combinations you expect a 3:1 ratio
of tall to short plants
– But 1/3 of the tall plants are
homozygous dominant and
2/3 are heterozygous
– Their phenotype ratio is 3:1
– Their genotype ratio is 1:2:1
– Is this what we’ve found?
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Probability and Punnett Squares
• Probabilities Predict Averages
– Probability predicts average outcome
of a large number of events
– It cannot predict the outcome of
a specific event
• Not all families with 4 children
have exactly 2 boys and 2 girls
• Not all families with 2 children
have 1 girl and 1 boy
– The samples are two small!
– Choose 1000 families with 4 children each and you’d find pretty
close to 2000 boys and 2000 girls among all of the children
– The larger the sample, the closer to the expected
outcome
Quick Lab – How are dimples inherited?
I. Title: How are Dimples Inherited (page 268)
II. Purpose: By using random number to assign a
genotype, students will be able to conclude how
dimples are inherited
III. Safety: none
IV. Procedure:
V. Data/Observations:
1. Write the last four digits of any
phone number. Odd digits
1.
represent the dominant allele; even
digits represent the recessive allele
2. Use the first two digits to represent
the father’s genotype. Write his
genotype.
2.
Quick Lab – How are dimples inherited?
IV. Procedure:
V. Data/Observations:
3. Use the last two digits to
represent the mother’s genotype.
Write her genotype.
3.
4. Construct a Punnett square for
the cross of these parents.
Then, using that Punnett
square, determine the
probability that their child will
have dimples. (remember the
4.
allele for dimples is dominant)
5. Determine the class average of
the percent of children with
dimples.
5.
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Quick Lab – How are dimples inherited?
VI. Calculations/Questions:
1.
2.
3.
How does the class average compare with your
results?
How does the class average compare with the
result of a cross of two heterozygous parents?
What percentage of the children will be
expected to have dimples if one parent is
homozygous for dimples (DD) and the other is
heterzygous (Dd)?
VII. Conclusion:
–
Summarize your findings. Do you see how a
dominant trait, such as dimples, is inherited?
Chapter 11 Section 3
EXPLORING MENDELIAN
GENETICS
Exploring Mendelian Genetics
• Independent Assortment
– Mendel knew from his studies of pea plants in the
F2 generation that alleles segregated during
reproduction but do they segregate
independently?
• Does a round seed always have to be yellow?
– Mendel designed an experiment to follow two
genes as they passed from one generation to
another
• This is called a two factor or dihybrid cross
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Exploring Mendelian Genetics
• Independent Assortment
– The Two-factor Cross
• Each allele segregates independently – half of all
gametes will have each allele
– For Smooth Yellow seeds, half will be smooth, half not
smooth
– Then half will be yellow, half will be not yellow
Exploring Mendelian Genetics
• The F1 plants are
heterozygous for
both characteristics
• Proving that genes
independently assort
can be seen in the
results shown using
a 4x4 Punnett square
– Four different
phenotypes observed
– Nine genotypes
observed
Exploring Mendelian Genetics
• Mendel’s F2 plants produced 556 seeds as
he studied round vs. wrinkled AND yellow vs.
green
– 315 plants were round and yellow
– 32 were wrinkled and green
– 209 were “mixed”
• 101 yellow and wrinkled
• 108 green and round
• Therefore, the genes had to sort
independently of one another!
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Exploring Mendelian Genetics
Exploring Mendelian Genetics
• Conclusions led to the Principle of
Independent Assortment
– When two traits are considered in the same cross,
the segregation of one pair of alleles is
independent of the segregation of the other pair of
alleles.
– There could be four possible gametes
• The Punnett square works on this assumption that each
gamete occurs about 1/4 of the time
– This is why the predicted ratio is 9:3:3:1.
– NOTE: Independent assortment is not always true
but since many do, this accounts for the
tremendous variation in living things!
Exploring Mendelian Genetics
• A Summary of Mendel’s Principles
– The inheritance of biological characteristics is
determined by genes
• Genes are passed from parents to their offspring during
reproduction
– When two or more alleles for a gene exist, some
forms may be dominant and others may be
recessive
– Generally, each organism has two copies of every
gene – one from each parent
– The alleles for different genes usually segregate
independently of one another
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Exploring Mendelian Genetics
• Beyond Dominant and Recessive Alleles
– There are exceptions to Mendel’s principles!
• Most genes have more than one allele
• Many important traits are controlled by more than one
gene
• Just because an allele is dominant doesn’t mean it is
common
• Some alleles are neither dominant nor recessive!
– Incomplete Dominance
• One allele is not completely dominant over another
• The heterozygous phenotype is somewhere in between
the two homozygous phenotypes
– Example – Four O’clock flowers; snapdragons, coat colors
in many animals, etc.
Exploring Mendelian Genetics
• The F1 generation
shows a phenotype that
is different than either P
generation parents
– Red and white
snapdragons produce all
pink flower in F1
• F2 generation shows the
F1 generation
phenotype as well as
both P generation
phenotypes
Exploring Mendelian Genetics
A
B
C
• Example A: CCRCCR – double
dose of cream gene
• Example B: CCRC – single
dose of cream gene
• Example C: CC – fully
pigmented; no cream gene
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Exploring Mendelian Genetics
• Co-dominance
– Both alleles will contribute to the phenotype
• Example – in some chickens there is an allele for black
feathers and an allele for white feathers. Chickens with
both alleles will NOT have gray feathers but will have
both black and white feathers (called erminette)
• Example – blood type
genes from both parents
combine to establish your
blood type
– A type A father and a
type B mother can have
offspring with type AB
blood
Exploring Mendelian Genetics
• Multiple Alleles
– Most genes have more than two alleles
• An individual can still have only two alleles though!
• Some of those alleles can be dominant to others, codominant, incomplete dominant or recessive!
• Example – Blood type – there are 3 alleles – IA, IB, and i
IA and IB are dominant to i but are co-dominant to each
other
• Example – (page 273 in text) – rabbit coat colors – 4
alleles – c has no color, producing an albino, recessive
to all others; ch restricts color to certain areas of the
body (making Himalayan), dominant to c and recessive
to all others; cch shows a partial color change (called
chinchilla), partially dominant to c and ch, recessive to C;
C is full color and dominant to all others
Exploring Mendelian Genetics
• Polygenic Traits
– Traits controlled by two or more genes
– Most traits fall under this
category!
– Top right horse is a
homozygous black
horse (like top left) with
TWO modifying genes
– Bottom right horse is a
bay color (normally like
lower left color) with 3
alleles of two color
modifying genes!
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Exploring Mendelian Genetics
• Applying Mendel’s Principles
– Law of Segregation
– Law of Independent Assortment
– Both apply to more than just plants!
• Humans, plants, insects, animals, etc.
• Genetics and the Environment
– Characteristics determined by genes inherited from parents
– Those same characteristics may be affected by the environment
as well
– Genes provide a “plan” but how the plan develops is often
determined by environment
• You may have genes for being tall but poor nutrition doesn’t let you
grow as tall as your genes would allow
Chapter 11 Section 4
MEIOSIS
General Information about Meiosis
• Mendel did not know exactly where genes
were located but it was fairly quickly
determined to be located on the
chromosomes in the nucleus of a cell.
• Mendel’s principles of genetics requires
– Each organism must inherit a single copy of every
gene from both its parents
– When an organism produces its own gametes,
those two sets of genes must separate from each
other so that each gamete contains only one set
of genes
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Meiosis
• Chromosome Number
– The two sets of chromosomes that every
organism has are called homologous
chromosomes
• Chromosomes appear in pairs; one of each pair from
each parent
• Gametes will have half the number of chromosomes
– The number that represents the number in the gametes is
N and is called haploid.
• All other cells have both sets of chromosomes are so
are called 2N or diploid.
– 2N=46 in humans; fruit flies 2N=8
Meiosis
• Phases of Meiosis
– During Meiosis the haploid gamete cells are
produced from diploid cells
– It involves two distinct divisions called meiosis I
and meiosis II.
• By the end of meiosis II, the diploid cell that entered
meiosis has become 4 haploid cells
• Meiosis I
– Each chromosome is replicated and the cells
begin to divide almost like they do in mitosis
• Instead of lining up individually in prophase
Meiosis
• Overview of Meiosis
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Meiosis
Meiosis I
• As in mitosis, cells grow and develop during
Interphase I, then duplicate their DNA!
Meiosis
Meiosis I
• Unlike mitosis, in Prophase I of meiosis, each
chromosome pairs with its analogous chromosome
to form a tetrad
• Crossing over occurs here!
Meiosis
• Crossing over
– Occurs during Prophase I of Meiosis I
• (1) Chromosomes line up in analogous pairs
• (2) They cross over one another
• (3) The crossed sections are exchanged
– This creates recombinant DNA
• It is different from either parent’s chromosomes
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Meiosis
• Overview of
the process of
Crossing over
Meiosis
Meiosis I
• In Metaphase I, spindle fibers attach to the chromosomes
at the centromere
• They are still lined up in pairs, unlike in mitosis.
Meiosis
Meiosis I
• In Anaphase I, the spindle fibers pull the homologous
chromosomes to opposite ends of cell.
• Still diploid but starting to become haploid!
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Meiosis
Meiosis I
• In Telophase I and Cytokinesis, the nuclear envelope
reforms and the cell divides in two.
• But neither daughter cell has two of each chromosome!
They have become haploid!
Meiosis
Figure 11-17 Meiosis II
• Meiosis I
– Took a diploid cell and created two haploid
daughter cells
– Two haploid daughter cells now enter a second
phase of meiotic division called Meiosis II
• Meiosis II
– Takes each of the Meiosis I daughter cells and
produces two more haploid daughter cells
– Entire process of Meiosis I and Meiosis II takes
one diploid cell and makes four haploid daughter
cells
Meiosis
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.
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
• In Prophase II, there is no chromosome replication this
time but nuclear envelope disappears and spindles form.
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Meiosis
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.
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
• In Metaphase II, chromosomes line up individually like in
mitosis
Meiosis
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.
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
• In Anaphase II, sister chromatids are pulled apart and
move to opposite ends of the cell
Meiosis
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.
Telophase II
Meiosis II results in four
haploid (N) daughter cells.
• In Telophase II and Cyntokinesis, four total daughter
cells are formed; nuclear envelope reappears.
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Meiosis
• Gamete Formation
– In males
• The four haploid daughter cells at the end of meiosis II
are four equally sized sperm
– In females
• Generally only one of the four haploid daughter cells at
the end of meiosis II forms an egg
– The egg takes nearly all of the cytoplasm
– The remaining 3 form polar bodies which are not used in
reproduction
» The polar bodies pick up the extra sets of
chromosomes so that too many are not contained in
the egg
Meiosis
Comparing Mitosis and Meiosis
Mitosis
Meiosis
• A diploid cell produces two
diploid daughter cells
• Asexual reproduction
• Allows an organism’s body
to grow and replace cells
• A diploid cell produces four
haploid daughter cells
• Sexual reproduction
• Produces gametes only
– Does not occur in organisms
that do not reproduce through
sexual reproduction!
Chapter 11 Section 5
LINKAGE AND GENE MAPS
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Linkage and Gene Maps
• Gene Linkage
– When genes for two different traits are always (or
nearly always) inherited together
– Linked genes are close together on the same
chromosome
• Rarely assort independently so they are inherited
together
• Still can be separated but not often
• Genes that are close together tend to stay together,
genes that are far apart tend to separate
– The chromosomes sort independently but not
individual genes!
Linkage and Gene Maps
• Gene Maps
– Crossing over separates genes on the same
chromosome
– Can sometimes separate linked genes
– How far apart genes were could be seen in how
frequently they were linked
• The farther apart, the less often they’d be linked
– The rate that linked genes were separated could
be used to produce a “map” showing where on a
chromosome a gene is located
Linkage and Gene Maps
• Gene map of chromosome 2 of the fruit fly
(as estimated in 1911)
– Genes are named after the abnormal problem
they cause
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Challenge Question
• The allele that causes galactosemia (g) is
recessive to the allele for normal lactose
metabolism (G). A normal woman whose
father had galactosemia marries a man with
galactosemia who had normal parents. They
have three children, two normal and one with
galactosemia.
• What are the genotypes of each: The
woman, her father, her husband, the
husbands parents, and their 3 children?
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