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© 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko
PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey
Chapter 9 Patterns of Inheritance
Dogs are one of man’s longest genetic experiments.
– Over thousands of years, humans have chosen and mated dogs with specific traits.
– The result has been an incredibly diverse array of dogs with distinct
– body types and
– behavioral traits.
Introduction
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Figure 9.0_1 Chapter 9: Big Ideas
Mendel’s Laws Variations on Mendel’s Laws
The Chromosomal Basis of Inheritance
Sex Chromosomes and Sex-Linked Genes
MENDEL’S LAWS
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9.1 The science of genetics has ancient roots
Pangenesis, proposed around 400 BCE by Hippocrates, was an early explanation for inheritance that suggested that – particles called pangenes came from all parts of the
organism to be incorporated into eggs or sperm and
– characteristics acquired during the parents’ lifetime could be transferred to the offspring.
Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited.
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9.1 The science of genetics has ancient roots
The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was – suggested in the 19th century by scientists studying
plants but
– later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.
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9.2 Experimental genetics began in an abbey garden
Heredity is the transmission of traits from one generation to the next.
Genetics is the scientific study of heredity.
Gregor Mendel
– began the field of genetics in the 1860s,
– deduced the principles of genetics by breeding garden peas, and
– relied upon a background of mathematics, physics, and chemistry.
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9.2 Experimental genetics began in an abbey garden
In 1866, Mendel – correctly argued that parents pass on to their offspring
discrete “heritable factors” and
– stressed that the heritable factors (today called genes), retain their individuality generation after generation.
A heritable feature that varies among individuals, such as flower color, is called a character.
Each variant for a character, such as purple or white flowers, is a trait.
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9.2 Experimental genetics began in an abbey garden
True-breeding varieties result when self-fertilization produces offspring all identical to the parent.
The offspring of two different varieties are hybrids.
The cross-fertilization is a hybridization, or genetic cross.
True-breeding parental plants are the P generation.
Hybrid offspring are the F1 generation.
A cross of F1 plants produces an F2 generation.
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Figure 9.2B
Stamen Carpel
Petal
Figure 9.2C_s1
Removal of stamens
Carpel
White
Stamens Transfer of pollen Purple Parents
(P)
2
1
Figure 9.2C_s2
Removal of stamens
Carpel
White
Stamens Transfer of pollen Purple
Carpel matures into pea pod
Parents (P)
2
3
1
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Figure 9.2C_s3
Removal of stamens
Carpel
White
Stamens Transfer of pollen Purple
Carpel matures into pea pod
Seeds from pod planted
Offspring (F1)
Parents (P)
2
3
1
4
Figure 9.2D Character Traits
Dominant Recessive
Flower color
Purple White
Flower position
Axial Terminal
Seed color Yellow Green
Seed shape Round Wrinkled
Pod shape Inflated Constricted
Pod color Green Yellow
Stem length
Tall Dwarf
9.3 Mendel’s law of segregation describes the inheritance of a single character
A cross between two individuals differing in a single character is a monohybrid cross.
Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.
– The F1 generation produced all plants with purple flowers.
– A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers.
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Figure 9.3A_s1 The Experiment P generation (true-breeding parents)
Purple flowers
White flowers
×
Figure 9.3A_s2 The Experiment P generation (true-breeding parents)
F1 generation
Purple flowers
White flowers
All plants have purple flowers
×
Figure 9.3A_s3 The Experiment P generation (true-breeding parents)
F1 generation
F2 generation
of plants have purple flowers
of plants have white flowers
Purple flowers
White flowers
All plants have purple flowers
Fertilization among F1 plants (F1 × F1)
×
3 4
1 4
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9.3 Mendel’s law of segregation describes the inheritance of a single character
The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis.
Mendel needed to explain why
– white color seemed to disappear in the F1 generation and
– white color reappeared in one quarter of the F2 offspring.
Mendel observed the same patterns of inheritance for six other pea plant characters.
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9.3 Mendel’s law of segregation describes the inheritance of a single character
Mendel developed four hypotheses, described below using modern terminology.
1. Alleles are alternative versions of genes that account for variations in inherited characters.
2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.
– A homozygous genotype has identical alleles.
– A heterozygous genotype has two different alleles.
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9.3 Mendel’s law of segregation describes the inheritance of a single character
3. If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.
– The phenotype is the appearance or expression of a trait.
– The genotype is the genetic makeup of a trait.
– The same phenotype may be determined by more than one genotype.
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9.3 Mendel’s law of segregation describes the inheritance of a single character
4. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.
Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation.
– The F1 hybrids all have a Pp genotype.
– A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.
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Figure 9.3B_s1 The Explanation
P generation Genetic makeup (alleles) Purple flowers White flowers
Gametes All p
pp PP
P All
Figure 9.3B_s2 The Explanation
P generation
F1 generation (hybrids)
Genetic makeup (alleles) Purple flowers White flowers
Gametes All p
pp PP
P p Gametes
All Pp
2 1
2 1
P All
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Figure 9.3B_s3 The Explanation
P generation
F1 generation (hybrids)
F2 generation
Genetic makeup (alleles) Purple flowers White flowers
Gametes P All p
pp PP
P
P
P
p
p
p
PP Pp
Pp pp
Eggs from F1 plant
Gametes
Fertilization
All Pp Alleles
segregate
Phenotypic ratio 3 purple : 1 white
Genotypic ratio 1 PP : 2 Pp : 1 pp
Sperm from F1 plant
2 1
2 1
All
Figure 9.3B_4
F2 generation P
P
p
p
PP Pp
Pp pp
Eggs from F1 plant
Phenotypic ratio 3 purple : 1 white
Genotypic ratio 1 PP : 2 Pp : 1 pp
Sperm from F1 plant
9.4 Homologous chromosomes bear the alleles for each character
A locus (plural, loci) is the specific location of a gene along a chromosome.
For a pair of homologous chromosomes, alleles of a gene reside at the same locus. – Homozygous individuals have the same allele on both
homologues.
– Heterozygous individuals have a different allele on each homologue.
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Figure 9.4
P
P
a
a
B
b
PP aa Bb
Dominant allele
Recessive allele
Gene loci
Homologous chromosomes
Genotype: Heterozygous, with one dominant and one recessive allele
Homozygous for the recessive allele
Homozygous for the dominant allele
9.5 The law of independent assortment is revealed by tracking two characters at once
A dihybrid cross is a mating of parental varieties that differ in two characters.
Mendel performed the following dihybrid cross with the following results: – P generation: round yellow seeds × wrinkled green seeds – F1 generation: all plants with round yellow seeds – F2 generation:
– 9/16 had round yellow seeds – 3/16 had wrinkled yellow seeds – 3/16 had round green seeds – 1/16 had wrinkled green seeds
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Figure 9.5A
4 1
4 1
4 1
4 1
4 1
4 1
4 1
4 1
16 9
16 3
16 3
16 1
2 1
2 1
2 1
2 1
F1 generation
F2 generation
P generation
Gametes
Sperm
Eggs Yellow round
Green round Yellow wrinkled Green wrinkled
×
RRYY rryy
RY ry
RrYy
The hypothesis of dependent assortment Data did not support; hypothesis refuted
The hypothesis of independent assortment Actual results; hypothesis supported
RY
RY
ry
ry
Eggs
RY
RY
rY
rY
Ry
Ry ry
ry
RRYY RrYY RRYy RrYy
RrYY rrYY RrYy rrYy
RRYy RrYy RRyy Rryy
RrYy rrYy Rryy rryy
Sperm
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Figure 9.5B
Phenotypes
Genotypes
Black coat, normal vision
B_N_
Black coat, blind (PRA)
B_nn
Blind
Chocolate coat, normal vision
bbN_
Blind
Blind Blind
Chocolate coat, blind (PRA)
bbnn
Mating of double heterozygotes (black coat, normal vision) BbNn BbNn ×
Phenotypic ratio of the offspring
9 Black coat,
normal vision
3 Black coat, blind (PRA)
1 Chocolate coat,
blind (PRA)
3 Chocolate coat, normal vision
9.5 The law of independent assortment is revealed by tracking two characters at once
Mendel needed to explain why the F2 offspring – had new nonparental combinations of traits and
– a 9:3:3:1 phenotypic ratio.
Mendel – suggested that the inheritance of one character has no
effect on the inheritance of another,
– suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and
– called this the law of independent assortment.
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9.5 The law of independent assortment is revealed by tracking two characters at once
The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers:
– black versus chocolate color,
– normal vision versus progressive retinal atrophy.
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9.6 Geneticists can use the testcross to determine unknown genotypes
A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.
A testcross can show whether the unknown genotype includes a recessive allele.
Mendel used testcrosses to verify that he had true-breeding genotypes.
The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes.
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Figure 9.6
What is the genotype of the black dog?
Two possibilities for the black dog:
× Testcross
Genotypes
Gametes
Offspring All black 1 black : 1 chocolate
or
B_? bb
Bb BB
B B
b b
b
Bb Bb bb
9.7 Mendel’s laws reflect the rules of probability
Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected – the segregation of allele pairs during gamete formation
and
– the re-forming of pairs at fertilization.
The probability scale ranges from 0 to 1. An event that is – certain has a probability of 1 and
– certain not to occur has a probability of 0.
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9.7 Mendel’s laws reflect the rules of probability
The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.
Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event.
The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition.
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Figure 9.7
F1 genotypes
Formation of eggs
Formation of sperm
Bb female Bb male
Sperm
F2 genotypes Eggs
B
B B B B
B
b
b
b b b b
×
2 1
2 1
2 1
2 1
2 1
2 1
4 1
4 1
4 1
4 1
( )
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
In a simple dominant-recessive inheritance of dominant allele A and recessive allele a,
– a recessive phenotype always results from a homozygous recessive genotype (aa) but
– a dominant phenotype can result from either
– the homozygous dominant genotype (AA) or
– a heterozygous genotype (Aa).
Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.
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Figure 9.8A Dominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
The inheritance of human traits follows Mendel’s laws.
A pedigree
– shows the inheritance of a trait in a family through multiple generations,
– demonstrates dominant or recessive inheritance, and
– can also be used to deduce genotypes of family members.
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Figure 9.8B
First generation (grandparents)
Second generation (parents, aunts, and uncles)
Third generation (two sisters)
Female Male Attached Free
Ff Ff Ff ff
Ff Ff ff ff ff
ff
FF or Ff
FF or Ff
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9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
Inherited human disorders show either
1. recessive inheritance in which
– two recessive alleles are needed to show disease,
– heterozygous parents are carriers of the disease-causing allele, and
– the probability of inheritance increases with inbreeding, mating between close relatives.
2. dominant inheritance in which
– one dominant allele is needed to show disease and
– dominant lethal alleles are usually eliminated from the population.
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Figure 9.9A
Parents
Offspring
Sperm
Eggs
Normal Dd
Normal Dd ×
D
D
d
d
DD Normal
Dd Normal
(carrier)
Dd Normal
(carrier)
dd Deaf
9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is
– recessive and
– carried by about 1 in 31 Americans.
Dominant human disorders include
– achondroplasia, resulting in dwarfism, and
– Huntington’s disease, a degenerative disorder of the nervous system.
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Table 9.9
New technologies offer ways to obtain genetic information – before conception,
– during pregnancy, and
– after birth.
Genetic testing can identify potential parents who are heterozygous carriers for certain diseases.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
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Several technologies can be used for detecting genetic conditions in a fetus. – Amniocentesis extracts samples of amniotic fluid
containing fetal cells and permits – karyotyping and
– biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease.
– Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
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Figure 9.10A Amniocentesis
Ultrasound transducer
Fetus
Placenta Uterus
Cervix
Amniotic fluid extracted
Centrifugation
Amniotic fluid Fetal cells
Cultured cells
Several hours
Several weeks
Several weeks
Biochemical and genetics tests
Several hours
Several hours
Fetal cells
Cervix Uterus
Chorionic villi
Placenta Fetus
Ultrasound transducer
Tissue extracted from the chorionic villi
Chorionic Villus Sampling (CVS)
Karyotyping
Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects.
Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus.
Newborn screening can detect diseases that can be prevented by special care and precautions.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
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New technologies raise ethical considerations that include – the confidentiality and potential use of results of
genetic testing,
– time and financial costs, and
– determining what, if anything, should be done as a result of the testing.
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
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VARIATIONS ON MENDEL’S LAWS
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9.11 Incomplete dominance results in intermediate phenotypes
Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.
For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which
– neither allele is dominant over the other and
– expression of both alleles occurs.
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Figure 9.11A P generation
F1 generation
F2 generation
2 1
2 1
2 1
2 1
2 1
2 1
Gametes
Gametes
Eggs
Sperm
×
Red RR
White rr
Pink hybrid Rr
R
R
R
R
r
r
r
r
RR rR
Rr rr
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9.11 Incomplete dominance results in intermediate phenotypes
Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation.
One example of incomplete dominance in humans is hypercholesterolemia, in which
– dangerously high levels of cholesterol occur in the blood and
– heterozygotes have intermediately high cholesterol levels.
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Figure 9.11B
Normal Mild disease Severe disease
Phenotypes
Cell
LDL receptor
LDL
HH Homozygous
for ability to make LDL receptors
hh Homozygous
for inability to make LDL receptors
Genotypes Hh
Heterozygous
9.12 Many genes have more than two alleles in the population
Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population.
Human ABO blood group phenotypes involve three alleles for a single gene.
The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.
The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.
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9.12 Many genes have more than two alleles in the population
In codominance,
– neither allele is dominant over the other and
– expression of both alleles is observed as a distinct phenotype in the heterozygous individual.
– AB blood type is an example of codominance.
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Figure 9.12
Blood Group (Phenotype) Genotypes
Carbohydrates Present on Red Blood Cells
Antibodies Present in Blood
A
B
AB
O
IAIA or IAi
IBIB or IBi
IAIB
ii
Carbohydrate A
Carbohydrate B
Carbohydrate A and Carbohydrate B
Neither
Anti-B
Anti-A
Anti-B
Anti-A
None
No reaction Clumping reaction
O A B AB
Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left
9.13 A single gene may affect many phenotypic characters
Pleiotropy occurs when one gene influences many characteristics.
Sickle-cell disease is a human example of pleiotropy. This disease
– affects the type of hemoglobin produced and the shape of red blood cells and
– causes anemia and organ damage.
– Sickle-cell and nonsickle alleles are codominant.
– Carriers of sickle-cell disease are resistant to malaria.
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Figure 9.13A Figure 9.13B An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
Sickled cell
The multiple effects of sickled cells
Damage to organs Other effects Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis)
Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections
9.14 A single character may be influenced by many genes
Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.
Human skin color is an example of polygenic inheritance.
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Figure 9.14 P generation
F1 generation
F2 generation
Eggs
Sperm
Skin color
Frac
tion
of p
opul
atio
n
aabbcc (very light)
AABBCC (very dark)
AaBbCc AaBbCc
×
×
8 1
64 15
64 20
64 6
64 1
64 15
64 6
64 1
8 1
8 1 8 1 8 1
8 1 8 1
8 1
8 1
8 1
8 1
8 1
8 1
8 1
8 1
8 1
9.15 The environment affects many characters
Many characters result from a combination of heredity and the environment. For example,
– skin color is affected by exposure to sunlight,
– susceptibility to diseases, such as cancer, has hereditary and environmental components, and
– identical twins show some differences.
Only genetic influences are inherited.
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THE CHROMOSOMAL BASIS OF INHERITANCE
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9.16 Chromosome behavior accounts for Mendel’s laws
The chromosome theory of inheritance states that
– genes occupy specific loci (positions) on chromosomes and
– chromosomes undergo segregation and independent assortment during meiosis.
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9.16 Chromosome behavior accounts for Mendel’s laws
Mendel’s laws correlate with chromosome separation in meiosis. – The law of segregation depends on separation of
homologous chromosomes in anaphase I.
– The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.
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Figure 9.16_s1 F1 generation All yellow round seeds
(RrYy)
Meta- phase I
of meiosis Y y
R r r
r R
R
Y
Y
y
y
Figure 9.16_s2 F1 generation All yellow round seeds
(RrYy)
Meta- phase I
of meiosis
Anaphase I
Metaphase II R
y
r
Y Y
R r
y
Y
Y
y
y
R
R
r
r r
r
r R
R
R
Y
Y
Y
y
y
y
Figure 9.16_s3 F1 generation
4 1
4 1
4 1
4 1
All yellow round seeds (RrYy)
Meta- phase I
of meiosis
Anaphase I
Metaphase II
Fertilization
Gametes
F2 generation 9 :3 :3 :1
RY ry rY Ry
R
R
R
y y
y
r r
r
Y Y
Y
Y Y
Y
R
R R
r
r r
y
y y
Y
Y
y
y
R
R
r
r r
r
r R
R
R
Y
Y
Y
y
y
y
Figure 9.16_4
Sperm
Eggs
Yellow round
Green round
Yellow wrinkled
Green wrinkled
RY
RY
rY
rY
Ry
Ry ry
ry
RRYY RrYY RRYy RrYy
RrYY rrYY RrYy rrYy
RRYy RrYy RRyy Rryy
RrYy rrYy Rryy rryy
4 1
16 9
16 3
16 3
16 1
4 1
4 1
4 1
4 1
4 1
4 1
4 1
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9.17 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together
Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which – are located close together on the same chromosome and
– tend to be inherited together.
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Figure 9.17_1
The Experiment
Purple flower
Long pollen × PpLl PpLl
Phenotypes Observed offspring
Prediction (9:3:3:1)
Purple long Purple round Red long Red round
284 21 21 55
215 71 71 24
Figure 9.17_2 The Explanation: Linked Genes
Parental diploid cell PpLl
Meiosis
P L
P L
p l
p l Most gametes
Fertilization
Sperm
Most offspring Eggs
3 purple long : 1 red round Not accounted for: purple round and red long
P L P L
P L
P L
PL
PL p l
p l
p l
p l
pl
pl
9.18 SCIENTIFIC DISCOVERY: Crossing over produces new combinations of alleles
Crossing over between homologous chromosomes produces new combinations of alleles in gametes.
Linked alleles can be separated by crossing over, forming recombinant gametes.
The percentage of recombinants is the recombination frequency.
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Figure 9.18A
P L
Tetrad (pair of
homologous chromosomes)
p l
p l p L
p L P l Crossing over
Parental gametes
Recombinant gametes
Figure 9.18C_1
The Experiment
Female Male
ggll GgLl
Black body, vestigial wings
Gray body, long wings (wild type)
Offspring: Gray long Black vestigial Gray vestigial Black long
Recombinant phenotypes
Parental phenotypes
Recombination frequency = = 0.17 or 17% 391 recombinants 2,300 total offspring
965 944 206 185
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Figure 9.18C_2
Offspring
Parental Recombinant
Eggs Sperm
Crossing over
G L g l g l
g l g l g l G l g L
G L g l G l g L g l
g l g l
g l The Explanation
GgLl Female
ggll Male
G L
9.19 Geneticists use crossover data to map genes
When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur.
Recombination frequencies can thus be used to map the relative position of genes on chromosomes.
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Figure 9.19A
Section of chromosome carrying linked genes
Recombination frequencies
17%
9% 9.5%
g c l
Figure 9.19B
Mutant phenotypes Short aristae
Black body (g)
Cinnabar eyes (c)
Vestigial wings (l)
Brown eyes
Red eyes
Normal wings (L)
Red eyes (C)
Gray body (G)
Long aristae (appendages on head)
Wild-type phenotypes
SEX CHROMOSOMES AND SEX-LINKED GENES
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9.20 Chromosomes determine sex in many species
Many animals have a pair of sex chromosomes, – designated X and Y,
– that determine an individual’s sex.
In mammals, – males have XY sex chromosomes,
– females have XX sex chromosomes,
– the Y chromosome has genes for the development of testes, and
– an absence of the Y allows ovaries to develop.
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Figure 9.20A
X
Y
Figure 9.20B
Parents (diploid)
Gametes (haploid)
Offspring (diploid)
Female
Female
Male
Male
Egg Sperm
44 +
XY 44 +
XX
22 + X
22 + Y
22 + X
44 +
XX 44 +
XY
Figure 9.21A Figure 9.21A_1
Figure 9.21A_2 Figure 9.21B Male Female
Sperm
Eggs
R = red-eye allele r = white-eye allele
XR
Xr
XrY XRXR
XRXr XRY
Y
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Figure 9.21C
R = red-eye allele r = white-eye allele
Male Female
Sperm
Eggs XR
xR
XRY XRXr
Y
Xr XrY
XRY XRXR
XrXR
Figure 9.21D
R = red-eye allele r = white-eye allele
Male Female
Sperm
Eggs
XrY
XRXr
XrXr Xr
XR
Xr Y
XrY
XRY
XRXr
9.22 CONNECTION: Human sex-linked disorders affect mostly males
Most sex-linked human disorders are
– due to recessive alleles and
– seen mostly in males.
A male receiving a single X-linked recessive allele from his mother will have the disorder.
A female must receive the allele from both parents to be affected.
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9.22 CONNECTION: Human sex-linked disorders affect mostly males
Recessive and sex-linked human disorders include
– hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,
– red-green color blindness, a malfunction of light-sensitive cells in the eyes, and
– Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.
© 2012 Pearson Education, Inc.
Figure 9.22
Female Male Hemophilia
Carrier
Normal Alexis
Alexandra Czar Nicholas II of Russia
Queen Victoria
Alice Louis
Albert
9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution
The Y chromosome provides clues about human male evolution because
– Y chromosomes are passed intact from father to son and
– mutations in Y chromosomes can reveal data about recent shared ancestry.
© 2012 Pearson Education, Inc.