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Chapter 9
Patterns of Inheritance
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Purebreds and Mutts-A Difference of Heredity
• Genetics is the science of heredity
• A common genetic background will produce
offspring with similar physical and behavioral
traits
– Purebred dogs show less variation than
mutts
– True-breeding individuals are useful in
genetic research
• Behavioral characteristics are also influenced
by environment
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
MENDEL'S LAWS
9.1 The science of genetics has ancient roots
• Early attempts to explain heredity have been
rejected by later science
– Hippocrates' theory of Pangenesis
• Particles from each part of the body travel to
eggs or sperm and are passed on
– Early 19th-century biologists' blending
hypothesis
• Traits from both parents mix in the offspring
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
9.2 Experimental genetics began in an abbey garden
Who Was Gregor Mendel?
Mendel studied botany and mathematics at the
university level before becoming a monk
Experimentation with pea plant inheritance took
place in the monastery garden
Mendel’s background allowed him to see patterns
in the way plant characteristics were inherited
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Gregor Mendel hypothesized that there are
alternative forms of genes, the units that
determine heritable traits
• Mendel crossed pea plants that differed in
certain characteristics
– Could control matings
– Developed true-breeding varieties
– Traced traits from generation to generation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Terminology of Mendelian genetics
– Self-fertilization: fertilization of eggs by
sperm-carrying pollen of the same flower
– Cross-fertilization (cross): fertilization of one
plant by pollen from a different plant
– True-breeding: identical offspring from selffertilizing parents; Plants homozygous for a
characteristic
– Hybrid: offspring of two different varieties
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
– P generation: true-breeding parents
– F1 generation: hybrid offspring of truebreeding parents
– F2 generation: offspring of self-fertilizing F1
parents
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-2b
Petal
Stamen
Carpel
LE 9-2c
Removed stamens
from purple
flower
White
Stamens
Carpel
Parents
(P)
Purple
Transferred pollen
from stamens of
white flower to
carpel of purple
flower
Pollinated carpel
matured into pod
Planted seeds
from pod
Offspring
(F1)
LE 9-2d
Flower color
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
Stem length
Alleles
From his experimental data, Mendel developed
several hypotheses
•
There are alternative forms (alleles) of genes
that account for variation in inherited
characteristics
– Let P stand for the purple flower allele
– Let p stand for the white flower allele
Homozygous and Heterozygous
– For each characteristic, an organism
inherits two alleles, one from each parent.
They can inherit the same form from each
parent or different forms from each parent.
• Homozygous: two identical alleles
• Heterozygous: two different alleles
Phenotype vs Genotype
•
•
The particular combination of the two
alleles carried by an individual is called
the genotype (PP, Pp, or pp)
The physical expression of the genotype
is known as the phenotype (e.g. purple
or white flowers)
Phenotype vs Genotype
•
•
The phenotype of the homozygous
genotype PP is purple flowers
The phenotype of the homozygous
genotype pp is white flowers
Dominant and Recessive Alleles
•
What is the phenotype of genotype Pp?
– The phenotype of Pp is purple flowers
– The P allele masks the presence of the p
allele
– P is the dominant allele while p is recessive
– The dominant allele is always written with a
capital letter while the recessive allele is
written in lower case
9.3 Mendel's law of segregation describes the
inheritance of a single characteristic
How Meiosis Separates Genes
• The two alleles for a characteristic separate
during gamete formation (meiosis)
– Homologous chromosomes separate in
meiosis anaphase I
– Each gamete receives one of each pair of
homologous chromosomes and thus one of the
two alleles per characteristic
How Meiosis Separates Genes
• The separation of alleles in meiosis is
known as Mendel’s Law of Segregation
– The law/principle of segregation: A sperm
or egg carries only one allele for each
inherited trait, because allele pairs
separate from each other during gamete
production
LE 9-3a
Mendel’s Flower Color Experiments
P generation
(true-breeding
parents)

Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 generation
3
4
of plants
have purple flowers
1
4
of plants
have white flowers
LE 9-3b
Genetic makeup (alleles)
PP
pp
P plants
Gametes
All P
All p
F1 plants
(hybrids)
All Pp
Gametes
1
P
2
1
p
2
Sperm
F2 plants
Phenotypic ratio
3 purple : 1 white
P
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
9.6 Geneticists use the testcross to
determine unknown genotypes
•
A test cross is used to deduce the actual
genotype of an organism with a dominant
phenotype (i.e., is the organism PP or
Pp?)
1. Cross the unknown dominant-phenotype
organism (P_) with a homozygous recessive
organism (pp)…
Practical Application: The Test Cross
2. If the dominant-phenotype organism is
homozygous dominant (PP), only dominantphenotype offspring will be produced (Pp)
3. If the dominant-phenotype organism is
heterozygous (Pp), approximately half of the
offspring will be of recessive phenotype (pp)
LE 9-6

Testcross:
Genotypes
bb
B_
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
9.4 Homologous chromosomes bear the two
alleles for each characteristic
• Alternative forms of a gene reside at the same
locus on homologous chromosomes
– Supports the law of segregation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-4
Gene loci
Dominant
allele
P
a
B
P
a
b
Recessive
allele
Genotype:
PP
Homozygous
for the
dominant allele
aa
Homozygous
for the
recessive allele
Bb
Heterozygous
9.7 Mendel's laws reflect the rules of probability
•
Events that follow probability rules are
independent events
– One such event does not influence the
outcome of a later such event
•
The rule of multiplication: The probability of two
events occurring together is the product of the
separate probabilities of the independent events
•
The rule of addition: The probability that an event
can occur in two or more alternative ways is the
sum of the separate probabilities of the different
ways
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-7
F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
2
1
2
1
2
B
B
B
b
B
b
B
1
4
1
4
F2 genotypes
1
2
b
b
B
1
4
b
b
1
4
9.5 The law of independent assortment is revealed
by tracking two characteristics at once
•
Mendel performed genetic crosses in which he
followed the inheritance of two traits at the same
time
• Seed color (yellow vs. green peas) and seed shape
(smooth vs. wrinkled peas) were the characteristics
studied
• The allele symbols were assigned:
– Y = yellow (dominant), y = green (recessive)
– S = smooth (dominant), s = wrinkled (recessive)
• Two trait cross was between two true breeding
varieties for each characteristic
– P: SSYY x ssyy
Mendel's law of independent assortment:
each pair of alleles segregates independently of
other allele pairs during gamete formation
– Genes of pea color and pea shape
(S, s and Y, y) separate independently during
meiosis
•
•
Possible gametes of parent SSYY are SY, SY, SY, and
SY (each S can combine with each Y)
Possible gametes of parent ssyy are sy, sy, sy, and sy
(each s can combine with each y)
Traits Are Inherited Independently
•
Punnett Square from SSYY x ssyy cross
Gametes
¼sy
1
16
¼SY
SsYy
¼sy
¼sy
¼sy
1
16
1
16
1
16
SsYy SsYy
SsYy
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
¼SY SsYy SsYy SsYy SsYy
¼SY SsYy
1
16
SsYy
1
16
¼SY SsYy SsYy
SsYy SsYy
1
16
1
16
SsYy SsYy
F1: All SsYy
Smooth yellow peas
Let’s look at crossing F1s
A dihybrid cross
SsYy x SsYy
LE 9-5a
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P generation
rryy
RRYY
RRYY
ry
Gametes RY
rryy
ry
RrYy
F1 generation
RrYy
Sperm
Sperm
1
2
F2 generation

Gametes RY
1
2
RY
1
2
1
4
rY
1
4
Ry
1
4
ry
ry
RY
Eggs
1
2
RY
1
4
ry
1
4
RY
1
4
rY
Eggs
1
4
Actual results
contradict hypothesis
1
4
RRYY RrYY
RRYy
RrYy
RrYY
RrYy
rrYy
rrYY
9
16
Ry
RRYy
RrYy
RRyy
Rryy
3
16
ry
RrYy
rrYy
Rryy
Actual results
support hypothesis
rryy
3
16
1
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
LE 9-5b
Blind
Phenotypes
Genotypes
Black coat, normal vision
B_N_
Mating of heterozygotes
(black, normal vision)
Phenotypic ratio
of offspring
Chocolate coat, normal vision Chocolate coat, blind (PRA)
bbN_
bbnn
Black coat, blind (PRA)
B_nn
BbNn
9 black coat,
normal vision
Blind
3 black coat,
blind (PRA)

BbNn
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
Steps to follow for a two trait cross
1. Determine the genotype of the parents
ex.
SsYy
x
SsYy
2. Use the product law (rule of multiplication) to determine
genotype and phenotype
- do a punnett square for each trait
- multiply fractions
S s
S SS Ss
s Ss ss
Y y
Y YY Yy
y Yy yy
What is the probability of SsYY?
2/4 * ¼ = 2/16
CONNECTION
9.8 Genetic traits in humans can be tracked
through family pedigrees
• The inheritance of many human traits follows
Mendel's laws
– The dominant phenotype results from either
the heterozygous or homozygous genotype
– The recessive phenotype results from only
the homozygous genotype
• Family pedigrees can be used to determine
individual genotypes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-8a
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
LE 9-8b
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D?
Abigail
Lambert
D?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Hearing
Dominant vs. Recessive
• Is it a dominant pedigree or a recessive pedigree?
• 1. If two affected people have an unaffected child, it must
be a dominant pedigree: D is the dominant mutant allele
and d is the recessive wild type allele. Both parents are
Dd and the normal child is dd.
• 2. If two unaffected people have an affected child, it is a
recessive pedigree: R is the dominant wild type allele
and r is the recessive mutant allele. Both parents are Rr
and the affected child is rr.
• 3. If every affected person has an affected parent it is a
dominant pedigree.
Dominant Autosomal Pedigree
I
2
1
II
1
2
3
4
5
6
III
1
2
3
4
5
6
7
8
9
10
Recessive Autosomal Pedigree
CONNECTION
9.9 Many inherited disorders in humans are
controlled by a single gene
• Thousands of human genetic disorders follow
simple Mendelian patterns of inheritance
– Recessive disorders
• Most genetic disorders
– Can be carried unnoticed by heterozygotes
• Range in severity from mild (albinism) to
severe (cystic fibrosis)
• More likely to occur with inbreeding
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Recessive Genetic Disorders
•
New alleles produced by mutation usually
code for non-functional proteins
•
Alleles coding for non-functional proteins are
recessive to those coding for functional ones
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Recessive Genetic Disorders
•
Heterozygous individuals are carriers of a
recessive genetic trait (but otherwise have a
normal phenotype)
•
Recessive genes are more likely to occur in a
homozygous combination (expressing the
defective phenotype) when related individuals
have children
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-9a
Parents
Normal
Dd
Normal
Dd

Sperm
Offspring
D
d
D
DD
Normal
Dd
Normal
(carrier)
d
Dd
Normal
(carrier)
dd
Deaf
Eggs
Albinism
• Melanin is the dark pigment that colors skin
cells
• Melanin is produced by the enzyme
tyrosinase
• An allele known as TYR (for tyrosinase)
encodes a defective tyrosinase protein in skin
cells, producing no melanin
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Albinism
• Humans and other mammals who are
homozygous for TYR have no skin, fur, or eye
coloring (skin and hair appear white, eyes are
pink)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Dominant disorders
– Some serious, but nonlethal, disorders
(achondroplasia)
– Lethal conditions less common than in
recessive disorders
• Cannot be carried by heterozygotes without
affecting them
• Can be passed on if they do not cause
death until later age (Huntington's
disease=degeneration of brain neurons)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
CONNECTION
9.10 New technologies can provide insight into
one's genetic legacy
• New technologies can provide insight for
reproductive decisions
• Identifying carriers
– Tests can distinguish parental carriers of
many genetic disorders
• Fetal testing
– Amniocentesis and chorionic villus sampling
(CVS) allow removal of fetal cells to test for
genetic abnormalities
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-10a
Amniocentesis
Ultrasound
monitor
Chorionic villus sampling (CVS)
Needle inserted
through abdomen to
extract amniotic fluid
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Ultrasound
monitor
Fetus
Fetus
Placenta
Placenta
Chorionic
villi
Uterus
Cervix
Cervix
Uterus
Amniotic
fluid
Centrifugation
Fetal
cells
Fetal
cells
Several
weeks
Biochemical
tests
Karyotyping
Several
hours
• Fetal imaging
– Ultrasound imaging uses sound waves to
produce a picture of the fetus
• Newborn screening
– Some genetic disorders can be detected at
birth by routine tests
• Ethical considerations
– How will genetic testing information be
used?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
VARIATIONS ON MENDEL'S LAWS
9.11 The relationship of genotype to phenotype is
rarely simple
• Mendel's principles are valid for all sexually
reproducing species
• However, most characteristics are inherited in
ways that follow more complex patterns
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
9.12 Incomplete dominance results in
intermediate phenotypes
• Complete dominance
– Dominant allele has same phenotypic effect
whether present in one or two copies
• Incomplete dominance
– Heterozygote exhibits characteristics
intermediate between both homozygous
conditions
– Not the same as blending
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-12a
P generation
Red
RR
White
rr

R
Gametes
r
F1 generation
Pink
Rr
Gametes
1
2
R
1
2
r
Sperm
1
2
F2 generation
R
1
2
r
1
2
R
Red
RR
Pink
rR
1
2
r
Pink
Rr
White
rr
Eggs
Incomplete Dominance
• Human hair texture is influenced by a gene
with two incompletely dominant alleles, C1 and
C2
• A person with two copies of the C1 allele has
curly hair
• Someone with two copies of the C2 allele has
straight hair
• Heterozygotes (C1C2 genotype) have wavy
hair
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Incomplete Dominance
• If two wavy-haired people marry, their
children could have any of the three hair
types: curly (C1C1), wavy (C1C2), or
straight (C2C2)
LE 9-12b
Genotypes:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes:
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
9.13 Many genes have more than two alleles in
the population
• In a population, multiple alleles often exist for a
single characteristic
• Each individual still carries two alleles for this
characteristic
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Multiple Alleles
• Examples of multiple allelism
– Thousands of alleles for eye color in fruit flies,
producing white, yellow, orange, pink, brown,
or red eyes
– Human blood group genes producing blood
types A, B, AB, and O
• Three alleles in this system: A, B, and O
LE 9-13
Blood
Group
(Phenotype)
Genotypes
O
ii
Anti-A
Anti-B
A
IAIA
or
IAi
Anti-B
B
IBIB
or
IBi
Anti-A
AB
IAIB
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed with
Antibodies from Groups at Left
O
A
B
AB
Codominance
•
•
Some alleles are always expressed even
in combination with other alleles
Heterozygotes display phenotypes of both
the homozygote phenotypes in
codominance
Codominance
•
Example: Human blood group alleles
– Alleles A and B are codominant
– Type AB blood is seen where individual has
the genotype AB
Example #2:
Roan Cattle
9.14 A single gene may affect many phenotypic
characteristics
• Pleiotropy: a single gene may influence
multiple characteristics
• Example: sickle cell disease
– Allele causes production of abnormal
hemoglobin in homozygotes
• Many severe physical effects
– Heterozygotes generally healthy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
– Most common inherited disorder among
people of African descent
• Allele persists in population because
heterozygous condition protects against
malaria
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-14
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle-cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
9.15 A single characteristic may be influenced by
many genes
• Polygenic inheritance is the additive effects of
two or more genes on a single phenotypic
characteristic
• Example: human skin color
– Controlled by at least three genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-15

P generation
aabbcc
AABBCC
(very light) (very dark)

F1 generation
AaBbCc
AaBbCc
1
64
Sperm
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
15
64
20
64
1
8
20
64
1
8
1
8
Eggs
1
8
Fraction of population
F2 generation
1
8
6
64
15
64
6
64
1
64
Skin color
15
64
6
64
1
64
9.16 The environment affects many
characteristics
• Many characteristics result from a combination
of genetic and environmental factors
– Nature vs. nurture is an old and hotly
contested debate
– Only genetic influences are inherited
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Environmental Influence
•
Example: Himalayan rabbit
– Himalayan rabbits have the genotype for black
fur all over the body
– Black pigment is only produced in colder
areas of the body: the nose, ears, and paws
Environmental Influence
•
Both heredity and environment play major
roles in the development of some
characteristics
– Identical twin studies in humans reveal
different IQ scores between twins
CONNECTION
9.17 Genetic testing can detect disease-causing
alleles
• Predictive genetic testing may inform people of
their risk for developing genetic diseases
– Used when there is a family history but no
symptoms
– Increased use of genetic testing raises
ethical, moral, and medical issues
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
THE CHROMOSOMAL BASIS OF INHERITANCE
9.18 Chromosome behavior accounts for
Mendel's laws
• Chromosome theory of inheritance
– Genes occupy specific loci on
chromosomes
– Chromosomes undergo segregation and
independent assortment during meiosis
– Thus, chromosome behavior during meiosis
and fertilization accounts for inheritance
patterns
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-18
F1 generation
R
r
All round yellow seeds
(RrYy)
y
Y
r
R
Y
R
r
Y
y
Metaphase I
of meiosis
(alternative arrangements)
y
r
Y
Metaphase II
of meiosis
y
Y
y
Gametes
R
R
1
4 RY
R
Y
y
Anaphase I
of meiosis
R
Y
r
r
1
4
F2 generation 9
y
Y
r
r
r
R
Y
y
r
R
Y
y
r
1
ry
4 rY
Fertilization among the F1 plants
:3
:3
:1
y
Y
(See Figure 9.5A)
y
R
R
1
4
Ry
9.19 Genes on the same chromosome tend to be
inherited together
• Linked genes
– Lie close together on the same
chromosome
– Tend to be inherited together
– Generally do not follow Mendel's law of
independent assortment
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genes on the Same Chromosome
•
Example of genetic linkage
– Flower color and pollen shape are on the
same chromosome in peas
– Gene assignments
•
•
Let P = purple flowers and p = red flowers
Let L = long pollen shape and l = round shape
Genes on the Same Chromosome
•
Example of genetic linkage
– What are the expected gametes from parent
PpLl, where P is linked with L and p is linked with
l?
•
•
Independent assortment would yield: ¼PL, ¼Pl, ¼
pL, ¼pl
Instead, the gametes are mostly PL and pl
LE 9-19
Experiment
Purple flower

PpLl
PpLl
Long pollen
Observed
offspring
Prediction
(9:3:3:1)
Purple long
284
215
Purple round
21
71
Red long
21
71
Red round
55
24
Phenotypes
Explanation: linked genes
PL
Parental
diploid cell
PpLl
pl
Meiosis
Most
gametes
pl
PL
Fertilization
Sperm
Most
offspring
PL
pl
PL
PL
PL
pl
pl
pl
PL
pl
PL
Eggs
pl
3 purple long : 1 red round
Not accounted for: purple round and red long
9.20 Crossing over produces new combinations
of alleles
• During meiosis, homologous chromosomes
undergo crossing over
– Produces new combinations of alleles in
gametes
– Percentage of recombinant offspring is
called the recombination frequency
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-20a
Tetrad
A
B
a
b
A
B
a
b
A
b
a
B
Crossing over
Gametes
• Thomas Hunt Morgan performed some of the
most important studies of crossing over in the
early 1900s
– Used the fruit fly Drosophila melanogaster
– Established that crossing over was the
mechanism that "breaks linkages" between
genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-20c
Experiment
Gray body,
long wings
(wild type)
Black body,
vestigial wings

GgLl
ggll
Female
Male
Offspring
Gray long
Black vestigial
Gray vestigial
Black long
965
944
206
185
Parental
phenotypes
Recombinant
phenotypes
Recombination frequency =
391 recombinants
2,300 total offspring
= 0.17 or 17%
Explanation
g l
GL
GgLl
(female)
GL
g l
g l
g l
G l
g L
Eggs
ggll
(male)
g l
Sperm
GL
g l
G l
g L
g l
g l
g l
g l
Offspring
9.21 Geneticists use crossover data to map
genes
• Morgan and his students greatly advanced
understanding of genetics
• Alfred Sturtevant used crossover data to map
genes in Drosophila
– Used recombination frequencies to map the
relative positions of genes on chromosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-21b
Chromosome
g
c
l
17%
9%
9.5%
Recombination
frequencies
LE 9-21c
Mutant phenotypes
Short
aristae
Long aristae
(appendages
on head)
Black
body
(g)
Gray
body
(G)
Cinnabar
eyes
(c)
Red
eyes
(C)
Vestigial
wings
(l)
Normal
wings
(L)
Wild-type phenotypes
Brown
eyes
Red
eyes
SEX CHROMOSOMES AND SEX-LINKED GENES
9.22 Chromosomes determine sex in many
species
• Many animals have a pair of chromosomes
that determine sex
– Humans: X-Y system
• Male is XY; the Y chromosome has genes
for the development of testes
• Female is XX; absence of a Y chromosome
allows ovaries to develop
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-22a
(male)
(female)
44
+
XY
22
+
X
Parents’
diploid
cells
44
+
XX
22
+
X
22
+
Y
Sperm
44
+
XX
Egg
Offspring
(diploid)
44
+
XY
• Other animals have other sex-determination
systems
– X-O (grasshopper, roaches, some other
insects)
– Z-W (certain fishes, butterflies, birds)
– Chromosome number (ants, bees)
• Different plants have various sexdetermination systems
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-22b
22
+
XX
22
+
X
LE 9-22c
76
+
ZW
76
+
ZZ
LE 9-22d
32
16
9.23 Sex-linked genes exhibit a unique pattern of
inheritance
•
Sex-linked genes are genes for characteristics
unrelated to sex that are located on either sex
chromosome
– In humans, refers to a gene on the X chromosome
•
Because of linkage and location, the inheritance of
these characteristics follows peculiar patterns
– Example: eye color inheritance in fruit flies follows
three possible patterns, depending on the genotype
of the parents
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 9-23b
Female
Male

XRXR
Xr Y
Sperm
Eggs
XR
R = red-eye allele
r = white-eye allele
Xr
Y
XRXr
XRY
LE 9-23c
Female
Male

XRXr
XRY
Sperm
XR
XR
Y
XRXR
XRY
XrXR
Xr Y
Eggs
Xr
LE 9-23d
Female
Male

XRXr
Xr Y
Sperm
Xr
Y
XR
XRXr
XRY
Xr
Xr Xr
Xr Y
Eggs
CONNECTION
9.24 Sex-linked disorders affect mostly males
• In humans, recessive sex-linked traits are
expressed much more frequently in men than in
women
– Most known sex-linked traits are caused by
genes (alleles) on the X chromosome
– Because the male has only one X
chromosome, his recessive X-linked
characteristic will always be exhibited
– Females with the allele are normally carriers
and will exhibit the condition only if they are
homozygous
– Examples: red-green color blindness,
hemophilia, Duchenne muscular dystrophy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-Linked Genetic Disorders
•
Examples of sex-linked (X-linked)
disorders
– Hemophilia (deficiency in blood clotting
protein)
•
Hemophilia gene in Queen Victoria of England was
passed among the royal families of Europe
LE 9-24b
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis