Download Cengage Learning

PowerLecture:
Chapter 20
Observing Patterns in
Inherited Traits
Learning Objectives




Be able to distinguish between “genes” and
“alleles.”
Know Mendel’s principles of dominance,
segregation, and independent assortment.
Understand how to solve genetics problems
that involve monohybrid and dihybrid
crosses.
Understand the variations that can occur in
observable patterns of inheritance.
Learning Objectives (cont’d)

Explain how a given pair of genes on
homologous chromosomes can separate
during meiosis.
Impacts/Issues
Designer Genes?
Designer Genes?
Our ability to tinker with genes
is growing all the time.



Mapping of the human genome is
pinpointing the locations of genes
on chromosomes.
One result of this effort could be
the correction of genetic defects,
but another could be eugenic
engineering.
Designer Genes?
There may be moral and ethical concerns
involved in deciding which forms of a trait
are more “desirable” or “acceptable” than
others.



Forty percent of Americans say it would be
acceptable to manipulate genes to make their
children smarter or better looking.
Eighteen percent of British parents said it would
be all right to use genetic enhancement to
prevent children from being aggressive.
Video: Genetics in Sports
 This
video clip is available in CNN Today
Videos for Genetics, 2005, Volume VII.
Instructors, contact your local sales
representative to order this volume, while
supplies last.
Useful References for Impacts/Issues
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 Washington
Post: Beyond Steroids: Designer
Genes For Unscrupulous Athletes
 InfoTrac: Designer Genes: Will DNA
Technology Let Parents Design Their Kids?
Ingrid Wickelgren. Current Science, Dec. 3,
2004.
How Would You Vote?
To conduct an instant in-class survey using a classroom response
system, access “JoinIn Clicker Content” from the PowerLecture main
menu.
 Would
you favor legislation that limits or
prohibits engineering genes except for health
reasons?


a. Yes, parents should accept their children as
they are.
b. No, parents should have the right to choose
the kind of child they want to raise.
Useful References for
How Would You Vote?
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
The Science and Politics of
Genetically Modified Humans. Richard
Hayes. World Watch, July–Aug. 2002.
 InfoTrac: Who Gets the Good Genes? Robert
Wright. Time, Jan. 11, 1999.
 InfoTrac: To Build a Baby. Fred Guterl.
Newsweek International, June 30, 2003.
Section 1
Basic Concepts
of Heredity
Basic Concepts of Heredity

Gregor Mendel used experiments in plant
breeding to investigate how sexually
reproducing organisms inherited traits; he
hypothesized that “factors” from each parent
were the units of heredity and formulated
early ideas concerning how they were
passed on.
Basic Concepts of Heredity
The following express Mendel’s ideas in
modern language.





Genes carry encoded information about
specific traits; each gene has a specific locus
on a chromosome.
Diploid cells have two genes (a gene pair) for
each trait—each on a homologous
chromosome.
Alleles are various molecular forms of a gene
for the same trait.
Identical alleles are said to be homozygous; if
the alleles differ, they are heterozygous.
a A pair of homologous chromosomes, each
in the unduplicated state (most often, one
from a male parent and its partner from a
female parent)
b A gene locus (plural, loci) the location for a
specific gene on a specific type of chromosome
c A pair of alleles (each being one chemical form of
a gene) at corresponding loci on a pair of
homologous chromosomes
d Three pairs of genes (at three loci on this pair of
homologous chromosomes) same thing as three
pairs of alleles
© 2007 Thomson Higher Education
Fig. 20.1, p. 374
Basic Concepts of Heredity



Dominant (A) alleles mask the effect of
recessive (a) alleles.
Thus, homozygous dominant = AA,
homozygous recessive = aa, and
heterozygous =Aa.
Genotype refers to
the sum of the genes
we inherit, and
phenotype is how
the genes are
expressed (what you observe).
Useful References for Section 1
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 Genetics
Society of America: Genetics
 InfoTrac: Darwin Would Have Loved It.
Michael J. Novacek. Time, April 17, 2006.
Section 2
One Chromosome, One
Copy of a Gene
One Chromosome, One Copy of a Gene

Mendel hypothesized that each diploid
organism inherits two units for each trait,
one from each parent.
Parents:
CC
cc
(meiosis)
Gametes:
(meiosis)
C
©2007 Thomson Higher Education
C
c
c
In-text Fig, p. 375
One Chromosome, One Copy of a Gene
His first experiment to show this was the
monohybrid cross.




Monohybrid crosses have two parents, P, that
are true-breeding for contrasting forms of a
trait, that is CC and cc.
Mendel discovered that each gene segregates
from the other during meiosis such that each
gamete will receive only one gene per trait.
This separation of genes is the principle of
segregation.
homozygous-dominant parent
homozygous-recessive parent
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
gametes
© 2007 Thomson Higher Education
gametes
fertilization
produces
heterozygous
offspring
Fig. 20.4, p. 375
homozygous-dominant parent
homozygous-recessive parent
(chromosomes
duplicated before
meiosis)
meiosis I
meiosis II
gametes
gametes
fertilization
produces
heterozygous
offspring
Stepped Art
Fig. 20.4, p. 375
Useful References for Section 2
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Human Chromosome 3 Is
Sequenced. UPI NewsTrack, April 27, 2006.
Section 3
Figuring Genetic
Probabilities
Figuring Genetic Probabilities


The parental generation in a cross is
designated P; the children are F1 (first filial);
the grandchildren are the F2 (second filial)
generation.
A Punnett square can be used to predict
the result of a genetic cross.
female gametes (n)
(eggs)
male gametes (n)
(sperm)
C
c
C
c
Cc
cc
C
c
C
c
cc
C
C
c
C
Cc
c
c
Cc
C
CC
Cc
cc
c
Cc
cc
Fig 20.5, p. 376
Figuring Genetic Probabilities

With a monohybrid cross for two heterozygous
parents (Cc), four outcomes are possible each
time a sperm fertilizes an egg.
•
•
Each parent produces C gametes and c gametes.
Put together, the offspring show a 3:1 phenotypic
ratio indicating that 75% of the time the child will
have the dominant trait (either CC or Cc).
F1
phenotypes
Parent:
homozygous
recessive
cc
Alleles segregate
c
c
Parent:
homozygous
dominant
c
c
C
C
Cc
Cc
C
C
Cc
Cc
Cc
Cc
Cc
Cc
CC
© 2007 Thomson Higher Education
Fig. 20.6 (1), p. 377
F1
offspring:
F2
phenotypes
Cc
C
c
F1
offspring:
C
c
C
C
CC
Cc
c
c
Cc
cc
CC
Cc
Cc
© 2007 Thomson Higher Education
Cc
cc
3 dominant (CC, Cc, Cc)
1 recessive (cc)
Fig. 20.6 (2), p. 377
Figuring Genetic Probabilities

Fertilization depends on probability.
•
•
Probability is a number between 0 and 1 that
indicates the likelihood that something will happen (if
0, it never happens; if 1, it always happens).
Thus, each new organism has a probability of three
chances in four of having at least one dominant
allele in the above example.
Figuring Genetic
Probabilities

It is important to
remember two things
about genetic probability:
•
•
Probability is not the same
as possibility; that is, the
outcomes predicted by
probability don’t have to
turn up in a given family.
Probability does not
change; that is, the
probability of having a son
or daughter is always 50%
no matter how many total
children you bear.
Figure 20.7
Figuring Genetic Probabilities
A testcross also can reveal genotypes.



To determine an unknown genotype (a question
of whether it is homozygous dominant [DD] or
heterozygous [Dd]) a testcross is done
between the organism in question and a known
recessive (dd).
If any recessive offspring are produced, then
the organism in question can be designated
heterozygous.
Useful References for Section 3
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac: Advances
in Preconception Genetic
Counseling. Marta C. Wille. Journal of
Perinatal & Neonatal Nursing, Jan.–Mar.
2004.
Section 4
How Genes for Different
Traits Are Sorted into
Gametes
How Genes for Different Traits Are Sorted
into Gametes


The Mendelian principle of independent
assortment states that each gene of a pair
tends to assort into gametes independently
of other gene pairs located on
nonhomologous chromosomes.
Evidence for independent assortment was
obtained from dihybrid crosses, crosses
involving two traits at a time where simple
dominance exists.
How Genes for Different Traits Are Sorted
into Gametes


There are 16 possible allele combinations in the
offspring when each parent is heterozygous for
two traits.
If we look at chin fissure and dimples as being
dominant, then the probable phenotypic ratio
for a cross between heterozygotes is 9:3:3:1 (9
with chin fissure and dimples; 3 with chin
fissure but no dimples; 3 with a smooth chin
and dimples; 1 with a smooth chin and no
dimples).
Nucleus of a diploid (2n) germ cells with
two pairs of homologous chromosomes
OR
a. Possible alignments of
the two homologous
chromosomes during
metaphase I of meiosis
b. The resulting
alignments at
metaphase II
c. Allele
combinations
possible in
gametes
© 2007 Thomson Higher Education
1/4 CD
1/4 cd
1/4 Cd
1/4 cD
Fig. 20.8, p. 378
Dyhibrid Cross
Figures 20.9 and 20.10
Useful References for Section 4
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Germline Susceptibility to
Colorectal Cancer Due to Base-Excision
Repair Gene Defects. Susan M. Farrington et
al. American Journal of Human Genetics,
July 2005.
Section 5
Single Genes,
Varying Effects
Single Genes, Varying Effects
One gene may affect several traits.



Pleiotropy occurs when a single gene affects
two or more aspects of the phenotype.
The recessive condition CHH (cartilage-hair
hypoplasia) occurs following mutation to a gene
called RMRP; individuals commonly have little
body hair, abnormally short limbs, loose
ligaments, and immunological dysfunction.
Mutation of RMRP gene
on chromosome 9
Skin
leads to multiple
effects
Skeleton
Immune
system
Sparse body Abnormally
Weak cellular
hair
short stature,
immunity,
loose ligaments susceptibility
to lymphatic cancer
Fig 20.11, p. 380
Single Genes, Varying Effects

In another example, the gene for sickle-cell
anemia codes for a variant form of hemoglobin,
which in turn not only affects the shape of the
red blood cells, but produces perhaps a dozen
other effects; individuals with sickle-cell trait
(i.e. they are heterozygous for the gene)
generally do not have symptoms.
Figure 20.12a
Normal HbA
Sickle-cell HbS
val
val
his
his
leu
leu
thr
thr
pro
pro
val
val
glu
glu
One amino acid
substituted in
hemoglobin
Fig 20.12c, p. 381
homozygous recessive individual ( HbS/ HbS)
abnormal hemoglobin
sickling of red blood cells
clumping of cells and
interference with
blood circulation
rapid destruction
of sickle cells
anemia
local failures in blood supply
overactivity of
bone marrow
heart
damage
increase in amount
of bone marrow
weakness
and fatigue
skull
deformation
collection of sickle
cells in the spleen
muscle and
joint damage
gastrointestinal
tract damage
dilation
of heart
lung
damage
brain
damage
kidney
damage
poor physical
development
pneumonia
paralysis
kidney
failure
impaired
mental
function
heart failure
rheumatism
abdominal
pain
enlargement,
then fibrosis
of spleen
Fig 20.12b, p. 381
Single Genes, Varying Effects
In codominance, more than one allele of a
gene is expressed.




In codominance, both of the alleles for a given
trait are expressed; this occurs in people
heterozygous for alleles that confer A and B
blood types.
In the ABO blood typing system, there are three
alleles: two that are dominant (IA and IB) and
one that is recessive (i).
In situations where there are more than two
forms of the gene, we call it a multiple allele
system.
Useful References for Section 5
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Pleiotropy and the Genomic
Location of Sexually Selected Genes. Mark J.
Fitzpatrick. The American Naturalist, June
2004.
 InfoTrac: Bone Area and Bone Mineral
Content Deficits in Children with Sickle Cell
Disease. Anne M. Buison et al. Pediatrics,
Oct. 2005.
Section 6
Other Gene Impacts
and Interactions
Other Gene Impacts and Interactions
Penetrance refers to the probability that
someone inheriting an allele will have the
phenotype associated with that allele.


A given phenotype can vary by different
degrees from one individual to the next in a
population—the result of interactions with other
genes and environmental influences.
Other Gene Impacts and Interactions

Several examples illustrate penetrance:
•
•
Cystic fibrosis, caused by a recessive gene, is
completely penetrant.
Polydactyly and campodactyly are incompletely
penetrant and show “variable expressivity.”
Figure 20.13
Other Gene Impacts and Interactions
Polygenic traits: several genes combined.


Most traits are polygenic—they result from the
combined expression of two or more genes;
skin and eye color are examples.
Other Gene Impacts and Interactions

Many traits show continuous variation
(example: height in humans).
Figure 20.15
Other Gene Impacts and Interactions
Do genes “program” behavior?



There is strong evidence that certain basic
human behaviors are genetically programmed.
Human behavior is so complex, however, that it
is difficult to design experiments to answer the
question conclusively.
Useful References for Section 6
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Disease Versus Disease. E.
Richard Stiehm. Pediatrics, Jan. 2006.
 InfoTrac: Mitochondrial Disease. Anthony
H.V. Schapira. The Lancet, July 1, 2006.
 American Psychological Association:
Searching for Genes That Explain Our
Personalities
 NPR: Genes and Behavior
Section 7
Searching for
Custom Cures
Searching for Custom Cures

Each of us, because of our
own personal mix of alleles,
responds differently to
therapeutic drugs; the field
of pharmacogenetics
aims at pinpointing the
relationship between
genetic variation and
response to medications.
Figure 20.16
Searching for Custom Cures

Once genes that control reactions to drugs
are identified, it will become easier and
easier to match therapy to need while at the
same time limiting side effects.
Useful References for Section 7
The latest references for topics covered in this section can be found at
the book companion website. Log in to the book’s e-resources page at
www.thomsonedu.com to access InfoTrac articles.
 InfoTrac:
Scientific, Ethical Questions Temper
Pharmacogenetics. Karen Young Kreeger.
The Scientist, June 11, 2001.
 InfoTrac: A Target for Iressa: The Fall and
Rise (And Fall) of a Pharmacogenetics
Poster Child. David Secko. The Scientist,
April 2006.