PowerLecture: Chapter 20

1
PowerLectureChapter 20

• Observing Patterns in Inherited Traits

2
Learning Objectives

• Be able to distinguish between genes and
  alleles.
• Know Mendels 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.

3
Learning Objectives (contd)

• Explain how a given pair of genes on homologous
  chromosomes can separate during meiosis.

4
Impacts/Issues

• Designer Genes?

5
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.

6
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.

7
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.

8
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 books 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.

9
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.

10
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 books e-resources page at
  www.thomsonedu.com to access InfoTrac articles.
• InfoTrac The Science and Politics of Genetically
  Modified Humans. Richard Hayes. World Watch,
  JulyAug. 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.

11
Section 1

• Basic Concepts
• of Heredity

12
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.

13
Animation Crossing Garden Pea Plants
CLICKTO PLAY
14
Basic Concepts of Heredity

• The following express Mendels 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 traiteach 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.

15
Animation Genetic Terms
CLICKTO PLAY
16
Fig. 20.1, p. 374
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
17
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).

18
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 books 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.

19
Section 2

• One Chromosome, One Copy of a Gene

20
One Chromosome, One Copy of a Gene

• Mendel hypothesized that each diploid organism
  inherits two units for each trait, one from each
  parent.

cc
CC
Parents
(meiosis)
(meiosis)
C
C
c
c
Gametes
In-text Fig, p. 375
21
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.

22
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
23
Fig. 20.4, p. 375
homozygous-dominant parent
homozygous-recessive parent
(chromosomes duplicated before meiosis)
meiosis I
meiosis II
fertilization produces heterozygous offspring
Stepped Art
24
Animation Chromosome Segregation
CLICKTO PLAY
25
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 books e-resources page at
  www.thomsonedu.com to access InfoTrac articles.
• InfoTrac Human Chromosome 3 Is Sequenced. UPI
  NewsTrack, April 27, 2006.

26
Section 3

• Figuring Genetic Probabilities

27
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.

28
Fig 20.5, p. 376
29
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 31 phenotypic
  ratio indicating that 75 of the time the child
  will have the dominant trait (either CC or Cc).

30
Fig. 20.6 (1), p. 377
F1 phenotypes
Parenthomozygous recessive
cc
c
c
Alleles segregate
Parent homozygous dominant
c
c
Cc
Cc
C
C
Cc
Cc
CC
C
C
Cc
Cc
Cc
Cc
31
Fig. 20.6 (2), p. 377
F2 phenotypes
F1 offspring
Cc
C
c
F1 offspring
C
c
Cc
CC
C
C
CC
Cc
Cc
Cc
c
c
Cc
cc
cc
3 dominant (CC, Cc, Cc) 1 recessive (cc)
32
Animation Monohybrid Cross
CLICKTO PLAY
33
Animation Three-to-One Ratio
CLICKTO PLAY
34
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.

35
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 dont
  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
36
Animation Genotypes Variation Calculator
CLICKTO PLAY
37
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.

38
Animation Testcross
CLICKTO PLAY
39
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 books 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.

40
Section 4

• How Genes for Different Traits Are Sorted into
  Gametes

41
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.

42
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 9331 (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).

43
Animation Independent Assortment
CLICKTO PLAY
44
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 atmetaphase II
c. Allele combinations possible in gametes
1/4 CD
1/4 cd
1/4 Cd
1/4 cD
Fig. 20.8, p. 378
45
Animation Dyhibrid Cross
CLICKTO PLAY
46
Dyhibrid Cross
Figures 20.9 and 20.10
47
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 books 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.

48
Section 5

• Single Genes,
• Varying Effects

49
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.

50
Mutation of RMRP gene on chromosome 9
leads to multiple effects
Immune system
Skin
Skeleton
Sparse body hair
Abnormally short stature, loose ligaments
Weak cellular immunity, susceptibility to
lymphatic cancer
Fig 20.11, p. 380
51
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
52
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
53
Animation Symptoms of Sickle-Cell Anemia
CLICKTO PLAY
54
Fig 20.12b, p. 381
homozygous recessive individual ( HbS/ HbS)
abnormal hemoglobin
sickling of red blood cells
collection of sickle cells in the spleen
clumping of cells and interference with blood
circulation
rapid destruction of sickle cells
local failures in blood supply
anemia
overactivity of bone marrow
gastrointestinal tract damage
muscle and joint damage
heart damage
kidney damage
dilation of heart
increase in amount of bone marrow
lung damage
brain damage
kidney failure
poor physical development
weakness and fatigue
pneumonia
paralysis
impaired mental function
enlargement, then fibrosis of spleen
abdominal pain
skull deformation
heart failure
rheumatism
55
Animation Overview of Sickle-Cell Anemia
CLICKTO PLAY
56
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.

57
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 books 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.

58
Section 6

• Other Gene Impacts
• and Interactions

59
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
  populationthe result of interactions with other
  genes and environmental influences.

60
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
61
Other Gene Impacts and Interactions

• Polygenic traits several genes combined.
• Most traits are polygenicthey result from the
  combined expression of two or more genes skin
  and eye color are examples.

62
Other Gene Impacts and Interactions

• Many traits show continuous variation (example
  height in humans).

Figure 20.15
63
Animation Height Bar Graph
CLICKTO PLAY
64
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.

65
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 books 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

66
Section 7

• Searching for
• Custom Cures

67
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
68
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.

69
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 books 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.