Lecture 8: Genetics and Heritable Disease

1
Lecture 8 Geneticsand Heritable Disease

• Objectives
• Understand the basis of genetic inheritance
• Understand the basis of genetic variation
• Relate meiosis, to sex and haploid cells
• Understand chromosome structure and how it
  affects general health
• Explain how small changes in DNA information
  result metabolic changes

Key Terms Gene, Chromosome, Allele, Locus, loci,
Mutation, Diploid and haploid, Phenotype and
genotype, Homologous vs. heterozygous, Meiosis
vs. Mitosis, Karyotype, X and Y chromosome, Sex
determination, Linkage, linkage groups, Full and
incomplete linkage, Genetic Markers, Crossover
(Recombination), Pedigree, Autosomal and
sex-linked, Recessive vs. Dominant, Duplication,
Inversion and Translocation, Down Syndrome,
Turner Syndrome, Klinefelter Syndrome, Prisoners
Syndrome.
Chapter 11 for background
2

• Published online February 12, 2004
• Evidence of a Pluripotent Human Embryonic Stem
  Cell Line Derived from a Cloned Blastocyst
• Woo Suk Hwang et al.
• 1 College of Veterinary Medicine, Seoul National
  University, Seoul 151-742, Korea
• Somatic cell nuclear transfer (SCNT) technology
  has recently been used to generate animals with a
  common genetic composition.
• In this study, we report the derivation of a
  pluripotent embryonic stem cell line (SCNT-hES-1)
  from a cloned human blastocyst.
• SCNT-hES-1 cells display typical ES cell
  morphology and cell surface markers and are
  capable of differentiating into embryoid bodies
  in vitro and of forming teratomas in vivo
  containing cell derivatives from all three
  embryonic germ layers in SCID mice. After
  continuous proliferation for gt70 passages,
  SCNT-hES-1 cells maintain normal karyotypes and
  are genetically identical to the somatic nuclear
  donor cells. Although we cannot completely
  exclude the possibility of a parthenogenetic
  origin of the cells, imprinting analyses provide
  support that the derived human ES cells have a
  somatic cell nuclear transfer origin.

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Nucleus of a diploid (2n) Reproductive cell
with two pairs of homologous chromosomes
OR
Possible alignments of the two homologous chromoso
mes during metaphase I of meiosis
A
A
A
A
a
a
a
a
B
B
B
B
b
b
b
b
The resulting alignments at metaphase II
A
A
A
A
a
a
a
a
B
B
B
B
b
b
b
b
allelic combinations possible in gametes
B
B
B
B
b
b
b
b
A
A
A
A
a
a
a
a
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
Fig. 10.8, p. 158
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part of the lumen of a seminiferous tubule
MITOSIS
MEIOSIS I
MEIOSIS II
Sertoli cell
spermatogonium (diploid)
primary spermatocyte
immature sperm (haploid)
late spermatid
secondary spermatocyte
early spermatids
head (DNA in enzyme-rich cap)
tail (with core of microtubules)
midpiece with mitochondria
Fig. 39.14, p. 659
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secondary oocyte
Ovulation. Mature follicle ruptures and releases
the secondary oocyte and the first polar body.
first polar body
antrum
A corpus luteum forms from remnants of the
ruptured follicle.
A primordial follicle meiosis I has been
arrested in the primary oocyte inside it.
When no pregnancy occurs, the corpus luteum
degenerates.
primordial follicle
Fig. 39.17b, p. 662
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oviduct
FERTILIZATION
ovary
OVULATION
uterus
opening of cervix
zona pellucida
vagina
follicle cell
granules in cortex of cytoplasm
sperm enter vagina
nuclei fuse
Fig. 39.20, p. 665
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inner cell mass
(see next slide)
oviduct
uterus
FERTILIZATION
ovary
IMPLANTATION
endometrium
Fig. 39.21a, p. 666
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start of amniotic cavity
start of embryonic disk
Trophoblast (surface layer of cells of the
blastoyst)
endometrium
blastocoel
inner cell mass
start of yolk sac
uterine cavity
DAYS 6-7
DAYS 10-11
chorionic cavity
chorion
chorionic villi
blood-filled spaces
amniotic cavity
start of chorionic cavity
connecting stalk
yolk sac
Fig. 39.21b, p. 667
DAY 14
DAY 12
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Fig. 39.25, p. 672
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Blastula
Cell migrations in early gastrula
Fig. 39.7, p. 652
11
a Dorsal lip is excised from donor embryo, then
grafted to an abnormal site in another embryo.
b Graft induces a second invagination.
Fig. 39.10, p. 654
c Gastrula develops into a double embryo. Most
of its tissues originated from the host embryo.
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Human Embryos Cloned for Stem Cells

• In work that observers call both remarkable and
  inevitable, scientists in Korea have produced an
  embryonic stem (ES) cell line from cloned human
  cells
• This advance holds promise for replacing cells
  damaged by diseases such as Parkinson's and
  diabetes.
• In doing so, the team has apparently overcome
  some of the obstacles that to date have hampered
  human cloning,
• This work is likely to reignite the smoldering
  debate over how such research should be
  regulated.

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How did they do it?

• The secret to their success may be the gentle way
  in which they removed the nucleus from a human
  egg.
• Then they added the nucleus from a Cumulus cell,
  a kind of cell that surrounds the developing eggs
  in an ovary.
• After prompting the reconstructed egg to start
  dividing, the team allowed it to develop for a
  week to the blastocyst stage, when the embryo
  forms a hollow ball of cells.
• They then isolated the inner-cell mass, which
  would develop into the fetus.
• When these cells are grown in culture, they can
  become ES cells.

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Whats an ES cell good for?

• ES reproduce indefinitely and can form all the
  cell types in the body.
• The ES cell line the team derived seems to form
  bone, muscle, and immature brain cells, for
  example.
• Scientists have hoped to create ES cells with
  genes that match those of a patient, an idea
  called therapeutic cloning or "cloning for stem
  cells."

15
Key Terms

• Cloned human embryo
• Embryonic stem cell
• Blastula, Blastocyst
• Pluripotent
• Karyotype
• How does cloning work
• Where does the egg come from
• Where does the DNA come from
• How many copies of each chromosome

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Ethical Questions

• Destroying Embryos is the Basis of the Ethical
  Debate
• Questions
• What is the moral status of the developing embryo

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Ethical Questions

• Destroying Embryos is the Basis of the Ethical
  Debate
• Questions
• Is this simply tissue or is it something more?

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Ethical Questions

• Destroying Embryos is the Basis of the Ethical
  Debate
• Questions
• Is this a twin? The genetic make up is identical

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Ethical Questions

• Destroying Embryos is the Basis of the Ethical
  Debate
• Questions
• What is the purpose?
• Making donor tissue?
• Making a baby?

20
Ethical Questions

• Destroying Embryos is the Basis of the Ethical
  Debate
• Questions
• Is regenerative medicine ethical?

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1- Scientific Imperialism

• Science is the Truth Arbiter
• Therefore, anything goes if scientists say so.

Objectivism is the belief that a scientist can be
removed from or independent of his surroundings
and experiences while making observations,
conclusions and recommendations.
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2- Postmodern Relativism

• Plurality of Truths
• Science is only one form of Subjective Truth
• Science has made errors in the past,
• Therefore, science and scientists should be
• Questioned
• Evaluated
• Regulated

Subjectivism holds that science and scientists
are not objective, but antecedents to
surroundings, training, personal experience, etc.
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3- Godisms

• Mankind is created and ultimately Truth is God
  Revealed.
• Science is a product of mankind, therefore
  science must be carefully evaluated for its
  potential good and/or bad outcomes.

Since Truth is ultimately Revealed and science
is error prone, science is subjective and an
ethical society must take care to evaluate and
judge sciences pursuits and products carefully.
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Lecture Outline

• The structure of our genes
• Intro to chromosomes
• Karyotypes
• Linkage and pedigree
• Genetic disorders
• The big problems
• Recombination
• Broken chromosomes
• Extra and missing chromosomes
• The small problems
• Mutations

25
Genes

• Units of information about heritable traits
• In eukaryotes, distributed among chromosomes
• Each has a particular locus
• Location on a chromosome

26
Homologous Chromosomes

• Homologous autosomes are identical in length,
  size, shape, and gene sequence
• Sex chromosomes are nonidentical but still
  homologous
• Homologous chromosomes interact, then segregate
  from one another during meiosis

27
Alleles

• Different molecular forms of a gene
• Arise through mutation
• Diploid cell has a pair of alleles at each locus
• Alleles on homologous chromosomes may be same or
  different

28
Sex Chromosomes

• Discovered in late 1800s
• Mammals, fruit flies
• XX is female, XY is male
• In other groups XX is male, XY female
• Human X and Y chromosomes function as homologues
  during meiosis

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Karyotype Preparation - Stopping the Cycle

• Cultured cells are arrested at metaphase by
  adding colchicine
• This is when cells are most condensed and easiest
  to identify

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Karyotype Preparation

• Arrested cells are broken open
• Metaphase chromosomes are fixed and stained
• Chromosomes are photographed through microscope
• Photograph of chromosomes is cut up and arranged
  to form karyotype diagram

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Human Karyotype
1 2 3 4 5
6 7 8 9 10
11 12
13 14 15 16 17 18
19 20 21 22 XX (or
XY)
32
Sex Determination
eggs
sperm
Female germ cell
Male germ cell
sex chromosome combinations possible in new
individual
33
The Y Chromosome

• Fewer than two dozen genes identified
• One is the master gene for male sex determination
  
• SRY gene (Sex-determining region of Y)
• SRY present, testes form
• SRY absent, ovaries form

34
Effect of YChromosome
appearance of structures that will give rise
to external genitalia
appearance of uncommitted duct system of
embryo at 7 weeks
7 weeks
Y present
Y absent
Y present
Y absent
testes
ovaries
10 weeks
ovary
testis
birth approaching
35
The X Chromosome

• Carries more than 2,300 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed in both
  males and females

36
Discovering Linkage
One cross
homozygous dominant female
recessive male
x
Gametes
heterozygous female
heterozygous male
All F1 offspring have red eyes
37
Discovering Linkage
Reciprocal cross
homozygous recessive female
dominant male
x
Gametes
X
X
X
Y
heterozygous females
recessive males
F1 offspring
Half are red-eyed females, half are white-eyed
males
38
Discovering Linkage

• Morgans crosses showed relationship between sex
  and eye color
• Females can have white eyes
• Morgan concluded gene must be on the X chromosome

39
Linkage Groups

• Genes on one type of chromosome
• Fruit flies
• 4 homologous chromosomes
• 4 linkage groups
• Indian corn
• 10 homologous chromosomes
• 10 linkage groups

40
Full Linkage
AB
ab
x
Parents
F1 offspring
All AaBb
meiosis, gamete formation
50AB
50ab
With no crossovers, half of the gametes have one
parental genotype and half have the other
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Incomplete Linkage
AC
ac
x
Parents
F1 offspring
All AaCc
meiosis, gamete formation
Unequal ratios of four types of gametes
a
a
A
A
C
c
C
c
Most gametes have parental genotypes
A smaller number have recombinant genotypes
42
Crossover Frequency
Proportional to the distance that separates genes
A
B
C
D
Crossing over will disrupt linkage between A and
B more often than C and D
43
Linkage Mapping in Humans

• Linkage maps based on pedigree analysis through
  generations
• Color blindness and hemophilia are very closely
  linked on X chromosome
• Recombination frequency is 0.167

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Pedigree

• Chart that shows genetic connections among
  individuals
• Standardized symbols
• Knowledge of probability and Mendelian patterns
  used to suggest basis of a trait
• Conclusions most accurate when drawn from large
  number of pedigrees

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Pedigree for Polydactly
male
female
5,5 6,6

5,5 6,6
6,6 5,5
6,6 5,5
6
7
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,6 6,7
12
6,6 6,6
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Genetic Abnormality

• A rare, uncommon version of a trait
• Polydactyly
• Unusual number of toes or fingers
• Does not cause any health problems
• View of trait as disfiguring is subjective

47
Genetic Disorder

• Inherited conditions that cause mild to severe
  medical problems
• Why dont they disappear?
• Mutation introduces new rare alleles
• In heterozygotes, harmful allele is masked, so it
  can still be passed on to offspring

48
Autosomal Recessive Inheritance Patterns

• If parents are both heterozygous, child will have
  a 25 chance of being affected

49
Galactosemia

• Caused by autosomal recessive allele
• Gene specifies a mutant enzyme in the pathway
  that breaks down lactose

enzyme 1
enzyme 2
enzyme 3
GALACTOSE-1- PHOSOPHATE
GALACTOSE-1- PHOSOPHATE
LACTOSE
GALACTOSE
glucose
intermediate in glycolysis
50
Autosomal Dominant Inheritance

• Trait typically appears in every generation

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Huntington Disorder

• Autosomal dominant allele
• Causes involuntary movements, nervous system
  deterioration, death
• Symptoms dont usually show up until person is
  past age 30
• People often pass allele on before they know they
  have it

52
Acondroplasia

• Autosomal dominant allele
• In homozygous form usually leads to stillbirth
• Heterozygotes display a type of dwarfism
• Have short arms and legs relative to other body
  parts

53
X-Linked Recessive Inheritance

• Males show disorder more than females
• Son cannot inherit disorder from his father

54
Examples of X-Linked Traits

• Color blindness
• Inability to distinguish among some of all colors
• Hemophilia
• Blood-clotting disorder
• 1/7,000 males has allele for hemophilia A
• Was common in European royal families

55
Fragile X Syndrome

• An X-linked recessive disorder
• Causes mental retardation
• Mutant allele for gene that specifies a protein
  required for brain development
• Allele has repeated segments of DNA

56
Hutchinson-Guilford Progeria

• Mutation causes accelerated aging
• No evidence of it running in families
• Appears to be dominant
• Seems to arise as spontaneous mutation
• Usually causes death in early teens

57
Duplication

• Gene sequence that is repeated several to
  hundreds of times
• Duplications occur in normal chromosomes
• May have adaptive advantage
• Useful mutations may occur in copy

58
Duplication
normal chromosome
one segment repeated
three repeats
59
Inversion

• A linear stretch of DNA is reversed
• within the chromosome

60
Translocation

• A piece of one chromosome becomes attached to
  another nonhomologous chromosome
• Most are reciprocal
• Philadelphia chromosome arose from a reciprocal
  translocation between chromosomes 9 and 22

61
Translocation
chromosome
nonhomologous chromosome
reciprocal translocation
62
Deletion

• Loss of some segment of a chromosome
• Most are lethal or cause serious disorder

63
Aneuploidy

• Individuals have one extra or less chromosome
• (2n 1 or 2n - 1)
• Major cause of human reproductive failure
• Most human miscarriages are aneuploids

64
Polyploidy

• Individuals have three or more of each type of
  chromosome (3n, 4n)
• Common in flowering plants
• Lethal for humans
• 99 die before birth
• Newborns die soon after birth

65
Nondisjunction
n 1
n 1
n - 1
n - 1
chromosome alignments at metaphase I
nondisjunction at anaphase I
alignments at metaphase II
anaphase II
66
Down Syndrome

• Trisomy of chromosome 21
• Mental impairment and a variety of additional
  defects
• Can be detected before birth
• Risk of Down syndrome increases dramatically in
  mothers over age 35

67
Turner Syndrome

• Inheritance of only one X (XO)
• 98 spontaneously aborted
• Survivors are short, infertile females
• No functional ovaries
• Secondary sexual traits reduced
• May be treated with hormones, surgery

68
Klinefelter Syndrome

• XXY condition
• Results mainly from nondisjunction in mother
  (67)
• Phenotype is tall males
• Sterile or nearly so
• Feminized traits (sparse facial hair, somewhat
  enlarged breasts)
• Treated with testosterone injections

69
XYY Condition

• Taller than average males
• Most otherwise phenotypically normal
• Some mentally impaired
• Once thought to be predisposed to criminal
  behavior, but studies now discredit

70
Phenotypic Treatments

• Symptoms of many genetic disorders can be
  minimized or suppressed by
• Dietary controls
• Adjustments to environmental conditions
• Surgery or hormonal treatments

71
Genetic Screening

• Large-scale screening programs detect affected
  persons
• Newborns in United States routinely tested for
  PKU
• Early detection allows dietary intervention and
  prevents brain impairment

72
Prenatal Diagnosis

• Amniocentesis
• Chorionic villus sampling
• Fetoscopy
• All methods have some risks

73
Preimplantation Diagnosis

• Used with in-vitro fertilization
• Mitotic divisions produce ball of 8 cells
• All cells have same genes
• One of the cells is removed and its genes
  analyzed
• If cell has no defects, the embryo is implanted
  in uterus

74
Chromosomes Cancer

• Some genes on chromosomes control cell growth and
  division
• If something affects chromosome structure at or
  near these loci, cell division may spiral out of
  control
• This can lead to cancer

75
Philadelphia Chromosome

• First abnormal chromosome to be associated with a
  cancer
• Associated with a chronic leukemia
• Overproduction of white blood cells

76
Sickle Cell Anemia

• Recessive trait
• Most common inherited blood disorder in US
• Symptoms-
• Chronic hemolytic anemia
• Severe pain
• Rapid septicemia (infection)
• Asplenia (no spleen left)

77
Inheritance of a Molecular Disease

• Sicklemia and Sickle Cell Anemia
• Tested blood from parents of patients
• Sicklemia- 1 sickled
• Sickle Cell Anemia- 30-60 sickled
• Molecular Disease
• Hemoglobin is the target
• Same size and weight
• Different charge! (Back to Biochemistry ?)

78
Hemoglobin and Sickle Cell Anemia

• Single base mutation in DNA
• A to T transversion
• Single amino acid change in the protein
• Glutamine to Valine
• Slightly increased positive charge

79
Sticky Situation

• Low Oxygen

Hemoglobin Polymerizes
80
Cell Sickling

• Polymers of hemoglobindeform red blood cells

Normal
Sickle
81
How Was the Mutation Selected?

• Malaria
• Mosquito born plasmodium parasite
• Some sickling is good
• Heterozygotes Have the Advantage!

82
A Reciprocal Translocation
1
2

• Chromosome 9 and chromosome 22 exchanged pieces

6
13
15
19
20
83
An Altered Gene

• When the reciprocal translocation occurred, a
  gene at the end of chromosome 9 fused with a gene
  from chromosome 22
• This hybrid gene encodes an abnormal protein that
  stimulates uncontrolled division of white blood
  cells

84
Understanding Chromosomes

• 1882 - Walter Fleming
• 1887 - August Weismann
• 1900 - Rediscovery of Mendels work

85
Ethical Questions

• Destroying Embryos is the Basis of the Ethical
  Debate
• Questions
• What is the purpose?
• Making donor tissue?
• Making a baby?