lecture 8: genetics and heritable disease objectives: understand the basis of genetic inheritance...
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Lecture 8: Geneticsand Heritable Disease
Objectives:Understand the basis of genetic inheritanceUnderstand the basis of genetic variationRelate meiosis, to sex and haploid cellsUnderstand chromosome structure and how it
affects general healthExplain 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
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 >70 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.
Fig. 10.8, p. 158
Nucleus of a diploid (2n)Reproductive cell with two pairs of homologouschromosomes
OR
Possible alignmentsof the two homologouschromosomes duringmetaphase I of meiosis
The resulting alignments at metaphase II:
allelic combinationspossible in gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
Sertoli cell
spermatogonium (diploid)
primary spermatocyte
MITOSIS MEIOSIS I MEIOSIS IIpart of the lumen of a seminiferous tubule
immature sperm (haploid)
late spermatid
secondary spermatocyte early spermatids
head (DNA in enzyme-rich cap)
midpiece with mitochondria
tail (with core of microtubules)
Fig. 39.14, p. 659
Fig. 39.17b, p. 662
first polar body
secondary oocyte
antrum
primordial follicle
Ovulation.Mature follicle ruptures and releases the secondary oocyte and the first polar body.
A primordial follicle; meiosis I has been arrested in the primary oocyte inside it.
A corpus luteum forms from remnants of the ruptured follicle.
When no pregnancy occurs, the corpus luteum degenerates.
zona pellucida
follicle cell
granules in cortex of cytoplasm
nuclei fuse
FERTILIZATION
OVULATION
oviduct
ovary
uterus
opening of cervix
vagina
sperm enter
vagina
Fig. 39.20, p. 665
uterus
ovary
oviduct
endometriumIMPLANTATION
FERTILIZATION
inner cell mass
(see next slide)
Fig. 39.21a, p. 666
Fig. 39.21b, p. 667
endometrium
uterine cavity
blastocoel
Trophoblast (surface layer of cells of the blastoyst)
inner cell mass
start of amniotic cavity
start of embryonic disk
start of yolk sac
blood-filled spaces
start of chorionic cavity
DAYS 6-7 DAYS 10-11
DAY 14 DAY 12
yolk sac
chorionic cavitychorionic
villi
chorion
amniotic cavity
connecting stalk
Fig. 39.25, p. 672
Fig. 39.7, p. 652
Blastula Cell migrations in early gastrula
a Dorsal lip is excised from donor embryo, then grafted to an abnormal site in another embryo.
b Graft induces a second invagination.
c Gastrula develops into a double embryo. Most of its tissues originated from the host embryo.
Fig. 39.10, p. 654
Human Embryos Cloned forHuman Embryos Cloned for Stem Cells Stem Cells
In work that observers call both remarkable and In work that observers call both remarkable and inevitable, scientists in Korea have produced an inevitable, scientists in Korea have produced an embryonic stem (ES) cell line from cloned human embryonic stem (ES) cell line from cloned human cellscells
This advance holds promise for replacing cells This advance holds promise for replacing cells damaged by diseases such as Parkinson's and damaged by diseases such as Parkinson's and diabetes. diabetes.
In doing so, the team has apparently overcome some In doing so, the team has apparently overcome some of the obstacles that to date have hampered human of the obstacles that to date have hampered human cloning, cloning,
This work is likely to reignite the smoldering This work is likely to reignite the smoldering debate over how such research should be debate over how such research should be regulated.regulated.
How did they do it?How did they do it?
The secret to their success may be the gentle way in which The secret to their success may be the gentle way in which they removed the nucleus from a human egg. they removed the nucleus from a human egg.
Then they added the nucleus from a Cumulus cell, a kind Then they added the nucleus from a Cumulus cell, a kind of cell that surrounds the developing eggs in an ovary. of cell that surrounds the developing eggs in an ovary.
After prompting the reconstructed egg to start dividing, the After prompting the reconstructed egg to start dividing, the team allowed it to develop for a week to the blastocyst team allowed it to develop for a week to the blastocyst stage, when the embryo forms a hollow ball of cells. stage, when the embryo forms a hollow ball of cells.
They then isolated the inner-cell mass, which would They then isolated the inner-cell mass, which would develop into the fetus. develop into the fetus.
When these cells are grown in culture, they can become ES When these cells are grown in culture, they can become ES cells.cells.
What’s an ES cell good for?What’s an ES cell good for?
ES reproduce indefinitely and can form all the cell ES reproduce indefinitely and can form all the cell types in the body.types in the body.
The ES cell line the team derived seems to form The ES cell line the team derived seems to form bone, muscle, and immature brain cells, for example.bone, muscle, and immature brain cells, for example.
Scientists have hoped to create ES cells with genes Scientists have hoped to create ES cells with genes that match those of a patient, an idea called that match those of a patient, an idea called therapeutic cloning or "cloning for stem cells." therapeutic cloning or "cloning for stem cells."
Key TermsKey TermsCloned human embryoCloned human embryoEmbryonic stem cellEmbryonic stem cellBlastula, BlastocystBlastula, BlastocystPluripotent Pluripotent KaryotypeKaryotype
How does cloning work:How does cloning work:– Where does the egg come fromWhere does the egg come from– Where does the DNA come fromWhere does the DNA come from– How many copies of each chromosomeHow many copies of each chromosome
Ethical QuestionsEthical Questions
Destroying Embryos is the Basis of Destroying Embryos is the Basis of the Ethical Debatethe Ethical Debate
Questions:Questions:
What is the moral status of What is the moral status of the developing embryothe developing embryo
Ethical QuestionsEthical Questions
Destroying Embryos is the Destroying Embryos is the Basis of the Ethical DebateBasis of the Ethical Debate
Questions:Questions:
Is this simply tissue or is it Is this simply tissue or is it something more?something more?
Ethical QuestionsEthical Questions
Destroying Embryos is the Destroying Embryos is the Basis of the Ethical DebateBasis of the Ethical Debate
Questions:Questions:
Is this a twin? The genetic Is this a twin? The genetic make up is identicalmake up is identical
Ethical QuestionsEthical Questions
Destroying Embryos is the Destroying Embryos is the Basis of the Ethical DebateBasis of the Ethical Debate
Questions:Questions:What is the purpose?What is the purpose?
Making donor tissue?Making donor tissue?Making a baby?Making a baby?
Ethical QuestionsEthical Questions
Destroying Embryos is the Destroying Embryos is the Basis of the Ethical DebateBasis of the Ethical Debate
Questions:Questions:
Is regenerative medicine Is regenerative medicine ethical?ethical?
1- Scientific Imperialism1- Scientific Imperialism
• Science is the Truth ArbiterScience is the Truth Arbiter– Therefore, anything goes if scientists Therefore, anything goes if scientists
say so.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.
2- Postmodern Relativism2- Postmodern Relativism• Plurality of TruthsPlurality of Truths
– Science is only one form ofScience is only one form of Subjective Subjective TruthTruth
– Science has made errors in the past, Science has made errors in the past, Therefore, science and scientists should be:Therefore, science and scientists should be:
– Questioned…Questioned…– Evaluated…Evaluated…– Regulated…Regulated…
SubjectivismSubjectivism holds that holds that science and scientists are not science and scientists are not objective, but antecedents to objective, but antecedents to surroundings, training, surroundings, training, personal experience, etc.personal experience, etc.
3- Godisms3- 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 subjectiveand an ethical society must take care toevaluate and judge science’s pursuits and products carefully.
Lecture Outline
The structure of our genesIntro to chromosomesKaryotypesLinkage and pedigree
Genetic disordersThe big problems
Recombination Broken chromosomesExtra and missing chromosomes
The small problemsMutations
Genes
• Units of information about heritable
traits
• In eukaryotes, distributed among
chromosomes
• Each has a particular locus
– Location on a chromosome
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
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
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
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
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
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)
Sex Determination
XX
XY
XX
XY
X X
Y
X
sex chromosome combinations possible in new individual
Y
X
sperm
X
X
eggs
Female germ cell Male germ cell
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
Effect of YChromosome
10 weeks
Y present
Y absent
7 weeks
birth approaching
appearance of structuresthat will give rise toexternal genitalia
appearance of “uncommitted” duct system
of embryo at 7 weeks
Y present
Yabsent
testis
ovary
testes ovaries
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
Discovering Linkage
homozygous dominant female
recessive male
Gametes:
XX X Y
All F1 offspring have red eyes
x
heterozygous male
heterozygousfemale
One cross
Discovering Linkage
homozygous recessive female
dominantmale
Gametes:
XX X Y
F1 offspring
x
recessive males
heterozygousfemales
Half are red-eyed females, half are white-eyed males
Reciprocal cross
Discovering Linkage
• Morgan’s crosses showed relationship
between sex and eye color
• Females can have white eyes
• Morgan concluded gene must be on the
X chromosome
Linkage Groups
• Genes on one type of chromosome
• Fruit flies
– 4 homologous chromosomes
– 4 linkage groups
• Indian corn– 10 homologous chromosomes
– 10 linkage groups
Full Linkage
xAB ab
50%AB
50%ab
All AaBb
meiosis, gamete formation
Parents:
F1 offspring:
With no crossovers, half of the gametes have one parental genotype and half have the other
AB
ab
AB
ab
ab
AB
Incomplete Linkage
Parents:
F1 offspring
Unequal ratios of four types of gametes:
All AaCc
x
meiosis, gamete formation
AC acA
C AC
AC
ac
ac
Ac
aC
ac
Most gametes have parental genotypes
A smaller number have recombinant genotypes
Crossover Frequency
Proportional to the distance that
separates genesA B C D
Crossing over will disrupt linkage between
A and B more often than C and D
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%
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
Pedigree for Polydactly
I
II
III
IV
V
6 7
12
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
5,5 6,6
6,6 5,5
6,6 5,5
5,6 6,7
6,6 6,6*Gene not expressed in this carrier.
*
malefemale
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
Genetic Disorder
• Inherited conditions that cause mild to
severe medical problems
• Why don’t they disappear?
– Mutation introduces new rare alleles
– In heterozygotes, harmful allele is masked,
so it can still be passed on to offspring
Autosomal Recessive Inheritance Patterns
• If parents are
both
heterozygous,
child will have a
25% chance of
being affected
Galactosemia
• Caused by autosomal recessive allele
• Gene specifies a mutant enzyme in the pathway that breaks down lactose
LACTOSE GALACTOSEGALACTOSE-1-PHOSOPHATE
GALACTOSE-1-PHOSOPHATE
enzyme 1 enzyme 2 enzyme 3
+glucose intermediate
in glycolysis
Autosomal Dominant Inheritance
Trait typically appears in every generation
Huntington Disorder
• Autosomal dominant allele
• Causes involuntary movements, nervous system deterioration, death
• Symptoms don’t usually show up until person is past age 30
• People often pass allele on before they know they have it
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
X-Linked Recessive Inheritance
• Males show disorder more than females
• Son cannot inherit disorder from his father
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
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
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
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
Duplication
normal chromosome
one segment repeated
three repeats
Inversion
A linear stretch of DNA is reversed
within the chromosome
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
Translocation
chromosome
nonhomologous chromosome
reciprocal translocation
Deletion
• Loss of some segment of a chromosome
• Most are lethal or cause serious disorder
Aneuploidy
• Individuals have one extra or less chromosome
• (2n + 1 or 2n - 1)
• Major cause of human reproductive failure
• Most human miscarriages are aneuploids
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
Nondisjunction
n + 1
n + 1
n - 1
n - 1chromosome alignments at metaphase I
nondisjunction at anaphase I
alignments at metaphase II anaphase II
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
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
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
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
Phenotypic Treatments
• Symptoms of many genetic disorders
can be minimized or suppressed by
– Dietary controls
– Adjustments to environmental conditions
– Surgery or hormonal treatments
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
Prenatal Diagnosis
• Amniocentesis
• Chorionic villus sampling
• Fetoscopy
• All methods have some risks
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
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
Philadelphia Chromosome
• First abnormal chromosome to be
associated with a cancer
• Associated with a chronic leukemia
– Overproduction of white blood cells
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)
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 )
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
NH2
CH
O
CH2
CH2
ONH2
OH
Glutamine
NH 2
CH
CHCH 3
O
CH 3
OH
Valine
Sticky Situation
Low Oxygen
Hemoglobin Polymerizes
Cell Sickling
Polymers of hemoglobindeform red blood cells
Normal
Sickle
How Was the Mutation Selected?
• Malaria– Mosquito born plasmodium parasite– Some sickling is good
• Heterozygotes Have the Advantage!
A Reciprocal Translocation1 2
6
13 15
19 20
Chromosome 9
and chromosome
22 exchanged
pieces
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
Understanding Chromosomes
• 1882 - Walter Fleming
• 1887 - August Weismann
• 1900 - Rediscovery of Mendel’s work
Ethical Questions
Destroying Embryos is the Basis of the Destroying Embryos is the Basis of the Ethical DebateEthical Debate
Questions:Questions:What is the purpose?What is the purpose?
Making donor tissue?Making donor tissue?Making a baby?Making a baby?