Lecture 8: Genetics and Heritable Disease Objectives: Understand the basis of genetic inheritance Understand the basis of genetic variation Relate meiosis,

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.

http://www.sciencemag.org/

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


Destroying Embryos is the Destroying Embryos is the Basis of the Ethical DebateBasis of the Ethical Debate


Is this simply tissue or is it Is this simply tissue or is it something more?something more?




Is this a twin? The genetic Is this a twin? The genetic make up is identicalmake up is identical



Questions:Questions:What is the purpose?What is the purpose?

Making donor tissue?Making donor tissue?Making a baby?Making a baby?




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?

Documents

Lecture 8: Genetics and Heritable Disease Objectives: Understand the basis of genetic inheritance Understand the basis of genetic variation Relate meiosis,