Transcript chapter26_lecture

Lecture PowerPoint to accompany
Inquiry into Life
Twelfth Edition
Sylvia S. Mader
Chapter 26
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
26.1 Counseling for Chromosomal
Disorders
• Genetic Counselors
– Help individuals understand modes of inheritance
– Medical consequences of genetic disorders
– Inform individuals of possible decisions they may
need to make
26.1 Counseling for Chromosomal
Disorders
• Karyotyping
– A karyotype is a visual
display of individual’s
chromosomes
– Can be performed on
any babies, children,
teens, adults and on a
fetus
26.1 Counseling for Chromosomal
Disorders
• Amniocentesis
– A prenatal test
– A sample of amniotic
fluid is collected
– Fetal cells can be
isolated and analyzed
– Risk of spontaneous
abortion is 0.25 - 0.5%
26.1 Counseling for Chromosomal
Disorders
• Chorionic Villi
Sampling
– A prenatal test
– Chorionic cells can be
collected and analyzed
– Can be performed
earlier than
amniocentesis
– Risk of spontaneous
abortion is 0.5 - 1.0%
26.1 Counseling for Chromosomal
Disorders
Normal Karyotype (male)
Down Syndrome Karyotype
26.1 Counseling for Chromosomal
Disorders
• Changes in Chromosome Number
– Too many or too few chromosomes
– Due to a process called nondisjunction
– Trisomy:
An extra chromosome is carried
by an egg or sperm
– Monosomy:
An egg or sperm lacks a
chromosome
Nondisjunction of Chromosomes
During Oogenesis
Abnormal Autosomal
Chromosome Number
Abnormal Sex
Chromosome Number
26.1 Counseling for Chromosomal
Disorders
• Changes in Chromosomal Structure
– Chromosomal Mutations
– Radiation, certain chemicals, and some viruses can
causes chromosomes to break apart
• Deletions
• Duplications
• Translocations
• Inversions
26.1 Counseling for Chromosomal
Disorders
• Chromosomal Mutations
– Deletions
• Williams Syndrome
– Chromosome 7 loses an end piece
» Individuals have a turned up nose and a wide mouth
with small chin
» Poor academic skills but well-developed verbal and
musical skills
• Cri Du Chat Syndrome
– Chromosome 5 loses an end piece
– Small head, mental retardation, cat-like cry
A Deletion
26.1 Counseling for Chromosomal
Disorders
• Chromosomal Mutations
– Duplication
• This example is
characterized by poor
muscle tone and autistic
traits
26.1 Counseling for Chromosomal
Disorders
• Chromosomal Mutations
– Translocations
• The exchange of chromosomal segments between two
nonhomologous segments
– A translocation of pieces of chromosomes 14 and 21 is another
phenomena that results in Down syndrome (5% of all cases)
A Translocation
• Alagille Syndrome
– Translocation of chromosomes 2 and 20
26.1 Counseling for Chromosomal
Disorders
• Inversion
– Segment is turned 180
degrees
– Leads to altered gene
activity
26.2 Counseling for Genetic Disorders
• Family Pedigrees
– Chart of Family’s History
• Key
– Males are indicated by squares
– Females are indicated by circles
– Shaded means individual is affected by disorder
Pedigrees for Autosomal Disorders
Autosomal Recessive
Autosomal Dominant
Autosomal Recessive Pedigree
Autosomal Dominant Pedigree
26.2 Counseling for Genetic Disorders
• Pedigrees for Sex-linked Disorders
– X-linked disorders
• X-linked recessive disorders
– To be affected, daughters must inherit it from both parents
– Sons can only inherit it from mother, therefore more males
affected than females
• X-linked dominant disorders
– Affected males pass the trait only to daughters
– Females can pass trait to both daughters and sons
– Y-linked disorders
• Present only in males
• Fathers pass trait to all sons
X-linked Recessive Pedigree
26.2 Counseling for Genetic Disorders
• Genetic Disorders of Interest
– Autosomal Recessive Disorders
• Tay-Sachs Disease
• Cystic Fibrosis (CF)
• Phenylketonuria (PKU)
• Sickle-Cell Disease
26.2 Counseling for Genetic Disorders
• Tay-Sachs Disease
– Lack of Hex A enzyme
– Results in defective
lysosomes (especially
in the brain)
– Symptoms appear in
the first year of life
– Disease is progressive
and fatal
26.2 Counseling for Genetic Disorders
• Cystic Fibrosis
– Most common genetic
disorder in Caucasians
in U.S.
– Defect in chloride
channel proteins in
cells
– Thick, abnormal
mucus production
• Lungs, bronchial tubes,
pancreatic ducts
affected
26.2 Counseling for Genetic Disorders
• Autosomal Recessive Disorders
– Phenylketonuria
• Lack enzyme for phenylalanine metabolism
• Affects nervous system development
• Can usually be controlled with diet
– Sickle-cell anemia
• Irregular red blood cells caused by abnormal hemoglobin
– Clog vessels- poor circulation
– Internal hemorrhaging
• Heterozygous individuals are normal unless dehydrated or
experience mild oxygen deprivation
26.2 Counseling for Genetic Disorders
• Autosomal Dominant Disorders
– Marfan Syndrome
• Defect in fibrillin-protein in elastic connective tissue
– Long limbs and fingers, weakened arteries, dislocated lenses in
the eyes
– Huntington Disease
• Progressive degeneration of brain cells
• Gene for defective protein called Huntington
– Too many copies of the amino acid glutamine
26.2 Counseling for Genetic Disorders
• Incompletely Dominant Disorders
– Familial hypercholesterolemia
• Affects the number of LDL-cholesterol receptors on cells
– Homozygous for defective gene- has no receptors and develops
cardiovascular disease in teenage years
– Heterozygous individual has half the normal number of receptors
26.2 Counseling for Genetic Disorders
• X-linked Recessive Disorders
– Color Blindness
• About 8% of Caucasian males have red-green color
blindness
– Duchene's Muscular Dystrophy
• Absence of a protein called dystrophin
– Causes calcium to leak into muscle cells which actives
enzymes that break down the cells
– Hemophilia
• Hemophilia A is due to a lack of clotting factor VIII
• Hemophilia B is due to a lack of clotting factor IX
• Blood clots slowly or not at all
26.2 Counseling for Genetic Disorders
• Testing for Genetic Disorders
– Testing for a Protein
• Some mutations lead to disorders caused by a missing
enzyme
– Tay-sachs disease-test for quantity of hex A enzyme present in
a sample of cells
» can determine if individual is homozygous normal, a
carrier, or has Tay-sachs
– PKU-blood test done on all newborns to detect the presence of
phenyalanine
Use of a Genetic Marker to Test for a
Genetic Mutation
26.2 Counseling for Genetic Disorders
• Testing for Genetic Disorders
– Testing the DNA
– Testing for genetic markers-similar to DNA fingerprinting
» Restriction enzymes cleave DNA
» Used to test for Huntington disease
– Testing with DNA probes
» DNA probe-single stranded piece of DNA that binds to
complementary DNA
» For genetic testing, the probe has a mutation of interest
– DNA chip can test for many abnormalities at once
» Has many DNA segments
» mutated genes bind if present and are detected by laser
scanner
Use of a DNA Chip
26.2 Counseling for Genetic Disorders
• Testing the Fetus
– Ultrasound
• The fetus is exposed to
high frequency sound
waves to produce a
picture
26.2 Counseling for Genetic Disorders
• Testing Fetal Cells
– Amniocentesis
• Performed at 15 weeks of gestation
• Can also test for alpha fetoprotein (AFP) which can indicate
neural tube defects
– Chorionic Villi Sampling
• Performed earlier than amniocentesis
• no amniotic fluid taken so cannot test for AFP
– Fetal Cells in Mother’s Blood
• A small number of fetal cells enters the mother’s blood
• These can be isolated and analyzed
26.2 Counseling for Genetic Disorders
• Testing the Embryo
– Following in vitro fertilization, the embryo is allowed to
develop. At the 8-cell stage, a cell can be removed
and tested.
– Only “normal” embryos are implanted
• Testing the Egg
– Test the polar bodies of women who are
heterozygous prior to in vitro fertilization
• If the polar body has the defect, then the ovum is normal
Counseling for genetic disorders:
the present cont’d.
• Testing for genetic disorders cont’d.
– Testing the embryo
• If both parents are carrier, they may want
assurance that embryo is normal
– Following in vitro fertilization, can remove a cell at 6-cell
stage and test for defect, then implant only those
embryos that are normal
– Testing the egg
• Test the polar bodies of women who are
heterozygous prior to in vitro fertilization
– If the polar body has the defect, then the ovum is normal
26.3 Genomics
• Genomics is the Study of Genomes.
– Humans have 20,000 - 25,000 genes that code for
proteins
– Many other organisms have more genes than do
humans
26.3 Genomics
• Sequencing the Bases
– The Human Genome project was a project to
sequence all of the base pairs in all of the DNA of
human chromosomes.
• Genome Comparisons
– Scientists can compare the genomes of different
species of organisms to gather data regarding the
cause of many diseases.
26.3 Genomics
• The HapMap Project
– The objective is to catalog common sequence
differences that occur in human beings.
– People have been found to inherit patterns of
sequence differences, these are called haplotypes.
– Information gained from this project will allow
scientists to link haplotypes to the risk of specific
illnesses.
26.3 Genomics
• Proteomics and Bioinformatics
– Proteomics
• Study of structure, function, and interaction of cellular
proteins
• Translation of all human genes results in a collection of
proteins called the proteome
• Computer modeling provides information about the threedimensional shape of protein molecules
– May be possible to correlate drug treatment to genetic profiles
26.3 Genomics
• Proteomics and Bioinformatics
– Bioinformatics
• Application of computer technologies to the study of the
genome
• Analysis of data produced by genomics and proteomics
• Cause and effect relationships between various genetic
profiles and genetic disorders caused by multifactor genes
• Current genome includes gene “deserts” with no known
function
– Bioinformatics may discover functions of these regions
26.3 Genomics
• Expanding Present-Day Genomics
– The Human Genome is described as threedimensional
• One dimension is the sections of DNA that encode proteins
– This accounts for less than 2% of the DNA
• A second dimension is what used to be called “junk” DNA
– This “junk” DNA may code for RNA, researchers are trying to
determine what this RNA does
• A third layer of our genome lies outside DNA.
– It consists of proteins and other chemicals that surround and
adhere to DNA. These may provide clues to the inheritance of
certain diseases.
26.3 Genomics
• Gene Therapy
– Insertion of genetic material into human cells for
treatment of a disorder
– Ex vivo gene therapy
• Cells are removed from the patient
– Treated outside the body and then returned to the patient
• Ex: SCID (Severe Combined Immunodeficiency)
– In vivo gene therapy
• Patient is given a foreign gene directly
• Gene is incorporated into the genome within the body
• Ex: Cystic fibrosis
Ex Vivo Gene Therapy in Humans