Transcript Chapter 14

Chapter 14
The Human Genome
Group 1: Julia Banas, Griffin Cason,
Olivia Katulka, Brianna Shinko,
Julianna Stella
Section 14-1 Human Heredity
By Olivia Katulka & Julia Banas
• Homo sapiens have always been interesting to research
and they have always made scientists wonder.
• Scientists are now beginning to understand human
genetics more than ever before.
• In order for biologists to analyze human
chromosomes, they photograph cells in mitosis when
the chromosomes are condensed and easier to see.
• Then, the biologists cut the individual chromosomes
from their photographs and pair them up with their
homologous chromosome.
• This picture of chromosomes grouped in order in pairs
is known as a karyotype.
Human Chromosomes
• A human body cell contains 46
chromosomes.
• A human begins life when a
haploid sperm with 23
chromosomes fertilizes a
haploid egg with 23
chromosomes.
• The result is a diploid zygote,
or fertilized egg, with 46
chromosomes total.
• Two of the 46 chromosomes are the sex chromosomes
because they determine a person’s sex.
• Females have two large X chromosomes.
• Males have one X and one small Y chromosome.
• The other 44 chromosomes that are not the sex
chromosome are autosomes.
• Both autosome and sex chromosomes are written
together:
Males- 46XY
Females- 46XX
Autosomes
(numbers 1- 22)
Sex
Chromosomes
Based on the Karyotype above, is the zygote a male or a female?
• The way in which sex chromosomes segregate during
meiosis allows males and females to be born in a 50:50
chance.
• ***All egg cells carry a single X chromosome (23X).
Although, half of all sperm cells carry an X
chromosome (23X) and half carry a Y chromosome
(23Y). This makes the ratio half and half, ensuring that
half of the zygotes will be 46XX and half will be
46XY.***
• Human chromosomes are just like the chromosomes
of other eukaryotes and human genes are coded
directly into the nucleotide sequence in DNA.
 Egg cells will always
have a single X
chromosome, while
half male sperm
cells have an X and
half have a Y.
 In every cross
between a male and a
female, the ratio will
always be half XX : half
XY.
• To apply Mendelian genetics to humans, biologists need to
identify an inherited trait controlled by a single gene.
• They need to first establish that the trait is inherited and not
influenced by the environment.
• Biologists then must study how the trait is passed from one
generation to the next.
• Pedigree charts, that show relationships within a family, help
biologists to do that job.
A circle
represents a
female.
A square
represents a
male .
A vertical line and a
bracket connect the
parents to their
children.
A horizontal line
connecting a male
and a female
represents a
marriage.
A half shaded
circle or square
indicates that a
person is a carrier
of the trait.
A completely shaded
circle or square
indicates that a
person expresses the
trait.
A circle or square that
is not shaded indicates
that the person neither
expresses the trait nor
is a carrier of the trait.
***Females who have one allele for the
trait are only carriers, while males who
have one allele express the trait.***
• In this pedigree chart, a male that
expresses red-green color blindness
marries a woman with normal vision.
• They have five children: A male with
normal vision, a female who carries the
color blindness trait, a male with normal
vision, a female who carries the color
blindness trait, and a male with normal
vision.
• The second female child (of the couple in
the first generation) that carries the color
blindness trait marries a man with
normal vision. Their children are a female
who carries the color blindness trait, a
female with normal vision, and a male
who expresses the color blindness trait.
Human Traits
• Some of the most obvious of human traits are impossible to
associate with single genes! Why?
• 1.) Things that we think are single traits, such as the shape of
your eyes or ears, are actually polygenic because they are
controlled by many genes.
• 2.) Many of our personal traits are only partly controlled by
genes because the phenotype is only partially determined by
the genotype. Many genes are even influenced by
environmental, nongenetic, factors such as exercise or
nutrition.
• An example of this is human height. Genes may control the
maximum height of a person, but nutritional improvements
in the United States and Europe in the 1800s have increased
the average height of these countries’ populations about 10
centimeters over their usual average height.
• However, environmental effects on gene expression are not
inherited as genes are.
• If genes are denied a proper environment to reach their full
potential in one generation, they can reach their full
potential in a later generation.
• Our complete set of genetic information is often
called the human genome. The human genome
includes tens of thousands of genes. The DNA
sequences of these genes determines certain
characteristics, such as eye color and structure of
proteins in cells.
• By 2000, the DNA sequence of the human
genome was nearly complete.
• Studying human genes are not easy, which is why
until recently the identification of a human gene
took years of research.
• Why? Humans have a complex life cycle and
produce very few offspring, compared to pea
plants or fruit flies.
• Some of the very first genes that were discovered that could
directly control a single human trait were those of blood type.
• Human blood comes in a variety of different blood groups.
• Knowing a person’s blood type is very important because
using the wrong type of blood for a transfusion during a
medical procedure can be fatal or harmful to that person.
• ABO blood groups and Rh blood groups are the best known
blood groups.
• Rh blood group is determined by a single gene with two alleles- one is
positive and the other is negative.
• Rh stands for “rhesus monkey” because that was the animal that the Rh
factor was discovered in.
• The positive allele (Rh+) is dominant, meaning people who are Rh+/Rh+
or Rh+/Rh- are Rh-positive.
• People with two Rh- alleles are Rh- negative.
• In humans, the Rh factor is not usually a threat until a woman is pregnant.
• If the mother of a child is Rh-negative and the father is Rh-positive it may result in a child
with Rh-positive blood.
• Although the child’s blood system is separated from the mother’s, the baby’s blood may
enter the mother’s system at different times. When the baby’s blood enters the mother’s
system,it will act as an intruder because of the difference in the Rh factor.
• If this happens, the mother will
become sensitized and her body
will make antibodies against the
Rh-positive baby .
• The antibodies will break down the
baby’s red blood cells. This is also
called a hemolytic disease and it
can cause brain damage, anemia,
and severe illness in the child.
Source
http://pregnancy.about.com/od/rhfactor/a/RhFactor-in-Pregnancy.htm
• Rh immunoglobulin (RhIg) can
prevent this hemolytic disease in
pregnant women
ABO Blood Groups- Four Blood Types:
•
•
•
•
A
B
AB
O
*There are three
alleles for this gene,
IA, IB, and i.
Alleles IA and IB are codominant. These alleles produce antigens on the surface
of red blood cells that can be recognized by the immune system.
The i allele is recessive.*
• People with alleles IA and IB produce both antigens, giving
them blood type AB.
• People with alleles IA IA or IA i produce only the A antigen,
giving them the blood type A.
• Those with alleles IB IB or IB i alleles produce only the B
antigen, giving them the blood type B.
• People homozygous for the i allele (ii) do not produce any
antigens, and are said to have blood type O. This is why blood
type O is the universal donor.
Blood Groups
Genotype
Antigen on Red
Blood Cell
Safe
Transfusion:
To
Safe
Transfusion:
From
A
IA IA or IA i
A
A, AB
A, O
B
IB IB or IB i
B
B, AB
B, O
AB
I AI B
A and B
AB
A, B, AB, O
O
ii
none
A, B, AB, O
O
Phenotype or
Blood Type
• Both Rh and ABO groups
are said together when
referring to a person’s
blood type.
• For example if a patient
has AB-negative blood, it
means they have IA and IB
alleles from the ABO gene
and the Rh- allele from the
Rh gene.
Chromosome
with Tay-Sachs
disease
• Through the study of genetic disorders, many
genes have become known.
• Usually, the presence of a normal, functioning
gene is revealed only when an abnormal or
nonfunctioning allele affects the phenotype.
• One of the first genetic disorders that was
understood this way was phenylketonuria (PKU),
which is when someone doesn’t have the enzyme
that breaks down phenylalanine. PKU is caused by
a recessive allele on chromosome 12.
• Another disorder, caused by autosomal recessive
alleles, is Tay-Sachs disease. This results in
nervous system breakdown and death in the first
few years of life.
Some genetic disorders are caused by
dominant alleles. Remember, dominant
alleles are always expressed, even if
there is a recessive allele present. One
example of a disorder caused by
autosomal dominant alleles is
achondroplasia, a form of dwarfism.
Another example is Huntington’s
disease, a progressive loss of
muscle control and mental
function until death occurs.
Usually no symptoms are shown
until middle age.
Sickle cell disease is found in 1 in 500
African Americans. It is a serious
disease that is caused by a codominant
allele.
Though scientists are still trying to figure out the
link between alleles for genetic disorders and the
genetic disorder, in cystic fibrosis and sickle cell
disease, they learned that there is a small change
in the DNA of a single gene that affects the
structure of a protein. This causes a serious
genetic disorder.
• Cystic fibrosis (CF) is caused by a recessive allele on chromosome 7.
• People with CF have serious digestive problems along with producing
heavy mucus that clogs their lungs and breathing passageways.
• CF involves a small genetic change, caused by the deletion of 3 bases
in the middle of a sequence for a protein.
This protein normally allows chloride ions to pass across
biological membranes.
The deletion of these bases removes an amino acid from the
protein, making it fold improperly.
Cells therefore do not transport the protein to the cell
membrane and the misfolded protein is destroyed.
Because the chloride ions can’t be transported, tissues
throughout the body malfunction.
Sickle cell disease is a genetic disorder
found in African Americans. It’s
characterized by a bent and twisted
shape of red blood cells, more rigid than
normal cells. They tend to get stuck in
capillaries, stopping blood from moving
through vessels, damaging cells and
tissues beyond the blockage. This
produces physical weakness and damage
in the brain, heart, and spleen.
Hemoglobin is the protein that carries oxygen through the blood.
The cause of sickle cell disease is one base in DNA—valine is
placed where glutamic acid should be. Because of this, abnormal
hemoglobin is somewhat less soluble than normal hemoglobin. A
decrease of blood oxygen levels causes hemoglobin molecules to
come out of the solution and stick together, making the shape of
the sickle cells.
Many African Americans carry the gene because malaria is
common in Africa. The sickle cell allele makes someone
resistant to malaria. This happens because when the body
destroys the sickled cells, it gets rid of the parasite at the
same time.
Sickle cell disease region
Malaria region
What makes an allele dominant, recessive, or codominant?
•The answer is the nature of a gene’s protein product and its
role in the cell.
• In CF, one copy of the normal allele can supply cells with
enough chloride channel proteins to function.
•Therefore, the normal allele is considered dominant over the
CF allele that is considered recessive.
• The sickle cell allele was once considered recessive, too, but
biologists have discovered that a person with both normal and
sickle cell alleles have a different phenotype from someone
with only normal alleles.
•Because of this, sickle cell alleles are codominant, because
both alleles contribute to the phenotype.
Autosomal Disorders
Type of Disorder
Disorder
Major Symptoms
Disorder caused
by recessive
alleles
Albanism
Lack of pigment in skin, hair, and eyes
Cystic Fibrosis
Excess mucus in lungs, digestive tract, liver; increased
susceptibility to infections; death in childhood, unless treated
Galactosemia
Accumulation of galactose in tissues; mental retardation; eye and
liver damage
Phenylketonuria (PKU)
Accumulation of phenylalanine in tissues; lack of normal skin
pigment; mental retardation
Tay-Sachs disease
Lipid accumulation in brain cells; mental deficiency; blindness;
death in early childhood
Achondroplasia
Dwarfism
Huntington’s disease
Mental deterioration and uncontrollable movements; appears in
middle age
Hypercholestorolemia
Excess cholesterol in blood; heart disease
Sickle cell disease
Sickled red blood in cells; damage to many tissues
Disorders caused
by dominant
alleles
Disorders caused
by codominant
alleles
14-2 Human Chromosomes
By Brianna Shinko
• Chromosomes 21 and 22 are the smallest human autosomes.
• Chromosome 22 contains approximently 43 million DNA bases.
• Chomosome 21 contains roughly 32 million bases.
•Chromosome 22 contains as many as 545 different genes, some of
which are very important for health.
•Genetic disorders on chromosome 22 include an allele that causes a
form of leukemia and another associated with neurofibromatosis, a
tumor causing disease of the nervous system.
•Chromosome 22 also contains long stretches of repetitive DNA
that don’t code for proteins.
• The
structure chromosome 21 is
similar. It contains about 225 genes,
including one associated with ALS or
amyotrophic lateral disease, also know
as Lou Gehrig’s disease.
Lou Gehrig
• Is there a special pattern of inheritance for genes located on the
X chromosome or the Y chromosome? The answer is Yes. Because
these chromosome determine sex, genes located on them are
said to be sex-linked genes.
•More than 100 sex-linked
genetic disorders have now been
mapped to the X chromosome.
•The human Y chromosome is much smaller than the X
chromosome and appears to contain only a few genes.
• 3 human chromosomes associated with color vision are located on
the X chromosome.
• In males, a defective version of any one of these genes produces
colorblindness, an inability to distinguish certain colors.
• The most common form of this disorder, red-green colorblindess, is
found in 1 and 10 males in the U.S.
• Among females, however, colorblindness is rare. Only 1 female in
100 has colorblindness.
• Hemophilia is another example of a sex-linked disorder.
• 2 important genes carried on the X chromosome help control blood
clotting.
• A recessive allele in either of these 2 genes may produce this disorder.
• In hemophilia, a protein necessary for normal blood clotting is
missing. About 1 in 10,000 males are born with a form of this disease.
People with hemophilia can bleed from bumps or bruises.
• Hemophilia can be treated by injections of normal clotting proteins.
• This disease is a sex-linked disorder that results in the progressive
weakening and loss of skeletal muscle.
• People with this disease nearly live past adulthood.
• In the U.S., 1 out of every 3,000 males are born with Duchenne
Muscular Dystrophy.
• Duchenne Muscualar Dystrophy is cause by a defective version
of the gene that codes for a muscle protein.
• Females have two X chromosomes, but males have only one.
• If just one X chromosome is enough for cells in males, how does the
cell “adjust” to the extra X chromosome in female cells? The answer
was discovered by the British geneticist Mary Lyon.
• In female cells, one X chromosome is randomly switched off. That’s
turned-off chromosome forms a dense region in the nucleus known as
a Barr body. Barr bodies are generally not found in males because their
single X chromosome is still alive.
Chromosomal Disorders
• The most common error in meiosis occurs when homologous
chromosomes fail to separate.
• This is known as nondisjunction, which means “not coming apart.”
• If nondisjunction occurs, abnormal numbers of chromosomes may
find their way into gametes, and a disorder of chromosomes
numbers may result.
• If
2 copies of an autosomal chromosome fail to separate during
meiosis, an individual may be born with 3 copies of a chromosome.
This is known as trisomy, meaning “three bodies”.
• The most common form of this involves 3 copies of chromome 21
and is called Down Syndrome.
• In the U.S., 1 baby in 800 is born with Down Syndrome.
• Disorders also occue amoung the sex chromosomes. two of these
abnormalities are Turner’s syndrome and Klinefelter’s syndrome.
• In females, nondisjunction can lead to Turner’s syndrome. A female
with Turner’s syndrome inherits only one X chromosome (genotype
XO). Women with Turner’s syndrome are sterile because their sex
organs don’t develop at puberty.
• In males, nondisjunction causes Klinefelter’s syndrome (genotype
XXY). The extra X chromosome interferes with meiosis and usually
prevents these individuals from reproducing. Cases of this syndrome
have been found in which individuals were XXXY or XXXXY. There have
been no reported instances of babies being born without an X
chromosome, indicating that the X chromosome contains genes that
are vital for normal development.
• These sex chromosome abnormalities point out the role of the Y
chromosome in sex determination. The Y chromosome contains a
sex-determining region that is necessary to produce male sexual
development, even in combination with several X chromosome.
However, in this region if the Y chromosome is absent, the embryo
develops as a female.
Chapter 14 Section 3
Human Molecular Genetics
By: Julianna Stella & Griffin Cason
• There are roughly 6 billion base pairs you carry in your DNA
that are like an encyclopedia with thousands of volumes
• Biologists search through volumes of human genomes using
DNA sequencing
• A variety of genetic tests have been develop to spot different
alleles & genetic disorders
• This makes it possible for parents to determine the risk for
passing on alleles to their children
• The great complexity of human genome
assures that no two individuals are
exactly alike
• DNA fingerprinting is one way to identify
individuals
• It has been used in the U.S. since 1980
• DNA fingerprinting does not analyze the
cells most important genes
• Instead, DNA fingerprinting analyzes the
sections of DNA that have little or no
function but vary widely from one
individual to another
• Advances in DNA sequencing
technology at the close of the
20th century have made it
possible for the first time to
sequence entire genomes
• The Human Genome Project is
an attempt to sequence all
human DNA
• The scientists laid ground work by sequencing
separated regions of DNA
• These “markers” made it possible to find proper
positions for DNA sequences
• Molecular biologists continue to search for
genes, they can locate genes in several
ways. One way is:
Open reading frame-a
series of DNA bases
that can produce part
of a working MRNA
sequence.
• Gene research has always been greatly supported
• Gene therapy is a process of changing the
gene that causes a genetic disorder
• In gene therapy, an absence or faulty gene
is replaced by a normal, working one
• This process gives the correct protein or
enzyme so it eliminates the disorder
• There have been questions about the ethics in
human genetics
• Some people are afraid if we gain too much
knowledge, we could genetically alter traits such as a
person’s hair/eye color, height, sex, blood type etc…
• No one will be the way they were suppose to be
made
• The goal of biology is to better understand life,
because the more we understand about life, the
more scientists will be able to manipulate it.