Download Human Chromosomes

What makes us human?
Human Chromosomes
• To analyze chromosomes, cell biologists
photograph cells in mitosis, when the
chromosomes are fully condensed and easy to
see.
• The biologists then cut out the chromosomes
from the photographs and group them together
in homologous pairs.
• A picture of chromosomes arranged in this way
is known as a karyotype (Homologous pairs)
Human Chromosomes
• A Human body cell contains 46 chromosomes.
• A haploid sperm, carrying just 23
chromosomes, fertilized a haploid egg, also
with 23 chromosomes.
– 22 autosomes (# 1-22)
– 1 sex chromosome (X or Y)
• The diploid zygote, or fertilized egg, contained
the full complement of 46 chromosomes.
Karyotype
• These human
chromosomes have
been cut out of a
photograph and
arranged to form a
karyotype.
Human Chromosomes
• Two of your 46 chromosomes are known as sex
chromosomes, because they determine an
individual’s sex. (Chromosomes # 23)
• Females have two copies of a large X
chromosome. Males have one X and one small
Y chromosome.
• The remaining 44 chromosomes are known as
autosomal chromosomes, or autosomes.
(Chromosomes # 1-22)
Sex Chromosomes:
• All egg cells carry a single X chromosome (23X).
• Half of all sperm cells carry an X chromosome
(23X) and half carry a Y chromosome (23Y).
• This ensures that just about half of the zygotes
will be 46XX and half will be 46XY.
Human Traits
• A pedigree chart, which shows the
relationships within a family, can be used
to help with this task.
• Many human traits are polygenic
(controlled by many genes)
• Environmental effects on gene expression
are not inherited; genes are.
Interest Grabber
Section 14-1
A Family Tree
• To understand how traits are passed on from generation to
generation, a pedigree, or a diagram that shows the relationships
within a family, is used. In a pedigree, a circle represents a
female, and a square represents a male. A filled-in circle or
square shows that the individual has the trait being studied. The
horizontal line that connects a circle and a square represents a
marriage. The vertical line(s) and brackets below that line show
the children of that couple.
Go to
Section:
Interest Grabber continued
1.
This pedigree shows the inheritance of
attached ear lobes. Which parent has
attached ear lobes?
2.
How many children do the parents have?
Which child has attached ear lobes?
3.
Which child is married? Does this child’s
spouse have attached ear lobes? Do any of
this child’s children have attached ear lobes?
Go to
Section:
Section 14-1
Figure 14-3 A Pedigree
A circle represents
a female.
A horizontal line connecting
a male and 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.
Go to
Section:
A square represents
a male.
A vertical line and a
bracket connect the
parents to their children.
A circle or square
that is not shaded
indicates that a
person neither
expresses the trait
nor is a carrier of
the trait.
Human Genes
• Some of the very first human genes to be
identified were those that control blood type.
• The Rh blood group is determined by a
single gene with two alleles —positive and
negative. Rh stands for “rhesus monkey,” the
animal in which this factor was discovered.
– The positive (Rh+) allele is dominant, so persons who
are Rh+/Rh+ or Rh+/Rh− are said to be Rh-positive.
– Individuals with two Rh− alleles are Rh-negative.
Blood Groups
Section 14-1
Phenotype
(Blood Type
Go to
Section:
Genotype
Antigen on
Red Blood Cell
Safe Transfusions
To
From
Human Alleles
• Many human genes have become known
through the study of genetic disorders.
• Genetic Disorders can be caused by
– recessive alleles
– dominant alleles
– Codominant alleles
• What makes an allele dominant, recessive, or
codominant?
– It all depends on the nature of a gene’s protein
product and its role in the cell.
Concept Map
Section 14-1
Autosomol
Disorders
caused by
Dominant alleles
Codominant
alleles
include
include
include
Huntington’s
disease
Sickle cell
disease
Galactosemia
Albinism
Cystic
fibrosis
Go to
Section:
Recessive
alleles
Phenylketonuria
Tay-Sachs
disease
Achondroplasia
Hypercholesterolemia
Section 14-1
Figure 14-8 The Cause of Cystic Fibrosis
Chromosome
#7
CFTR
gene
Go to
Section:
The most common
allele that causes
cystic fibrosis is
missing 3 DNA
bases. As a result,
the amino acid
phenylalanine is
missing from the
CFTR protein.
Normal CFTR is a
chloride ion
channel in cell
membranes.
Abnormal CFTR
cannot be
transported to the
cell membrane.
The cells in the
person’s airways are
unable to transport
chloride ions. As a
result, the airways
become clogged with
a thick mucus.
Sickle Cell Disease
• Sickle cell disease is a
common genetic disorder
found in African
Americans.
• Sickle cell disease is
characterized by the bent
and twisted shape of the
red blood cells
• These sickle-shaped red blood
cells are more rigid than
normal cells and tend to get
stuck in the capillaries, the
narrowest blood vessels in the
body.
• As a result, blood stops
moving through these vessels,
damaging cells and tissues
beyond the blockage.
• Sickle cell disease produces
physical weakness and
damage to the brain, heart,
and spleen. In some cases, it
may be fatal.
Sickle Cell Disease
• Hemoglobin is the protein
that carries oxygen in the
blood.
• Mutation: the amino acid
valine in place of glutamic
acid.
• As a result, the abnormal
hemoglobin is somewhat
less soluble than normal
hemoglobin. Blood gets
stuck in cappillaries.
Why do so many African Americans carry
the sickle cell allele?
• Most African Americans can trace their ancestry
to west central Africa.
• Malaria, a serious parasitic disease that infects
red blood cells, is common in this region of
Africa.
• People who are heterozygous for the sickle
cell allele are generally healthy and are
resistant to malaria.
Checkpoint Questions:
1. What are sex chromosomes? What determines
whether a person is male or female?
2. Using an example, explain how a small change in a
person’s DNA can cause a genetic disorder.
3. How does studying genetic disorders such as PKU help
biologists understand normal alleles?
4. What are some problems biologists face in studying
human inheritance?
5. If a woman with type O blood and a man with type AB
blood have children, what are the children’s possible
genotypes?
Sex-Linked Genes
• Is there a special pattern
of inheritance for genes
located on the X
chromosome or the Y
chromosome?
• The answer is yes.
Because these
chromosomes determine
sex, genes located on
them are said to be sexlinked genes.
Sex-Linked Genes
• Males have just one X
chromosome. Thus, all Xlinked alleles are expressed
in males, even if they are
recessive.
• This means that the recessive
phenotype of a sex-linked
genetic disorder tends to be
much more common among
males than among females.
• In order for a recessive allele,
such as the one for
colorblindness, to be
expressed in females, there
must be two copies of the
allele, one on each of the two
X chromosomes.
• In addition, because men pass
their X chromosomes along to
their daughters, sex-linked
genes move from fathers to
their daughters and may then
show up in the sons of those
daughters.
Expression of X-Linked Alleles:
Colorblindness
•
X-linked alleles are
always expressed in
males, because males
have only one X
chromosome.
• Males who receive the
recessive Xc allele all
have colorblindness.
• Females, however, will
have colorblindness only
if they receive two Xc
alleles.
Sex-Linked Genes: Hemophilia
•
Hemophilia is another
example of a sex-linked
disorder.
• Two important genes carried
on the X chromosome help
control blood clotting.
• A recessive allele in either of
these two genes may
produce a disorder called
hemophilia
• In hemophilia, a protein
necessary for normal blood
clotting is missing.
• About 1 in 10,000 males is
born with a form of hemophilia.
• People with hemophilia can
bleed to death from minor cuts
and may suffer internal
bleeding from bumps or
bruises.
• Fortunately, hemophilia can be
treated by injections of normal
clotting proteins.
Sex-Linked Genes: Duchenne
Muscular Dystrophy
• Duchenne muscular
dystrophy is a sex-linked
disorder that results in the
progressive weakening and
loss of skeletal muscle.
• People with Duchenne
muscular dystrophy rarely live
past early adulthood. In the
United States, one out of every
3000 males is born with
Duchenne muscular dystrophy.
• Duchenne muscular
dystrophy is caused by a
defective version of the gene
that codes for a muscle
protein.
• Researchers in many
laboratories are trying to find a
way to treat or cure this
disorder, possibly by inserting
a normal allele into the muscle
cells of Duchenne muscular
dystrophy patients.
X-Chromosome Inactivation
• Females have two X
chromosomes, but males
have only one.
• In female cells, one X
chromosome is randomly
switched off.
• If just one X chromosome
is enough for cells in
males, how does the cell
“adjust” to the extra X
chromosome in female
cells?
• That 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 active.
X-Chromosome Inactivation: Cats
• In cats, for example, a gene that controls the
color of coat spots is located on the X
chromosome.
• One X chromosome may have an allele for
orange spots and the other may have an allele
for black spots.
• In cells in some parts of the body, one X
chromosome is switched off. In other parts of the
body, the other X chromosome is switched off.
X-Chromosome Inactivation: Cats
• As a result, the cat’s fur will
have a mixture of orange and
black spots, as shown in the
figure below.
• Male cats, which have just one
X chromosome, can have
spots of only one color.
• By the way, this is one way to
tell the sex of a cat. If the cat’s
fur has three colors—white
with orange and black spots,
for example—you can almost
be certain that it is female.
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.” Nondisjunction
can occur either during meiosis I, as
shown in the figure below, or in meiosis II,
Chromosomal Disorders
• Nondisjunction
causes gametes to
have abnormal
numbers of
chromosomes.
• The result of
nondisjunction may
be a chromosome
disorder such as
Down syndrome.
Down Syndrome
•
If two copies of an autosomal
chromosome fail to separate
during meiosis
(nondisjunction) an individual
may be born with three copies
of a chromosome.
• This is known as a trisomy,
meaning “three bodies.” The
most common form of trisomy
involves three copies of
chromosome 21 and is
called Down syndrome.
• In the United States,
approximately 1 baby in 800 is
born with Down syndrome.
• Down syndrome produces mild
to severe mental retardation.
• It is also characterized by an
increased susceptibility to
many diseases and a higher
frequency of some birth
defects.
Sex Chromosomal Disorders:
• Disorders also occur
among 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
do not develop at puberty.
Sex Chromosomal Disorders:
• 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 Klinefelter’s 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.
Checkpoint Questions:
1. Why are sex-linked disorders more common in males
than in females?
2. How does nondisjunction cause chromosome number
disorders?
3. List at least two examples of human sex-linked
disorders.
4. Describe two sex chromosome disorders.
5. Distinguish between sex-linked disorders and sex
chromosome disorders.