Transcript Prentice Hall Biology

Human Chromosomes
• The 46 human chromosomes contain 6
billion base pairs (bp) of DNA.
• Genes make up only 2 percent of the DNA.
• Average human gene is about 3,000 bp.
The largest gene is more than 2 million bp.
Genetic Disorders
• Recessive alleles – phenotype expressed in
homozygous recessive individuals only.
• Dominate alleles – phenotype expressed if
the dominate allele is present (homozygous
and heterozygous dominant).
• Codominant alleles – both alleles are
dominant.
Human Genes and Chromosomes
• Chromosome 22 – approx. 43 million bp;
545 different genes. Genetic disorders
include a form of leukemia and
neurofibromatosis. Also contains long
stretches of repetitive DNA that are
unstable sites where rearrangements can
occur (breakage).
• Chromosome 21 – approx. 32 million bp;
about 225 genes. Genetic disorders include
amyotrophic lateral sclerosis (ALS). Also
contains many regions with no genes at all.
• Many genes that are located close together
on the same chromosome are linked,
meaning that they tend to be inherited
together. May be separated by crossing-over
during meiosis.
Chromosomal Disorders
• Errors in chromosomal separation can occur
during meiosis.
• Nondisjunction – homologous chromosomes
fail to separate. If this occurs, abnormal
numbers of chromosomes may find their
way into gametes, and a disorder of
chromosome numbers may result.
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Down Syndrome
• Trisomy 21 – an individual is born with three
copies of autosomal chromosome 21.
• In U.S., occurs in about 1 in 800 babies
born.
• Produces mild to severe mental retardation
and an increased susceptibility to many
diseases and a higher frequency of birth
defects.
Sex Chromosome Disorders
• Turner’s syndrome (karyotype 45,X) – female that
inherits only one X chromosome. The women are
sterile and their sex organs do not develop at
puberty.
• Klinefelter’s syndrome (karyotype 47,XXY) – Males
with an extra chromosome X. (Y chromosome has
sex-determining region.) Cannot reproduce.
• XXXY and XXXXY have been reported.
Super Male or XYY syndrome
• Affected individuals are usually very tall and
thin.
• Many experience severe acne during
adolescence.
• Additional symptoms may include antisocial
or behavioral problems and learning
disabilities. Intelligence is usually normal,
although IQ, on average, is 10 to 15 points
lower than siblings.
The Human Genome
Karyotyping
• To analyze chromosomes, cells are
photographed during mitosis when
chromosomes are fully condensed and
easy to see.
• Chromosomes are cut out of
photographs and lined up with
homologous chromosome and in order
of largest to smallest.
• Sex chromosome = one of two
chromosomes that determine an
individual’s sex; females have two X
chromosomes; males have one X
chromosome and one Y chromosome.
• Autosomes = chromosome that is not a
sex chromosome.
• 46,XX = female
• 46,XY = male
Polygenic traits
• Trait controlled by more than one gene.
• Examples: height, skin color, and eye
color.
• None of the genes for a polygenic trait are
dominant. Each gene has an active allele
and an inactive allele.
• Active alleles have an additive effect on the
phenotype.
• Inactive alleles do not affect the phenotype.
• Because of the additive effects, a
continuous range of phenotypes is
possible.
Sex-Linked Genes
• Genes located on the X or Y chromosomes.
• Males have one X chromosomes. All Xlinked alleles are expressed in males, even
if they are recessive.
• Because males pass their X chromosomes
along to their daughters, X-linked genes
move from fathers to daughters and then to
sons of the daughters.
Colorblindness
• Three human genes associated with color
vision are located on the X chromosome.
• In males, a defective version of any one of
these genes produces colorblindness.
• The most common form, red-green
colorblindness, is found in about 1 in 10
males and 1 in 100 females.
Figure 14-13 Colorblindness
Section 14-2
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
Figure 14-13 Colorblindness
Section 14-2
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
Hemophilia
• X-linked disorder. Two genes that help
control blood clotting are found on the X
chromosome.
• Recessive allele of either gene causes
hemophilia. A protein necessary for normal
blood clotting is missing.
• Occurs in about 1 in 10,000 males.
• Treated by injections of clotting proteins.
Duchenne Muscular Dystrophy
• Sex-linked disorder that results in the
progressive weakening and loss of skeletal
muscles.
• In the U.S., 1 in 3,000 males affected.
• Caused by a defective version of the gene
that codes for a muscle protein.
Concept Map
Section 14-1
Autosomol
Disorders
caused by
Recessive
alleles
Dominant alleles
Codominant
alleles
include
include
include
Huntington’s
disease
Sickle cell
disease
Galactosemia
Albinism
Cystic
fibrosis
Phenylketonuria
Tay-Sachs
disease
Achondroplasia
Hypercholesterolemia
Blood Types: A, B, AB, O
Rh factor: +, -
Section 14-1
Blood Types
Multiple alleles &
Codominance
Phenotype
(Blood Type
Genotype
Antigen on
Red Blood Cell
Safe Transfusions
To
From
Transfusion Reaction
Symptoms:
fever
chills
rash
flank pain or back pain
bloody urine
fainting or dizziness
Treatment:
transfusion is stopped
corticosteroids
antihistamines
acetominophen (Tylenol)
Figure 14-3 A Pedigree
Section 14-1
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.
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.
A pedigree chart shows the relationships within a family.
From Gene to Molecule
• Cystic fibrosis – caused by the deletion of
three bases in the DNA of the gene
resulting in an amino acid (phenylalanine)
missing from the protein. The protein
produced by the gene is a chloride ion
pump. The deletion of phenylalanine
prevents the protein from being transported
to the cell membrane.
Figure 14-8 The Cause of Cystic Fibrosis
Section 14-1
Chromosome
#7
CFTR
gene
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 – characterized by sickleshaped red blood cells that are more rigid
than normal cells and tend to get stuck in
capillaries leading to damage to cells,
tissues, and organs.
• The normal allele differs from the sickle cell
allele by one DNA base leading to a
substitution of valine for glutamic acid.
Sickle cell disease and malaria:
• In parts of the world
where malaria is a
major health threat,
the presence of the
sickle cell allele in
heterozygous
individuals confers
resistance to malaria.
X-Chromosome Inactivation
• In female cells, one X chromosome is
randomly switched off. The turned-off
chromosome forms a dense region in the
nucleus called a Barr body.
• Also occurs in cats and other mammals (ex,
white cat with orange and black spots).
Go Online
Links from the authors on DNA samples
Career links on geneticists
More information on the Human Genome Project
Interactive test
For links on pedigrees, go to www.SciLinks.org and enter the Web
Code as follows: cbn-4141.