Transcript Chapter 14 Notes

Humans are difficult to study
-produce few offspring
-mature slowly with long reproductive cycle
-controlled breeding is unethical
-scientists have studied human genes directly or
by looking for patterns in population studies
Human Chromosomes
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To analyze chromosomes, biologists
photograph cells in mitosis (chromosomes are
condensed)
Biologists then cut out the chromosomes from
the photograph and group them together in
pairs
Karyotype- picture of chromosomes arranged
in pairs
Human Chromosomes
Human body cells have 46 chromosomes
 2 of the 46 are sex chromosomes which
determine an individual’s sex
 Females have two large X chromosomes
 Males have 1 X and 1 small Y
 44 of the 46 chromosomes are autosomal
chromosomes or autosomes
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-Males and females are born in a roughly 50:50
ratio because of segregation in meiosis
-All human egg cells carry a
single X chromosome
-Half of the sperm cells carry
an X and half carry a Y
-Thus, half the zygotes will be
XX and half XY
X
X
X
XX
XX
Y
XY
XY
-Human genes are inherited according to the principles of
Gregor Mendel and his work with garden peas
-Biologists study how traits are passed from one generation to
the next using a Pedigree chart
Pedigrees
-show relationships within a family
-genetic counselors analyze pedigrees to infer genotypes of
family members
Pedigrees
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Many traits can’t be
traced easily because they
are polygenic and
influenced by the
environment
For those that can,
pedigrees can predict
how a trait and the genes
that control it are
inherited
Square-male
Circle-female
Full color- trait expressed
Half color- carrier of the trait
Roman Numerals- generation
Numbers- individuals
?- unknown genotype
Horizontal line between parents- marriage
Vertical line- children
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.
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.
What do you know about person II-2?
 Is the trait for hitchhiker’s thumb dominant or
recessive? How do you know?
 Which individuals are carriers?
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Tt
T_
Tt
tt
T_
Tt
tt
Tt
-1st genes to be identified were those that
control blood type
-Blood Types are A, B, AB, and O
-There are also Rh blood groups
-Single gene with two alleles
-Positive or Negative
-Rh is from the Rhesus monkey, the animal in
which this factor was discovered
-Many human genes have become known through
the study of genetic disorders
-Recessive genetic disorders are not expressed in
the phenotype of the heterozygote
-Heterozygous individuals are carriers
-Existence of carriers allows the defective gene to
remain in the gene pool, whereas many dominant
alleles are reduced in numbers since homozygous
individuals often do not survive
- Being a carrier usually does not affect health of
individual
- Children can’t break down phenylaline found in
milk (lack the enzyme)
- Buildup damages nerve cells and causes severe
retardation
-May be controlled by diet
-Testing at birth is now required
-Caused by an autosomal recessive allele on
chromosome 12
PKU Missing Gene to make PAH
(Phenylalanine hydroxylase)
Where is the PAH gene located?
Cytogenetic Location: 12q22-q24.2
Molecular Location on chromosome 12:
base pairs 101,756,233 to 101,835,510
The PAH gene is located on the long (q)
arm of chromosome 12
between positions 22 and 24.2.
More precisely, the PAH gene is located
from base pair
101,756,233 to base pair 101,835,510
on chromosome 12.
-Fatal genetic disorders among Jewish community
from central Europe
-Inability to breakdown a lipid, causing
accumulation of a substance in the brain
-Blindness, seizures, and short life
-There is no treatment, but there is a test for the
allele, so parents can learn if they are at risk
Tay Sachs
-Found predominately in Caucasians
-Recessive allele found on chromosome number 7
-Caused by a small genetic change- deletion of 3
bases in the middle of protein sequence
-Excessive secretion of thick mucus which
accumulates in the pancreas, lungs and other organs
-The deletion removes 1 amino acid causing the
protein to fold incorrectly
Cystic Fibrosis
The Cause of Cystic Fibrosis
Chromosome # 7
The most
common
CFTR
gene
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.
-Always expressed
-Individuals may die before they pass the trait
-Less common than recessive traits
-Dwarfism- achondroplasia
-Huntington’s Disease
- fatal- deterioration of nervous system
-symptoms begin in early 40’s- because
symptoms appear later in life, alleles may be
passed to offspring
Huntington’s Disease
-both alleles express themselves in the heterozygous
-heterozygous may show milder or fewer symptoms
Examples: Sickle Cells Anemia
-Abnormal hemoglobin causes a sickle shape
of red blood cell
-Red blood cells with sickle cell anemia have
a bent and twisted shape, they are more rigid
and easily get stuck in capillaries
-Heterozygous- mix of normal and sickle cells- they are carriers and
have some attacks
-Homozygous- all red blood cells sickle shaped and they have
painful attacks and blood clots
-Caused by a change in one base for making hemoglobin
-Heterozygous can be beneficial because they are resistant to
malaria
Comparing Sickle Trait Distribution
to that of Malaria
Historical distribution of
Malaria shown in green
Distribution of sickle
cell trait shown in pink
and purple
Sickle Cell Hemoglobin
A single amino acid change
causes hemoglobin proteins to
form fibers
Concept Map
Autosomol
Disorders
caused by
Recessive
alleles
Dominant alleles
Codominant
alleles
include
include
include
Huntington’s
disease
Sickle cell
disease
Galactosemia
Albinism
Cystic
fibrosis
Tay-Sachs
disease
Phenylketonuria
Achondroplasia
Hypercholesterolemia
-Chromosomes 21 and 22 are the smallest autosomes
-22 has 43 million base pairs & 21 has 32 million base pairs
Chromosome 22
-As many as 545 different genes
-Disorders
-Form of leukemia
-Neurofibromatasis- tumor causing nervous system disease
-Long stretches of repetitive DNA that does not code
for proteins
-Area is unstable and rearrangement occurs
Chromosome 21
- 225 genes
- Disorders
-ALS- amyotrophic lateral sclerosis (Lou
Gehrig’s disease)
-Many regions with no genes
As we discover what the larger chromosomes contain, we can learn more about
how the arrangement of genes on a chromosome affect gene expression and
development
As we learned in Chapter 11, some genes are linked-they are located on the same
chromosome
This is true for human genes
-Genes located on the sex chromosomes
-Most are found on the X
-The Y chromosome is smaller and has only a few genes
-There are more than 100 sex-linked genetic disorders
-There are 3 genes associated with color vision and
they are found on the X chromosome
-Colorblindness is the inability to distinguish between
certain colors
-The most common is red-green colorblindness, found in 1
in 100 males
-X-linked recessive allele
-Females must be homozygous recessive to be
colorblind (XcXc)
-Males only need 1 allele for the condition (XcY)
Colorblindness
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
Colorblindness
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
-2 genes control blood clotting
-Found in 1 in 10,000 males
- Individuals can bleed to death from minor cut or suffer
internal bleeding from bruises
-Females are usually carriers
-It is believed to have begun in the Royal family of Europe
in the 19th century
-It can be treated by giving normal clotting protein
-Progressive weakening and loss of muscle tissue
-Almost all cases are male
-Death by age 20
-Genes consists of about 3,000,000 nucleotides (longest known human
gene)
-First symptoms appear in childhood when child has difficulty standing
up
-It is caused
by a
defective
muscle
protein
-Females have two X chromosomes
-If males only need 1, why not females? Mary
Lyon, a British genetists, discovered in female
cells, one X chromosome will randomly be
switched off
-The turned off chromosome becomes a dense
region in the nucleus called a Barr body
Cats
-The gene for coat color is found on the X chromosome
-One chromosome may have the allele for orange spots and
another for black spots
-Cells in some parts of the body will inactivate one X and in
other parts the other X
-As a result a female cat may be a mix of orange and black
spots
-Mistakes made in meiosis
-Most common mistake
Nondisjunction- failure of
homologous chromosomes to
separate
-Occurs when 2 copies fail to
separate
-As a result, an individual has
three copies of a chromosome
- trisomy “3 bodies”
-Trisomy 21- individual has 3
copies of chromosome 21
-1 in 800 babies
-Mild to severe retardation, an
increase in the susceptibility to
disease and birth defects
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Down syndrome is correlated with the age of
the mother; older mothers have an increased
risk of giving birth to a child with Down
syndrome
Turner’s Syndrome
Klinefelter’s Syndrome
-Female has only 1 X
chromosome
- Male with XXY
-Sterile- unable to reproduce
-Short
-Sex organs do not develop
at puberty
-Extra X interferes with meiosis
and usually prevents individuals
from reproducing
-Individuals have been found with
XXXY and XXXXY
There have been no cases of babies born with just a
Y which indicates that the X chromosome contains
genes necessary for survival and development
Human DNA Analysis
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6 billion base pairs exist in the human genome
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Genetists “look up” gene sequences to find
disorders and traits
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Can’t read them like a book, there are too many
To test for disorders, genetists look for changes in
the normal sequence
DNA fingerprinting- used to identify people
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Look at DNA with little or no function, but varies
from person to person
Human Genome Project
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Project to analyze the
human DNA
sequence
Scientists have found
there are very few
genes despite all of
the base pairs that
exist
Scientists when
searching for genes
look for promotersbinding sites for
transcription
Promoter
Start
signal
Gene
Stop
signal
Gene Therapy
Bone
marrow
Normal hemoglobin gene
cell
Chromosomes
Genetically engineered virus
Nucleus
Bone
marrow
Cure for genetic disorders
Absent or faulty gene replaced by a normal working
gene
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http://biomedicum.ut.ee/armpgb/1kursus/Ani
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