Transcript Ch 14- Human Heredity

14-1 Human Heredity
Vocabulary:
•Karyotype
•Sex chromosome
•Autosome
•Pedigree
•Polygenic
•Carrier
Key Concept:
How is sex determined?
How do small changes in DNA cause
genetic disorders?
Human Chromosomes
• To examine human chromosomes, biologist
photograph cells in mitosis when the chromosomes
are fully condensed
• Cut out the chromosomes from the picture and
group them together in pairs
• This type of picture is called a
KARYOTYPE
Karyotypes
•
•
A karyotype can tell you THREE
things:
1. Sex (male or female)
2. Irregular numbers of chromosomes
3. Any mutations in the chromosomes
Basically: all the chromosomes in a
cell are displayed and can be
examined for any abnormalities
Remember…
• Humans have 46 chromosomes (23
pair)
• 2 of them are sex chromosomes 
they determine what sex you are
– XX = female
– XY = male
• 44 of them are autosomes  they do
not determine what sex you are
Who determines the sex of
a child?
The mother or the father?
THE FATHER!!!!
Why does the father determine
the sex of the offspring???
• Mom is XX, she can donate either one X
chromosome or the other X chromosome
• Dad is XY, he can donate either an X
chromosome or a Y chromosomes.
• If the offspring receives the father’s X, it
is female
• If the offspring receives the father’s Y, it
is male
The Y chromosome
• If a Y chromosome is present, the
person is male
• X chromosomes contain genes
necessary for growth/development
• No cases of a person born with being
45,0Y
– Probably spontaneously aborted
(miscarriage)
Human Genes
• The human genome is the complete
set of genetic information
– Determines characteristics such as
eye color and how proteins function
within cells
Recessive and Dominant
Alleles
• Some common genetic disorders are
autosomal recessive
– This means that you need two recessive alleles
(on any of the 44 chromosomes—NOT the sex
chromosomes) to express the disease
• EX: Cystic Fibrosis
• Other genetic disorders are autosomal
dominant
– Only one allele is needed for the trait to be
expressed
• EX: Huntington’s Disease
From Gene to molecule
• In both cystic fibrosis and sickle cell
anemia, a small change in the DNA of
a single gene affects the structure of
a protein, causing a serious genetic
disorder
Cystic Fibrosis
• Caused by a recessive allele on chromosome
#7
– It is an autosomal genetic disorder
• Causes digestive and respiratory problems
• Death around 20-30 years of age
• How does it happen?
– Three bases are deleted from the protein,
which removes one amino acid
– The protein cannot fold properly anymore, and
is destroyed
– Result: airway is clogged with mucus
Sickle Cell Anemia
• One DNA base has been changed
• Amino acid is valine, instead of
glutamic acid
• Result=Abnormal hemoglobin
Pedigree Chart
• Shows how a trait is transmitted
from generation to generation
• Each row is a generation
• Circles represent females
• Squares represent males
– Shaded in: person expresses that trait
– Half shaded in: person is only a
carrier
– Clear: person does not carry or
express that trait
Definitions
• Carrier= a person who carries the allele for
the trait but does not express it
• EX: Cystic Fibrosis is an autosomal
recessive disorder
– Carrier’s genotype= Ff
– For a person to have Cystic Fibrosis
(genotype=ff), he or she must have inherited an
“f” from both parents
– Therefore, BOTH parents must be carriers
(both Ff)
• Can’t trace every human trait through a
pedigree because some genes are polygenic
–
–
–
–
Shape of your eyes
Shape of your ears
Height
Eye color
• Also, phenotype is influenced by your
environment (ex: nutrition & exercise)
– Average height is 10cm more than it was in the
1800’s in the US & Europe due to nutritional
improvements
– Genes that are denied a proper environment in
which to reach full expression in 1 generation,
can achieve full potential in a later generation
(genes are inherited, the environment is not)
What makes an allele
dominant or recessive?
• Gene=sequence of DNA that codes for a
protein
• Usually:
– Dominant alleles code for the correct
production of that protein
– Recessive alleles code for no/wrong protein
production
– Heterozygous condition—the normal (dominant)
allele will still cause correct protein production
Example
• Cystic fibrosis heterozygotes (Ff)
– just one copy of the normal (dominant) allele is
enough to supply the cell with the proper
proteins to function.
– Because of this, the normal allele is considered
dominant over the recessive allele
• Therefore, a person who is heterozygous
does not suffer from Cystic Fibrosis
Review…
If a person is a carrier for a
trait, what does that
mean?
What is the difference
between an autosome
and a sex chromosome?
• They have the allele for
the trait but do NOT
express it
• Sex chromosome
determines the
individual’s sex (2), while
autosomes are the rest of
the chromosomes (44)
Human Blood Groups
Blood Groups
• Human blood comes in a variety of
genetically determined blood groups
• Using the wrong blood during a blood
transfusion can be fatal
• A number of genes help determine
blood type but we will focus on two:
– ABO blood groups
– Rh blood groups
Blood Groups—the easy one first
• The Rh blood group is determined by
a single gene with 2 alleles—positive
and negative
• The positive allele is dominant
• You need two Rh- alleles (Rh-/Rh-) to
be Rh negative
ABO Blood Groups
• This is a case of multiple alleles
• There are 3 alleles for this gene—A, B, and
O.
• AND…A and B are codominant!
• O is recessive to A and B
• Alleles A and B produce antigens (which are
carbohydrates) on the surface of red blood
cells
• O produces NO antigens
ABO Blood Groups—the wrong blood can be
FATAL
• Antigens are recognized by the immune
system and induce an immune response
• If the wrong blood is transfused, the body
will respond to these antigens by
producing antibodies
– Antibodies are named for what they attack
• Antibodies bind to the foreign molecule
(the antigen) and blood clumping will occur,
which leads to blood clotting, which leads
to death
ABO Blood Groups
• If you have blood type A, then you have:
– The “A” antigen on the surface or your RBCs
– You have anti-B antibodies
– You can receive type A blood and type O blood
• Remember: Your antibodies are named for
what they attack—so if you received type
AB or type B blood then clumping would
occur.
ABO Blood Groups
• Try this cross
• Cross a person who’s genotype is AA
with a person who is AB.
• Give the possible genotypes and
phenotypes
• Cross a person who’s genotype is AA
with a person who is AB.
• Gametes= A, A and A, B
• Cross:
A
A
A
B
AA
AB
AA
AB
• Genotypes= ½ AA, ½ AB
• Phenotypes = ½ A, ½ AB
14-2 Human Chromosomes
(and 12-4)
Vocabulary:
• Sex-linked gene
• Sex-influenced gene
• Sex-limited gene
• Nondisjunction
Key Concepts:
• Why are sex-linked disorders more
common in males than in females?
• What is nondisjunction, and what
problems does it cause?
Lou Gehrig: died at
37 of ALS
Human Chromosomes
• Your chromosomes contain about
30,000 genes and 46 chromosomes
– Once thought to be over 100,000 genes
• That’s about 3 billion base pairs
• Remember: genes on the same
chromosome are linked together
– They tend to be inherited together
Human Genes & Chromosomes
• Chromosomes 21 & 22 are the smallest
human autosomes
– Chromosome 22 contains approximately 43
million DNA bases (approx. 545 genes)
– Chromosome 21 contains approximately 32
million DNA bases (approx.225 genes)
• 1 of these genes is associated with ALS (Amyotrophic
Lateral Sclerosis)
• Causes a progressive loss of muscle control due to the
destruction of nerves in the brain and spinal chord
Sex Linked Genes
• What are linked genes?
• They can be separated during meiosis
through CROSSING OVER
• Genes carried on the X or Y chromosome
are sex linked because they are on the sex
chromosomes
• Many sex-linked genes are
found on the X-chromosome
Sex Linked Genes
• The X chromosome has many genes that
are important for growth and development
• The Y-chromosome only contains a few
genes
• All X-linked traits are expressed in males
• WHY???????
• Males only have 1 copy of the X
chromosome, while females need 2 copies
of the defective gene
Sex-Linked Gene Disorders
Colorblindness
•
3 human genes associated with color
vision are located on the X-chromosome
• In males, a defective version of any one
of these produces colorblindness
• Females must receive 2 copies of the
allele to be colorblind
CXc
X
CY
XCXC
X
XcY
The Test
http://www.toledo-bend.com/colorblind/Ishihara.html
Sex-Linked Gene Disorders
cont.
2. Hemophilia
• 2 important genes on the X-chromosome
that code for proteins control blood
clotting
• A recessive allele in either of these 2
genes may lead to hemophilia
–
–
“bleeders disease”
Injections of normal clotting proteins prevent
death
Sex-Influenced Genes
• A person’s phenotype is affected by their
sex
• Ex: Pattern Baldness
WOMEN:
BB: not bald
MEN:
BB: not bald
Bb: not bald
Bb: bald
bb: bald
bb: bald
X-Chromosome Inactivation
• Females have 2 X-chromosomes… if 1
is enough for males, how does the cell
“adjust” to the extra x-chromosome
in females?
• One X-chromosome is randomly
switched off
– Condenses and is called a Barr body
Chromosomal Disorders
• Most of the time, the mechanisms that
separate human chromosomes in meiosis
work very well, but things can go wrong
• The most common error:
NONDISJUNCTION
– The chromosomes fail to separate
– The result? Abnormal numbers of
chromosomes
Nondisjunction
Prefixes:
dis = absence of / opposite of
non = not
Root Word:
Junction = joining together
• disjunction = the act of separating
• nondisjunction = the act of not
separating
Examples of Chromosomal
Disorders
•
•
•
•
Down Syndrome
Klinefelter’s Syndrome
Turner’s Syndrome
Triple X Syndrome
Examples of Nondisjunction resulting in
Chromosomal Disorders
• Down's Syndrome: 47 chromosomes with 3
#21 chromosomes.
• Triple-X Syndrome: 47 chromosomes
caused by 3 X chromosomes.
• Klinefelter's Syndrome: 47 chromosomes
caused by 2 X chromosomes and 1 Y
chromosomes.
• Turner's Syndrome: 45 chromosomes with
1 X chromosome (caused by the absence of
one of the X chromosomes or a Y
chromosome).
Chromosomal Mutations
(from 12-4)
• Chromosomal Mutation
–
–
–
–
Deletion
Duplication
Inversion
Translocation
Chromosomal Mutation
(structural change)
• Deletion: lose all or part of a
chromosome
ABC-DEF
AC-DEF
• Duplication: segment of a
chromosome is repeated
ABC-DEF
ABBC-DEF
Chromosomal Mutation
(structural change)
• Inversion: chromosome segment
becomes oriented in reverse
direction
ABC-DEF
ABE-DCF
Chromosomal Mutation
(structural change)
• Translocation: part
of 1 chromosome
breaks off &
attaches to
another nonhomologous
chromosome
(segment is usually
exchanged)
ABC-DEF
ABC-JKL
GH-IJKL
GH-IDEF
Chromosomal Mutation
(change in number)
• If an organism is
born with the
incorrect number
of chromosomes
– Ex: 47 human
chromosomes
instead of 46
• Result of
nondisjunction
Down Syndrome
• 3 copies of chromosome #21
• 1 in 800 babies in the US is born with this
disease
• produces mild to severe mental retardation
• why should an extra copy of one
chromosome cause so much trouble?
Klinefelter’s Syndrome
• This is a sex chromosomal disorder
associated with males.
• Nondisjunction causes an extra X
chromosome to be passed along during
meiosis (XXY).
• Resulting male cannot reproduce
• Cases have been found in which
individuals were (XXXY) or (XXXXY)
Turner’s Syndrome
• This is a sex chromosomal disorder
associated with females.
• Nondisjunction causes offspring to
inherit only one X chromosome
(genotype = XO).
• Resulting female is sterile due to
underdeveloped sex organs.
14-3 Human Molecular
Genetics
DNA FINGERPRINTING
• DNA fingerprinting is a technique
developed to analyze sections of DNA
that have little or no known function.
– these areas vary widely from one individual
to another.
• This technique can be used to identify individuals
based on banding patterns in the resulting DNA
fragments
• uses restriction enzymes
– proteins that cuts DNA at specific sites along
the nucleotide chain (cuts at specific base
pairs).
• Is used to determine paternity or match a
suspect to a crime scene (like on CSI!)
THE PROCESS
1. human DNA is cut with restriction
enzymes; this produces DNA
fragments of different lengths
2. mix the DNA fragments with a
probe that glows in UV light (the
probe will stick to certain segments
of DNA)
3. run the DNA fragments through a
clear gel
DNA goes
in wells at
the top
negative end
•DNA has a "-"
charge
 DNA will move to
the "+" end of the gel
positive end
**smaller fragments will travel through the gel faster
4.
Smaller fragments of DNA will travel
through the gel faster (remember the
gel is porous).
Large DNA
fragment
Small DNA
fragment
negative end
positive end
5. In order to see these
fragments, the gel is
viewed under UV light.
The result: The DNA
fragments with the probe
attached will glow & you
will see a banding pattern
**Because each person's DNA is different, the
restriction enzyme will cut the DNA in different
places for different people and every person will have
a different band pattern.
DNA_DetectivePC.exe
Gene Therapy
• Gene therapy is the process of changing the gene that
causes a genetic disorder. This way, the body can
make the correct protein or enzyme it needs which
eliminates the cause of the disorder.
• 1st attempt to cure a genetic disorder by gene transfer:
1990
– Then, 1999: French girl apparently cured of an inherited
immune disorder after cells from her bone marrow were
removed, modified in the laboratory, and then placed back in
her body
•
Uses a vector to get the “new” DNA inside your cell 
viruses are often used cause they infect cells easily.
How does it work?….
1. The virus DNA is modified so it doesn’t cause
disease
2. DNA containing “good” gene is spliced to viral
DNA
3. The patient is then infected with this newly modified virus
particles which should carry the “good” gene into the cells
and correct the disorder.
Gene therapy today remains a high
risk,experimental procedure.
Ethical Questions raised as we learn
more about the human genome and
gene therapy:
1. If we can manipulate genes to
cure disease, does this also give us
the right to engineer our bodies?
(determine eye color, whether you
are tall/short, hair color, sex, blood
type...)
2. What will be the consequences if
biologists successfully clone human
beings?
Science meets
Hollywood:
Issues like these were
the basis of the movie,
GATTACA
Review…
What is the purpose of
gene therapy?
What is a restriction
enzyme?
Gene therapy is a process
designed to change
(replace) the damaged
or defective gene with
a normal, functional
A restriction enzyme
is a protein that cuts
DNA at specific
sites along the
nucleotide chain
(cuts at specific
base pairs).
gene.