Transcript Genetic Diseases

Genetic Diseases
Chapter 27
Outline
1.
2.
3.
4.
Genetic Diseases Background
Recessive Genetic Diseases
Dominant Genetic Diseases
Pedigrees
What is a Genetic Disease?
Definition: A genetic disease is a disease that is
inherited from one’s parents and is present
from the moment the zygote is created.
Genetic Disease vs. Acquired Disease
Genetic Disease
• A genetic disease is one that
is caused by having
defective DNA in some way.
• The disease is present
throughout the entire life of
the individual.
Acquired Disease
• An acquired disease (like
Strep Throat or the Flu) is a
disease that a person
acquired during his or her
life.
• Often caused by a foreign
organism (virus/bacteria).
• It is not caused by a
person’s DNA.
Recessive Genetic Diseases
• In most cases, genetic diseases are recessive.
• This means that if a person must have two
defective copies of that gene (homozygous
recessive) for the person to be affected.
How do you get a child with two
defective (recessive) copies?
1. Both parents are homozygous recessive- This is
fairly uncommon because most of these genetic diseases are
lethal and a person who has homozygous recessive will likely
die without reproducing.
2. One parent is heterozygous and one parent is
homozygous recessive- Same as above
3. Both parents are heterozygous – This is BY FAR
the most common way to have a child with a genetic
disease.
Example: Tay-Sachs Disease
• We will look at Tay-Sachs
Disease (TSD).
• TSD is caused by having
defective copies of a gene in
chromosome 15.
• Babies who are born with this
disease develop normally until
6 months. At they begin to lack
an enzyme in the brain.
• They suffer from mental and
physical handicaps and mostly
will die by the age of 4.
Vocabulary
• We will use the letter T to be a normal copy of the
gene. We will use the letter t to be a defective copy of
the gene.
• A person who is TT (homozygous dominant)is called
non-carrier. This person is not affected by the disease
and contains both normal copies of the gene.
• A person who is Tt (heterozygous) is called a carrier.
This person is not affected by the disease at all, but
contains one abnormal copy of the gene and one
normal copy.
• A person who is tt (homozygous recessive) is called
affected. This means that this person actually has TaySachs Disease.
Inheritance of Tay-Sachs
• The only way to have a child
who has TSD is if two carriers
marry.
• If either parent is a non-carrier
then it is impossible to have a
child with TSD. (Punnett Square)
• If both parents are carriers:
¼
• Then there is a _____
chance of
having a child who is a noncarrier.
½
• There is a ______
chance of
having a child who is a carrier.
• There is a _______
chance of
¼
having a child who has TSD.
How common are these genetic
diseases?
Carrier rate: (not DISEASE rate)
• Tay Sachs: 1/25
• Canavan: 1/40
• Niemann-Pick: 1/90
• Cystic-Fibrosis: 1/25
• And many others…
Note: Not all genetic diseases are as devastating
as TSD.
Genetic Testing
• Unlike acquired diseases, genetic diseases are
preventable.
• A definite way to prevent genetic diseases is
by undergoing genetic testing before
marriage.
• Genetic testing looks at a person’s genes to
see whether they are a non-carrier or a
carrier.
Example
• Meir and Chaya are both Ashkenazi Jews. They
are thinking about getting married. They both go
for genetic testing. They are tested to see
whether they are carriers (heterozygous) for any
of the common Ashkenazi genetic diseases.
• If they find that they are both non-carriers then
they can go ahead without worry.
• If either one finds out that he or she is not a
carrier, the other one need not be tested.
• If they both find out that they are carriers for the
same disease, then they have a serious question
on their hands. (Show Punnett for carriers for 2
different diseases)
Genetic Testing
• Every person should be genetically tested
before they get married to ensure that their
children will not be at risk for one of these
diseases.
• Many Rabbis will refuse to perform a marriage
unless the bride and/or groom have been
tested.
Dominant Genetic Diseases
• There are rare genetic diseases that are
actually dominant.
• The reason is that if a dominant disease killed
the individual before they reproduce, the
disease will eliminate itself from the world.
Huntington’s Disease
• An example of a dominant genetic
disease is Huntington’s Disease
(HD).
• People with HD develop an excess of
a substance in the brain called
Huntingtin.
• Around the age of 30, this begins to
affect them.
• It is caused by a defect on a gene on
chromosome 4.
• They begin getting personality
changes and in later stages develop
jerky, uncontrollable movements of
the limbs.
• As of today, there is no known cure.
• The key point about HD is that it does not
begin to affect the person until age 30-40. This
means that the person very likely had a
chance to reproduce and pass on the disease
before they necessarily realize that they have
it.
Vocabulary
• We will use the letter H to be a defective copy of the
Huntington’s gene. We will use the letter h to be a
normal copy of the gene.
• A person who is HH (homozygous dominant) is
homozygous for Huntington’s and will have HD. (This is
extremely rare.) This person received a defective gene
from both parents.
• A person who is Hh (heterozygous) has HD. This person
received one defective copy and one normal copy from
his or her parents.
• A person who is hh (homozygous recessive) is normal.
This means that this person does not have HD.
• We will not deal with HH people because they
are extremely rare.
• Much more common is an Hh person who
marries a normal person (hh).
• What are the odds that this person will have a
child with HD?
½
• [If an Hh person marries another Hh person,
what are the odds to have a child with HD?
¾ ]
• http://video.nytimes.com/video/2007/03/15/
health/1194817106661/when-a-dna-testshows-a-lethalfate.html?scp=20&sq=huntington's%20diseas
e&st=cse
HD- Ethical Dilemma
• Sammy is 15 years old. His father is 41. His father begins to
suffer from personality changes and then eventually
devastating chorea. The doctors discover that Sammy’s
father had HD. He dies at 42 years old. His mother is still
perfectly healthy at age 41.
• Sammy has a number of options.
1. He can do nothing and choose to marry and have children.
2. He can do nothing and choose not to marry and have
children.
3. He can choose to be tested himself for HD:
a) If he is negative, he will obviously have no problem having
children.
b) If he is positive, he can choose to have children or not.
Pedigrees
• Pedigrees are ways of
tracing genetic traits
through generations of
families. This is often done
with genetic diseases but
can also be done with
other traits (widow’s peak,
blue eyes, etc).
• We will use pedigrees to
figure out if a trait is
dominant or recessive and
also to deduce different
people’s genotypes.
Circles
represent
females
Downward line
is children
Black means affected
Squares represent
males
Horizontal
lines is
marriage
White means
unaffected
Albinism
• An albino cannot produce
normal pigment.
• Often, pedigrees will use
the example of albinism,
which is a recessive trait.
• Worksheet
• Tips
• Key hints to remember about pedigrees:
1. Fill in the genotypes of the ones you do know
and then work on the ones you’re not sure
about.
2. If it does not appear in every generation, it is
Recessive
________
3. If you have two affected parents with an
Dominant
unaffected kid, then the trait is _____
4. If you have two unaffected parents with an
Recessive
affected kid, then the trait is ______
• http://www.zerobio.com/drag_gr11/pedigree
/pedigree1.htm