Genetic Diseases Chapter 27. Outline 1.Genetic Diseases Background 2.Recessive Genetic Diseases 3.Dominant Genetic Diseases 4.Pedigrees.

Genetic Diseases

Chapter 27

Outline

1. Genetic Diseases Background2. Recessive Genetic Diseases3. Dominant Genetic Diseases4. 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 Tay-Sachs 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: • Then there is a _____ chance of

having a child who is a non-carrier.

• There is a ______ chance of having a child who is a carrier.

• There is a _______ chance of having a child who has TSD.

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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?

• [If an Hh person marries another Hh person, what are the odds to have a child with HD?

]

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• http://video.nytimes.com/video/2007/03/15/health/1194817106661/when-a-dna-test-shows-a-lethal-fate.html?scp=20&sq=huntington's%20disease&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.

Squares represent males

Circles represent females

Downward line is children

Horizontal lines is marriage

Black means affected 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 ________

3. If you have two affected parents with an unaffected kid, then the trait is _____

4. If you have two unaffected parents with an affected kid, then the trait is ______

Dominant

Recessive

Recessive

• http://www.zerobio.com/drag_gr11/pedigree/pedigree1.htm