Genetic Diseases Chapter 27 - PowerPoint Presentation

Slide1

Genetic Diseases

Chapter 27

Slide2

Outline

Genetic Diseases Background

Recessive Genetic Diseases

Dominant Genetic Diseases

Pedigrees

Slide3

What is a G

enetic

D

isease?

Definition: A

genetic disease

is a disease that is inherited from one’s parents and is present from the moment the zygote is created.

Slide4

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.

Slide5

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.

Slide6

How do you get a child with two defective (recessive) copies?

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.

One parent is heterozygous and one parent is homozygous recessive-

Same as above

Both parents are heterozygous –

This is

BY FAR

the most common way to have a child with a genetic disease.

Slide7

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.

Slide8

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.

Slide9

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.

¼

½

¼

Slide10

How common are these genetic diseases?

Carrier rate:

(not DISEASE rate)

Tay

Sachs: 1/25

Canavan

: 1/40

Niemann

-Pick: 1/90

Cystic-Fibrosis: 1/25And many others…

Note: Not all genetic diseases are as devastating as TSD.

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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.

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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

A

shkenazi 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)

Slide13

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.

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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.

Slide15

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.

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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.

Slide17

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

.

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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?

]

½

¾

Slide19

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

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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.

He can do nothing and choose to marry and have children.

He can do nothing and choose not to marry and have children.

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.

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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

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Albinism

An

albino

cannot produce normal pigment.

Often, pedigrees will use the example of albinism, which is a recessive trait.

Slide23

Slide24

Worksheet

Tips

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Key hints to remember about pedigrees:

Fill in the genotypes of the ones you do know and then work on the ones you’re not sure about.

If it does not appear in every generation, it is ________

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

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

Dominant

Recessive

Recessive

Slide26

http://

www.zerobio.com/drag_gr11/pedigree/pedigree1.htm