Inherited Diseases Cystic fibrosis Huntington’s disease Sickle cell anaemia Contents What are inherited diseases? Haemophilia Down’s syndrome Summary quiz

Inherited Diseases

Cystic fibrosis

Huntington’s disease

Sickle cell anaemia

Contents

What are inherited diseases?

Haemophilia

Down’s syndrome

Summary quiz

Each person inherits characteristics from their parents.

How are inherited characteristics passed on from one generation to the next?

What are inherited characteristics?

Inherited characteristics are passed on in the chromosomes inherited from your parents.

Each chromosome contains genes that code for characteristics such aseye colour, hair type and height.

Can a disease also be an inherited characteristic?

Genes pass on characteristics from one generation to the next.

Each chromosome contains hundreds of different genes.

A mutation in just one genecan change a characteristic.Sometimes this change has a harmful effect on the body and can lead to disease.

This type of condition is called an inherited disease.

Is an inherited disease contagious?

What is an inherited disease?

Examples of inherited diseases

What causes an inherited disease?

Each matching pair of chromosomes contains one chromosome inherited from each parent.

Each pair of chromosomes contains the same type of genes that code for the same characteristics.

What are different versions of a gene called?

gene for a characteristic

matching gene

chromosome from female parent

chromosome from male parent

For some genes, there is an allele that can cause disease.

Does inheriting an allele that causes disease always lead to the development of that disease?

Different versions of a gene, that code for different versions of a characteristic, are called alleles.



matching gene

disease allele

normalallele

Inherited diseases, like other inherited characteristics, can be caused by dominant or recessive alleles.

What is the difference between dominant and recessive alleles?

The development of an inherited disease depends on the combination of alleles that a person has inherited.



matching gene

disease allele

normalallele

If an inherited disease is caused by a dominant allele, how many alleles will cause the disease to develop?


normalallele

disease allele

1 dominant disease allele (from either parent)

inherited disease

=

Does it matter which parent a dominant disease allele is inherited from?

If an inherited disease is caused by a recessive allele, how many alleles will cause the disease to develop?


disease allele

disease allele

2 recessive disease alleles (one from each parent)

inherited disease

=

The inheritance of an inherited disease depends on two main factors:

the genes carried by the parents;

the particular sex cells that combine during fertilization.

How can you work out the chance that genes for inherited disease will be passed on to the next generation?

A genetic cross can be used to determine the possible offspring and the likelihood that genes are passed on.

Are inherited diseases always passed on?

Why is it useful to be able to predict the probability of inherited disease?

It’s useful for people who know that there is a history of inherited disease in their family.

Being able to make predictions about genetic disease, they can work out what might happen if they have children.

People often need advice about such an important issue. This is called genetic counselling.

Genetic counsellors can help people discuss any concerns they might have and help them make informed decisions for themselves.

Understanding inherited diseases

What information would a genetic counsellor need to know about a couple seeking their advice?

Inherited Diseases

Cystic fibrosis


Sickle cell anaemia

Contents


Haemophilia

Down’s syndrome

Summary quiz

Cystic fibrosis is an inherited disease that affects many body organs, particularly the lungs and digestive system.

The air passages in the lungs are coated with a constant flow of mucus, which traps and removes bacteria.

What is cystic fibrosis?

In a person with cystic fibrosis, mucus is thicker than normal.

This thick, sticky mucus can block the lungs and airways. How will this affect a person with cystic fibrosis?


People with cystic fibrosis need daily physiotherapy, which involves massage to help loosen the sticky mucus.

Antibiotics help to treat andcontrol lung infections and prevent lung damage.

A person with cystic fibrosis has breathing difficulties.

They are at risk of repeated chest infections which can lead to lung damage.


Treatment for the cystic fibrosis also includes special inhalants.In severe cases, a lung transplant operation is required.

The thick, sticky mucus that affects the lungs of a person with cystic fibrosis can also affect their digestive system.What problems might this cause?

People with cystic fibrosis can have problems digesting food. Most take enzyme pills when they eat to supply missing enzymes and allow digestion.

Over 7,500 people in the UK are affected by cystic fibrosis.

The quality of life for people with cystic fibrosis is much better than in earlier times, due to improved treatment and understanding of this inherited disease.

Children born with cystic fibrosis do not have a normal life expectancy, although this is improving all the time.

Living with cystic fibrosis

Cystic fibrosis is caused by a recessive allele.

What symbols could stand for the different forms of this gene?

How is cystic fibrosis inherited?

What alleles has a person with cystic fibrosis inherited?

A person with cystic fibrosis has the genotype cc. They have inherited a cystic fibrosis gene from both parents.

normal allele normal allele==CC

cystic fibrosis allele=cc

cc cc

How is cystic fibrosis inherited?

How do you represent the genotype of a person who inherits one cystic fibrosis allele?

Does a person with this genotype have cystic fibrosis?

A heterozygous person (Cc) is called a carrier. They do not have the disease but they do carry the cystic fibrosis allele which they might pass on to their children.

In the UK, 2.3 million people carry the cystic fibrosis gene.

If both parents are carriers of the faulty gene, what is the chance that their children will have cystic fibrosis?

CC cc

Cystic fibrosis inheritance activity 1

Cystic fibrosis inheritance activity 2

Inherited Diseases

Cystic fibrosis


Sickle cell anaemia

Contents


Haemophilia

Down’s syndrome

Summary quiz

Sickle cell anaemia is an inherited disease that affects red blood cells.

What is sickle cell anaemia?

Normal red blood cells contain haemoglobin, a protein that carries oxygen around the body.

In sickle cell anaemia, red blood cells have abnormal haemoglobin.This distorts red blood cells to form curved, sickle shapes.

How will sickle cells affect the bloodstream and the body?

Normal red blood cells are shaped so that they move easily through blood vessels.

This ensures that oxygen is delivered to the body.


Sickle-shaped red blood cells can get stuck in narrow blood vessels and cause blockages.

Oxygen is prevented from getting to the body’s tissues and organs. This leads to severe pain and can cause permanent organ damage.

sickle cells get stuck

normal cellsmove freely

Red blood cells normally have a lifetime of about 4 months.


Sickle cells are fragile and only last for 10–20 days.

How will this affect the level of red blood cells?

The shorter lifetime of sickle cells reduces the number of red blood cells leading to anaemia.

People with anaemia feel weak and tired.

Children with sickle cell anaemia are able to lead normal lives but need regular medical attention. Treatment sometimes includes blood transfusions.

Sickle cell anaemia mostly affects people of African descent, but people from India, the Mediterranean and Middle East are also affected.

Who is affected by sickle cell anaemia?

It is estimated that there are around 6,000 people affected by sickle cell disorders in Britain.

What causes sickle cell anaemia?

Sickle cell anaemia is caused by a recessive allele.


What alleles has a person with sickle cell anaemia inherited?

A person with sickle cell anaemia has inherited two sickle cell alleles, one from each parent.

normal allele=SS

sickle cell allele=ss

ss ss

What causes sickle cell trait?

What the genotype of a person who has inherited one sickle cell allele?

Does a person with this genotype have sickle cell anaemia?

The sickle cell allele is recessive so this person does not have the disease. They are a carrier of the sickle cell allele and have what is called sickle cell trait.

People with sickle cell trait must avoid becoming dehydrated and short of oxygen.

But having sickle cell trait isn’t all bad news…

SS ss

Sickle cell trait and malaria resistance

Carriers of the sickle cell allele (Ss) are more resistant to malaria.

Malaria is a serious disease caused by parasites. It is spread by bites from infected mosquitoes.

Malaria parasites enter red blood cells. The effects include fever and vomiting and can be fatal.

Why is the sickle cell allele common in areas of the world where malaria is or has been widespread?

Sickle cell trait and malaria resistance

The sickle cell allele may be due to a genetic mutation that occurred in malaria-prone areas many years ago.

People with sickle cell trait were more likely to survive a malaria outbreak and live long enough to have children, so the trait might be passed on.

The disadvantage of the sickle cell allele is the possibility that children may be born with sickle cell anaemia.

If both parents are sickle cell carriers, what is the chance that their children have sickle cell anaemia?

Sickle cell anaemia inheritance activity

Inherited Diseases

Cystic fibrosis


Sickle cell anaemia

Contents


Haemophilia

Down’s syndrome

Summary quiz

Huntington’s disease is an inherited disease that affects the brain and central nervous system.

The wide-ranging symptoms of this inherited condition usually develop in adults between the ages of 30 and 50 .

What is Huntington’s disease?

Huntington’s disease causes damage to nerve cells in the areas of the brain involved in the control of movement, planning and motivation.

How will this affect a person with Huntington’s disease?

People with Huntington’s disease are affected by gradual physical, mental and emotional changes.


People can have symptoms for several years before finding out they have Huntington’s disease. Why can it take so long for this disease to be diagnosed?

The early symptoms of Huntington’s disease include: mild tremor clumsiness lack of concentration problems remembering things depression mood changes, sometimes including aggressive behaviour.

Currently there is no cure for Huntington’s disease. Medication is available to help treat some symptoms such as involuntary movements, depression and mood swings.

Around 4 800 people in the UK have Huntington’s disease.


The physical, mental and emotional symptoms of Huntington’s disease become progressively worse.

As the disease continues, people may experience involuntary movements, weight loss, speech problems and extensive memory loss. Eventually a person with Huntington’s disease needs complete nursing care.

Huntington’s disease is caused by a dominant allele.


How is Huntington’s disease inherited?

Huntington’s allele Huntington’s allele==HH

normal allele=hhIf someone inherits one Huntington’s disease allele will they develop the disease?

The Huntington’s disease allele is dominant, so inheriting one allele from either parent will lead to this condition.

HH hh

Huntington’s disease is caused by a dominant allele and can be passed on by one parent who has the disease.

Anyone who inherits the Huntington’s allele will develop the disease, usually between the ages of 30 and 50.

How is Huntington’s disease inherited?

People with the Huntington’s allele may have had a family before realizing that they have inherited the disease.

If one parent has the Huntington’s allele and the other is a non-carrier, what is the chance that their children will have Huntington’s disease?

Huntington’s disease inheritance activity

Inherited Diseases

Cystic fibrosis


Sickle cell anaemia

Contents


Haemophilia

Down’s syndrome

Summary quiz

Haemophilia is an inherited disease in which blood cannot clot properly.

A person with this condition is called a haemophiliac.

They do not produce the clotting factor needed for the blood to clot and so bleed for longer than normal.

What is haemophilia?

Cuts and grazes are not a major problem – a little pressure and a plaster are usually enough to stop bleeding.

Internal bleeding is a much more serious problem affecting joints, muscles and soft tissue.

Treatment for haemophilia usually involves injections to replace the missing clotting factor.

Haemophiliacs can be treated on a regular basis to prevent bleeding or when bleeding occurs.

What is haemophilia?

About 6 000 people in the UK are affected by haemophilia.Most of these are males, while females are carriers of this condition.

Haemophilia is called a sex-linked characteristic.

Which chromosomes do you think are involved in the inheritance of this blood disorder?

Haemophilia is caused by a recessive allele on the X chromosome (one of the sex chromosomes in pair 23).

How is haemophilia inherited?

X chromosome Y chromosome

haemophilia allele

no gene to match haemophilia allele

The other sex chromosome, the Y chromosome, is much shorter than the X chromosome.

It does not carry a gene to match the haemophilia allele.

Why is haemophilia known as a sex-linked condition?

Haemophilia is caused by a recessive allele carried on the X chromosome.

What symbols could stand for the different forms of the X chromosome?

How is haemophilia inherited?

Why does inheritance of the haemophilia allele affect males and females differently?

XhXh haemophilia alleleon X chromosomehaemophilia alleleon X chromosome==

XHXH normal alleleon X chromosome=

XHXH

What happens if a female inherits one haemophilia allele?

Haemophilia in females

XhXh

The recessive haemophilia allele is masked by the dominant normal allele on the other Y chromosome, so the woman is not affected by the disease.

Why is this type of female called a carrier?

== femalecarrierfemalecarrier

Haemophilia in females is an extremely rare condition. This occurs if the woman inherits two haemophilia alleles.

XhXh == female withhaemophiliafemale withhaemophiliaXhXh

What happens if a male inherits one haemophilia allele?

Haemophilia in males

XhXh YY

If a man inherits one haemophilia allele, he will be affected by haemophilia.

The haemophilia allele is normally recessive but in this case the Y chromosome does not carry a matching allele that might mask the disease.

The inheritance of one haemophilia allele is enough for a man to be affected by the disease.

Which parent will have passed on the haemophilia allele?

== male withhaemophilia

male withhaemophilia

Haemophilia genotypes

© Boardworks Ltd 2004

In about one third of all cases of haemophilia there is no family history of the disease.

The condition is caused by a spontaneous mutation in the person with haemophilia or their mother.

The most famous example of such a case was Queen Victoria. None of her ancestors had haemophilia but her son, Leopold, was born with the condition.

Two of Victoria’s daughters were carriers. They married into European royal families and haemophilia was called the royal disease.

What is the chance that a carrier woman and an unaffected man will have a son with haemophilia?

Haemophilia – the royal disease

Haemophilia inheritance activity 1

Haemophilia inheritance activity 2

Inherited Diseases

Cystic fibrosis


Sickle cell anaemia

Contents


Haemophilia

Down’s syndrome

Summary quiz

Down’s syndrome is an inherited condition that results in characteristic physical featuresand various medical problems.

It is not a disease or illness.

A person with Down’s syndrome has an extra chromosome in their body cells.

This additional chromosome is an extra copy of chromosome 21.

What is the total number of chromosomes in the body cells of a person with Down’s syndrome?

What is Down’s syndrome?

1 2 3 4 5

21 22 23

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

21

The extra chromosome 21 in people with Down’s syndrome leads to certain physical characteristics.

However, a person with Down’s syndrome looks more like their family than someone else with the condition. Why is this the case?

What is Down’s syndrome?

People with Down’s syndrome have various medical problems including learning difficulties and heart problems.

With the right support and medical care, they can take part in education and have full and varied lives.

1 2 3 4 5

21 22 23

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20

Down’s syndrome affects people of all ages, races, religious and economic situations.

For every 1,000 babies born, 1 baby will have Down’s syndrome.

It is estimated that there are around 60,000 people with Down’s syndrome living in the UK.

Nothing done before or during pregnancy can cause Down’s syndrome. It can happen to anyone.

So where does the extra chromosome 21 come from?

What causes Down’s syndrome?

1 2 3 4 5

21 22 23

6 7 8 9 10

11 12 13 14 15

16 17 18 19 20


The extra chromosome 21 can come from either parent and is present at fertilization.

Which type of cells will the extra chromosome be found in?

What has not happened when the sex cells are made to result in an extra chromosome?

The extra chromosome is introduced by mistake when sex cells are made.

The extra chromosome can be in an egg cell or sperm cell.


Usually, pairs of chromosomes separate when sex cells are made.

Sometimes, these pairs don’t separate properly.

If it happens that a sex cell with an extra chromosome 21 is involved in fertilization, it will lead to Down’s syndrome.

This type of chromosome mutation can occur when either egg cells or sperm cells are made.

The extra chromosome can be in an egg cell or sperm cell.


For example, a woman might produce an egg cell in which chromosome pair 21 have not separated.

How many chromosomes will the egg cell contain?

If this egg is fertilized by a normal sperm cell, how many chromosomes will a baby with Down’s syndrome have in each body cell?

24 chromosomes 23 chromosomes

++

47 chromosomes

Inherited Diseases

Cystic fibrosis


Sickle cell anaemia

Contents


Haemophilia

Down’s syndrome

Summary quiz

Multiple-choice quiz

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Inherited Diseases Cystic fibrosis Huntington’s disease Sickle cell anaemia Contents What are inherited diseases? Haemophilia Down’s syndrome Summary quiz