Inheritance (OCR GCSE Biology A (Gateway))

Hypercholesterolemia (HC) is the result of a mutation in the genome. It is caused by a dominant allele on chromosome 19. The mutation involved causes a change in the DNA nucleotides.

Write the words allele, chromosome, genome and nucleotide in the boxes to show their size from smallest feature to largest feature.

One in 500 people are heterozygous for HC.

There are 66000000 people in the UK.

Calculate how many people in the UK are heterozygous for HC.

Number of people = ......................

A woman who does not have HC and a man who is heterozygous are expecting a baby.

What is the probability of the baby having HC?

Complete the genetic diagram to explain your answer.

Use D for the dominant HC allele and d for the recessive allele.

Probability = ........................

Higher tier content only

The allele that is affected by HC codes for a protein called LDL receptor protein. The faulty allele often has four extra nucleotides, making a total of 2521 nucleotides.

Calculate the number of amino acids found in the healthy, unaffected protein.

Number of amino acids = .....................

People with HC are more likely to develop heart disease. Fig. 21.1 shows the heart of a person who has heart disease.

Fig. 21.1

The LDL receptor protein is found on the cell membrane of liver cells. The receptor picks up cholesterol from the blood and transports it into the liver cell. Inside the liver cell the cholesterol is broken down or used.

Explain why people who have the mutation in the allele for the LDL receptor are much more likely to develop heart disease.

A drug is being developed that might help protect people from heart disease.

To test the drug patients were divided into three groups.

Each group was given a different treatment and their blood cholesterol measured.

The results are shown in Fig. 21.2.

Fig. 21.2

Evaluate the results of the test.

Wolfram’s Syndrome is a genetic disorder.

It is caused by a recessive allele (n).

In people with Wolfram’s syndrome, a protein does not function correctly.

Explain how a change in an allele can stop a protein functioning correctly.

The diagram shows a pair of chromosomes from a person called Tim.

Fill in the table to show Tim’s genotype and phenotype.

Choose your answers from this list.

does not have Wolfram’s syndrome
has Wolfram’s syndrome
heterozygous
homozygous dominant
homozygous recessive

(i) Meena is expecting a baby.

Tim is the father.

Complete this genetic diagram.   Tim     Meena N     n    

[2]

(ii) Wolfram’s syndrome can affect the pancreas.

Meena and Tim’s doctor tells them that there is a chance that their baby will have problems controlling their blood glucose level.

Explain why the doctor thinks this.

Use information from part (c) (i) and your biological knowledge.

 [4]

Huntington’s disease is a genetic condition. It is caused by a dominant allele.

Explain what is meant by the term dominant allele.

The symptoms of Huntington’s disease usually appear after the age of 40.

There is no cure and people with the disease usually die after 10–15 years.

Scientists now know that there are a number of different forms of the allele that causes Huntington’s disease. The allele has a sequence of three bases, CAG, that repeats many times. The number of repeats varies between patients.

Patients can be tested to see if they have the allele and how many repeats it has.

Doctors have studied many patients to see:

• The number of CAG repeats a patient has

• The age that the patient starts to show symptoms of the disease.

The results are shown on the graph.

Discuss how useful the gene test and the graph are for people who have a history of Huntington’s disease in their family.

Higher Tier only

A new study is giving hope for a treatment for Huntington’s disease.

Doctors gave patients an injection of a drug that blocks the action of mRNA that is produced by the Huntington allele.

Explain how this drug could prevent the symptoms of Huntington’s disease.

Wolfram’s syndrome is a genetic disorder.

• It is caused by a recessive allele (n).

• In people with Wolfram’s syndrome, a protein does not function correctly.

The diagram shows a pair of chromosomes from a person called Kai.

Complete these sentences.

Kai does not have Wolfram’s syndrome.

This is because his genotype is .........................................

This means that the probability of any one of his sperm having the Wolfram’s allele is ...........................

Retinitis pigmentosa is a genetic condition that affects the eyes.

It is caused by a mutation to a gene.

This mutation produces a recessive allele.

The condition causes rod cells in the retina to break down.   

Explain the meaning of these terms.

Gene .................................

Allele .................................

(i) Two people who are heterozygous for retinitis pigmentosa are expecting a baby.

Draw a genetic diagram to calculate the probability that the baby will have the condition.

Use R for the normal allele and r for the allele for retinitis pigmentosa.

Answer = ......................... [3]

(ii) If the baby has retinitis pigmentosa, it will have normal colour vision but will not be able to see well in dim light.

Explain why.

[3]

(i) Explain why stem cells could be used as a treatment for this condition.

[2]

(ii) Why is it an advantage to use stem cells from the patient rather than from another person?

[1]

The data in the table shows the ratio of males to females in England and Wales.

Describe how sex is determined in humans.

You may use a genetic diagram in your answer.

In 2015 there were approximately 698 000 babies born in England and Wales.

Calculate how many of these were male.

Answer = .....................

There are more females than males living in England and Wales as an average over the whole population.

Suggest one reason why there are more females.

Characteristics can be examples of continuous or discontinuous variation.

Write the four characteristics below in the correct columns of the table.

Sex    Blood group    Height    Eye colour

Identical twins occur when an embryo splits into two.

In the diagram below, some of the chromosome numbers in the different cells have been given to you.

Complete the diagram to show the number of chromosomes in:

• The sperm cell

• The zygote

• The cell in Twin B’s body.

The sex chromosomes determine the sex of a baby.

What are the sex chromosomes of a male and of a female?

Male ...............    Female ...............

The data in the table shows the ratio of males to females in England and Wales.

(i) What percentage of babies are male at birth?

Answer = .............................. % [1]

(ii) In a hospital 410 babies are born in a week. Calculate how many of them are likely to be male.

Answer = .................................. [1]

(iii) On average, men do not live as long as women. How does the data in the table show this?

[2]

Retinitis pigmentosa is a genetic condition.

It is caused by a mutation to a gene. This mutation produces a recessive allele.

If people have retinitis pigmentosa then the cells in their retina are damaged.

Explain the meaning of these terms.

• Mutation

• Gene

If a person has two alleles for retinitis pigmentosa, they will not be able to see properly.

(i) Why does a person need two affected alleles to have the condition?

[1]

(ii) Why does the condition affect the ability to see properly?

[1]

Two people are heterozygous for retinitis pigmentosa and are expecting a baby.

Complete the genetic diagram to work out the probability that the baby will have the condition.

R is the normal allele and r is the allele for retinitis pigmentosa.