OCR GCSE Combined Science
Topic questions Genetics and inheritance Monohybrid inheritance
18 questions · 89 marks · about 1.5 hours
The diagram shows a genetic cross for inheriting cystic fibrosis.
Cystic fibrosis is a recessive inherited condition.
What percentage of the offspring will have cystic fibrosis?
A 0%
B 25%
C 75%
D 100%
Inheritance is passing genetic material from one generation to the next.
Which terms describe sex cells in humans?
Tick (✓) two boxes.
Peas develop inside pods.
Pea pods can be green or yellow.
A pea plant with green pods is crossed with a pea plant with yellow pods. All the pods on the new plant are green.
Complete the sentences about the pea pods.
Use words from the list.
dominant haploid heterozygous
homozygous recessive
The results of this cross show that the allele for green is........ and the
allele for yellow is........
The genotype of the new plant is Gg. This means it is........ for pod
colour.
The new plants with green pods are then crossed.
What is the probability that a plant from this cross will have yellow pods?
Complete the genetic diagram to explain your answer.
Genetic material is required for inheritance in eukaryotic and prokaryotic cells.
Compare the genetic material found in eukaryotic and prokaryotic cells.
Peas develop inside pods.
Pea pods are either green or yellow. The allele for green is dominant over the allele for yellow.
The table shows some information about three different pea pods.
Complete the table.
| Phenotype | Genotype | Description of genotype |
| yellow | gg | homozygous ......................................... |
| green | GG | homozygous dominant |
| ............................ | Gg | ......................................... |
A gardener crosses a plant that has green pods with a plant that has yellow pods. The gardener thinks both these plants have a homozygous genotype.
The result is 15 new plants with green pods and 3 plants with yellow pods.
Show that the gardener’s green plant did not have a homozygous genotype by completing the two genetic diagrams.
Use letters G and g for the alleles.
If the green plant has a homozygous genotype:
If the green plant does not have a homozygous genotype:
The ratio the gardener achieved was 5 green to 1 yellow.
Suggest why this does not match either of the offspring ratios.
A gardener grows a species of plant that has either red or white flowers. The colour of the flowers is controlled by a single pair of alleles, R and r.
The gardener crosses a plant that is homozygous for red flowers with a plant that is homozygous for white flowers. The diagram shows the results.
Explain why the offspring in the diagram are all red.
The offspring are all heterozygous for flower colour.
What is meant by the term heterozygous ?
The gardener grows another species of plant that can produce flowers of five different colours.
The gardener counts the number of plants for each flower colour.
The graph shows their results.
The gardener counted 97 plants.
Calculate the percentage of these plants that have blue flowers.
Give your answer to the nearest whole number.
What evidence is there in the graph that flower colour is an example of discontinuous variation in this species?
The male gametes of a plant are called pollen.
The sentences in the text box are about pollen. There are two words in the sentences that are not correct.
Pollen cells are haploid. This means that the pollen cells of a plant have twice
the number of chromosomes as the leaf cells of the plant.
Pollen cells are formed by a type of cell division called mitosis.
A gardener grows a species of plant that has either red or white flowers. The colour of the flowers is controlled by a single pair of alleles, R and r.
The gardener crosses a plant that is homozygous for red flowers with a plant that is homozygous for white flowers. The diagram shows the results.
Explain why the offspring in the diagram are all red.
The offspring are all heterozygous for flower colour.
What is meant by the term heterozygous ?
The gardener crosses one of these offspring plants with a plant that is homozygous for white flowers.
Predict the probability that the next generation of plants will have white flowers.
Complete the genetic diagram to explain your answer.
The gardener grows another species of plant that can produce flowers of five different colours.
The gardener counts the number of plants for each flower colour.
The graph shows their results.
The gardener counted 97 plants.
Calculate the percentage of these plants that have blue flowers.
Give your answer to the nearest whole number.
What evidence is there in the graph that flower colour is an example of discontinuous variation in this species?
The male gametes of a plant are called pollen.
The sentences in the text box are about pollen. There are two words in the sentences that are not correct.
Pollen cells are haploid. This means that the pollen cells of a plant have twice
the number of chromosomes as the leaf cells of the plant.
Pollen cells are formed by a type of cell division called mitosis.
The diagram shows how a condition caused by a recessive gene mutation is inherited.
Which child, A, B, C or D, will inherit and develop the condition?
The diagram shows a genetic cross for seed shape in peas.
Which prediction about the offspring is most likely?
A All the offspring will be heterozygous for seed shape.
B All the offspring will be homozygous for seed shape.
C The ratio of heterozygous to homozygous offspring will be 1: 1.
D The ratio of heterozygous to homozygous offspring will be 3: 1.
The diagram shows a genetic cross for seed shape in peas.
Which prediction about the offspring is most likely?
A All the offspring will be heterozygous for seed shape.
B All the offspring will be homozygous for seed shape.
C The ratio of heterozygous to homozygous offspring will be 1:1.
D The ratio of heterozygous to homozygous offspring will be 3:1.
Some forms of breast cancer are linked to specific mutated genes called BRCA.
BRCA is a dominant allele.
Inheriting the BRCA allele means in both males and females, the risk of developing breast cancer is affected. It does not mean a person has been diagnosed with cancer.
A male who is heterozygous for BRCA has children with a female who is homozygous recessive.
How are they affected by the BRCA allele?
Put ticks (✓) in the correct boxes.
What is the percentage probability of them having a child with an increased risk of developing breast cancer?
Use a labelled genetic diagram to explain your answer.
Use the letter B for the dominant BRCA allele.
Complete Table 13.1 by choosing words from this list to identify the genetic term used to describe cell A and cell B.
diploid genotype haploid phenotype
Give an explanation for each choice of word in Table 13.1.
Huntington’s disease can cause death.
Fig. 13.3 shows the occurrence of HD by age when symptoms first appear.
What is the modal age range for the data shown in Fig. 13.3 ?
Early studies in the UK showed one in 15 000 have the HD allele. There are 66 000 000 people in the UK.
Calculate the number of people in the UK with the HD allele.
Read the information about HD in the box.
• The HD allele is a dominant allele that can lead to death.
• Alleles that cause death rarely persist in populations.
• About 10% of people with HD acquire the HD allele from a newly formed mutation and not through inheriting it from their parents.
Use this information and the data in Fig. 13.3 to suggest why the HD allele still persists in the human population.
A flower homozygous dominant for colour is crossed with a homozygous recessive flower.
What is the predicted percentage of heterozygous offspring?
A 25%
B 50%
C 75%
D 100%
A scientist crossed a white flower with a purple flower. The white flower is homozygous recessive. The purple flower is heterozygous.
The scientist uses this Punnett square to predict the expected ratio of offspring.
| P | p | |
| p | Pp | pp |
| p | Pp | pp |
What is the expected ratio of phenotypes in the offspring?
A 1 purple: 1 white
B 1 purple: 2 white
C 2 purple: 1 white
D 4 purple: 0 white
The phenotype of rabbits can be black or white fur. The allele for black fur is dominant.
Which genetic cross identifies the genotype of a rabbit with black fur (rabbit X) as Bb?
A Cross rabbit X with a black rabbit. All offspring have black fur.
B Cross rabbit X with a black rabbit. 50% of offspring have black fur and 50% have white fur.
C Cross rabbit X with a white rabbit. All offspring have white fur.
D Cross rabbit X with a white rabbit. 50% of offspring have black fur and 50% have white fur.
Huntington’s disease is a single gene disorder caused by a dominant allele (H).
The Punnett square shows the genetic cross between two heterozygous parents.
| H | h | |
| H | HH | Hh |
| h | Hh | hh |
What is the probability of these parents having a child with Huntington’s disease?
A 0 in 4
B 1 in 4
C 2 in 4
D 3 in 4
A genetic cross occurs between a heterozygous individual and a homozygous recessive individual.
What is the probability of these individuals having offspring that are homozygous recessive?
A 0 in 4
B 1 in 4
C 2 in 4
D 3 in 4
Some children can inherit a rare kidney condition called ARPKD. It is caused by a recessive allele.
A pregnant woman has ARPKD. The baby’s father is heterozygous for ARPKD.
What is the probability of their baby having ARPKD?
Complete the genetic diagram to explain your answer.
Use the letters:
• R for the dominant allele
• r for the recessive allele.
It is estimated that 1 in 70 people in the UK is a carrier of ARPKD.
The estimated population of the UK in 2011 was 63.2 million.
Estimate the number of people in the UK in 2011 who were carriers.
Explain why this number is only an estimate and not the real number of carriers.
The kidneys in people with ARPKD have not developed normally.
Stem cells could be used to repair the damaged kidneys.
Discuss reasons for and against using stem cells to treat ARPKD.
Type 1 diabetes is a disorder caused by lack of insulin production in the pancreas. Type 1 diabetes has a genetic link.
In humans the haploid number of chromosomes is 23. Insulin is a hormone controlled by a single gene found on chromosome number 11.
How many chromosomes are there in a single human pancreas cell?
Some individuals have a copy of the gene that would cause diabetes but do not have the disorder.
Suggest a reason why.
Some people can inherit a rare condition called ADPKD that results in kidney disease.
The PKD1 gene provides instructions for making a protein called polycystin-1. Polycystin-1 controls kidney cell differentiation.
In ADPKD, polycystin-1 does not work properly.
What name is given to a change in the PKD1 gene that causes ADPKD?
Explain why the ADPKD version of polycystin-1 prevents the kidney working properly.
A pregnant woman is heterozygous for ADPKD. The baby’s father does not have ADPKD and is homozygous recessive.
ADPKD is caused by a dominant allele (D).
What is the probability of their baby having ADPKD?
Use a labelled genetic diagram to explain your answer.
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