Exam questions · Biology
Inheritance
- 42 exam questions
- 125 marks
- 63 quick checks
Genes, the Genome and Phenotype
Just this lesson-
1 State [1 mark]
State what is meant by the genome of an organism.
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Model answer
The entire genetic material of an organism.
Mark scheme
- Entire genetic material — 1 mark
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2 Give [2 marks]
Give the meaning of the terms homozygous and heterozygous.
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Model answer
Homozygous means having two identical alleles of a gene. Heterozygous means having two different alleles of a gene.
Mark scheme
- Homozygous: two identical alleles — 1 mark
- Heterozygous: two different alleles — 1 mark
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3 Describe [2 marks]
Pea plants with the genotype Tt are tall, and plants with tt are dwarf. State the genotype that is homozygous recessive, and state the phenotype of Tt plants.
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Model answer
The homozygous recessive genotype is tt. The phenotype of Tt plants is tall.
Mark scheme
- tt — 1 mark
- Tall — 1 mark
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4 Explain [3 marks]
Two plants have identical genes, but one is much taller than the other. Explain how this can happen.
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Model answer
The phenotype depends on the genome and on the environment. The taller plant may have had more light, water or minerals in its environment, so it grew more than the other plant, even though they have the same genes.
Mark scheme
- Phenotype depends on genome and environment — 1 mark
- Environment differs, such as light, water or minerals — 1 mark
- So one plant grows taller — 1 mark
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5 Calculate [2 marks]
The human genome has about 3 billion base pairs, in 23 pairs of chromosomes. Estimate the mean number of base pairs in one chromosome, to the nearest 10 million.
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Model answer
There are 46 chromosomes in 23 pairs. 3 000 000 000 ÷ 46 = 65 million, which is 70 million to the nearest 10 million.
Mark scheme
- 46 chromosomes — 1 mark
- About 70 million — 1 mark
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6 Evaluate [6 marks]
A student says that a person's height is decided only by the gene that they inherit for height. Evaluate this statement.
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Model answer
The statement is not correct. Height is a phenotypic feature, and most features are the result of many genes, rather than a single gene, so there is no one gene for height. The genome sets what the person can become, but the environment, including diet and illness in childhood, also has an effect, and the phenotype is the result of the interaction between the two. For example, identical twins have the same genome, but may be different heights if they have been fed differently. Genes are important, and tall parents often have tall children, but the claim that they alone decide height ignores the role of the environment and of many genes.
Mark scheme
- Most features involve many genes — 1 mark
- Not a single gene for height — 1 mark
- The genome sets a possible range — 1 mark
- The environment affects the phenotype — 1 mark
- An example such as diet or identical twins — 1 mark
- A reasoned conclusion — 1 mark
Quick check
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1
What is a gene?
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B: A section of DNA that codes for a protein
A gene is a section of DNA that codes for a protein.
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2
What is an allele?
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C: A different version of the same gene
An allele, or variant, is a different version of the same gene.
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3
A plant has the genotype Tt. Which word describes it?
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A: Heterozygous
It has two different alleles, so it is heterozygous.
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4
Which genotype shows the recessive phenotype?
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D: tt
The recessive phenotype only shows when there are two recessive alleles.
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5
What is the genome?
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B: All of the genetic material of an organism
The entire genetic material of an organism.
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6
What decides the phenotype of an organism?
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C: The genome and the environment
The genome and its interaction with the environment.
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7
Which feature is controlled by a single gene?
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A: Cystic fibrosis
Cystic fibrosis is controlled by one gene. Height and skin colour involve many genes.
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8
About how many genes are in the human genome?
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D: 20 000
About 20 000.
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9
Two plants of the genotype Tt have different heights. What is the most likely explanation?
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B: The environment has affected their growth
They have the same genotype, so the difference is probably caused by the environment.
Variation and Mutations
Just this lesson-
1 State [1 mark]
State what is meant by continuous variation.
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Model answer
Variation in which a feature can have any value within a range.
Mark scheme
- Any value in a range — 1 mark
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2 Give [2 marks]
Give one example of discontinuous variation and one example of continuous variation in humans.
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Model answer
Discontinuous: eye colour, or blood group. Continuous: height, or weight.
Mark scheme
- Discontinuous example — 1 mark
- Continuous example — 1 mark
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3 Explain [3 marks]
Explain why a bar chart with gaps is used to show the data for blood groups, but a histogram is used for heights.
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Model answer
Blood groups are discontinuous, as they fall into separate categories, so each is shown by a separate bar. Heights are continuous, as they can have any value in a range, so the bars touch in a histogram.
Mark scheme
- Blood group is discontinuous — 1 mark
- Height is continuous — 1 mark
- Gaps for categories, touching bars for a range — 1 mark
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4 Calculate [3 marks]
The masses of six mice were 21, 24, 22, 25, 22 and 26 g. Calculate the mean mass and state the mode.
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Model answer
21 + 24 + 22 + 25 + 22 + 26 = 140. 140 ÷ 6 = 23.3 g. The mode is 22 g.
Mark scheme
- Total of 140 — 1 mark
- Mean 23.3 g — 1 mark
- Mode 22 g — 1 mark
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5 Describe [3 marks]
Describe the effects that mutations can have on the phenotype of an organism.
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Model answer
Most mutations have no effect on the phenotype. Some mutations influence the phenotype, by changing a protein or the amount of it. A very few mutations determine the phenotype.
Mark scheme
- Most no effect — 1 mark
- Some influence the phenotype — 1 mark
- A very few determine the phenotype — 1 mark
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6 Explain [6 marks]
Explain how both genes and the environment are responsible for the differences in body mass between people, and explain where the genetic differences came from.
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Model answer
Body mass is an example of continuous variation, which is the result of many genes, and also the environment. A person's genes affect how they store fat and how fast they use energy, which sets a range for their mass. However, the environment is also important, because the person's diet and the amount of exercise they take change where in the range they end up. This is an interaction between the genome and the environment. The genetic differences between people are because they have different variants, or alleles, of genes. All variants arose from mutations, which are random changes in the DNA. Sexual reproduction then mixes the variants in each generation, so that each person has a different combination.
Mark scheme
- Body mass is continuous, from many genes — 1 mark
- Genes set a range — 1 mark
- Diet and exercise affect it — 1 mark
- Interaction of the genome and the environment — 1 mark
- Variants arose from mutations — 1 mark
- Sexual reproduction mixes the variants — 1 mark
Quick check
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1
Which is an example of discontinuous variation?
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C: Eye colour
Eye colour falls into distinct categories.
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2
Which is an example of continuous variation?
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A: Height
Height can have any value in a range.
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3
Which type of chart is best for blood groups?
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D: A bar chart
A bar chart with gaps shows separate categories.
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4
What does all variation in the phenotype depend on?
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B: The genome and the environment
The genome, the environment, or both.
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5
What produces new variants?
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C: Mutations
Mutations are the source of new variants.
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6
What effect do most mutations have on the phenotype?
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A: No effect
Most have no effect.
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7
Which of these is a mutagen?
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D: Ionising radiation
Ionising radiation increases the rate of mutation.
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8
The leaf lengths were 4, 5, 5, 6 and 10 cm. What is the mean?
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B: 6 cm
\((4 + 5 + 5 + 6 + 10) \div 5 = 6\) cm.
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9
Identical twins differ in body mass. What does this show?
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C: The environment affects body mass
They have the same genome, so the difference must be caused by the environment.
Genetic Variants and Phenotype (Higher tier)
Just this lesson-
1 State [1 mark]
State what is meant by gene expression.
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Model answer
The way that a gene is used to make its protein.
Mark scheme
- Gene used to make a protein — 1 mark
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2 State [2 marks]
State two ways in which a genetic variant can influence the phenotype of an organism.
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Model answer
A variant in coding DNA can alter the activity of a protein. A variant in non-coding DNA can alter how genes are expressed.
Mark scheme
- Coding DNA, altering a protein — 1 mark
- Non-coding DNA, altering gene expression — 1 mark
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3 Explain [3 marks]
A variant changes one base in a gene that codes for an enzyme. Explain how this could stop the enzyme from working.
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Model answer
The change in the base can change an amino acid in the enzyme. This changes the shape of the protein, including the active site. The substrate no longer fits in the active site, so the enzyme cannot catalyse the reaction.
Mark scheme
- Amino acid changed — 1 mark
- Shape of the active site changes — 1 mark
- Substrate does not fit, so no reaction — 1 mark
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4 Explain [3 marks]
Explain how a variant in non-coding DNA, next to a gene, could stop a protein from being made, when the gene itself is normal.
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Model answer
The variant could stop the transcription of the gene, so that no mRNA is made. Without mRNA, the protein cannot be made, although the coding DNA is normal.
Mark scheme
- Variant in non-coding DNA affects expression — 1 mark
- Transcription is stopped, so no mRNA — 1 mark
- So the protein is not made — 1 mark
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5 Describe [2 marks]
Describe why a variant in the DNA does not always change the phenotype.
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Model answer
Some variants are in non-coding DNA that does not control any gene. Others are in coding DNA, but do not change the amino acid, because more than one triplet codes for the same amino acid.
Mark scheme
- Variant in DNA with no effect — 1 mark
- Same amino acid is coded — 1 mark
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6 Evaluate [6 marks]
Compare the effects on an organism of a variant in the coding DNA of a gene and a variant in non-coding DNA next to the gene.
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Model answer
A variant in the coding DNA of a gene can change one or more amino acids in the protein. This can change the shape of the protein and its activity, and if it is an enzyme, the shape of the active site, so the substrate no longer fits. The protein that is made is different. A variant in non-coding DNA next to the gene does not change the amino acids, so the protein itself is normal. Instead it can change how the gene is expressed, for example by stopping transcription of the mRNA, so that less of the protein is made. In both cases the phenotype may change, but the cause is different, either a different protein or a different amount of it.
Mark scheme
- Coding variant changes the amino acid — 1 mark
- Shape or activity of the protein changes — 1 mark
- Enzyme active site example — 1 mark
- Non-coding variant leaves the protein normal — 1 mark
- Gene expression changes, such as stopped transcription — 1 mark
- Different cause, same type of result — 1 mark
Quick check
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1
What does coding DNA contain?
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B: Genes that code for proteins
Genes that code for proteins.
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2
How can a variant in coding DNA affect an enzyme?
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C: It can change the shape of the active site
It can change the shape of the active site.
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3
What can a variant in non-coding DNA affect?
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A: How genes are expressed
How genes are expressed.
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4
Which stage can be stopped by a variant in non-coding DNA?
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D: Transcription of mRNA
Transcription of mRNA.
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5
What is gene expression?
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B: The use of a gene to make its protein
The use of a gene to make its protein.
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6
Why might a change in a base have no effect on the protein?
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C: More than one triplet codes for the same amino acid
More than one triplet codes for the same amino acid.
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7
A variant changes the active site of an enzyme. What happens?
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A: The substrate no longer fits
The substrate no longer fits, so the reaction is not catalysed.
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8
Sickle cell disease is caused by a variant in which type of DNA?
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D: Coding DNA
It is a variant in the coding DNA for haemoglobin.
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9
A variant stops the transcription of a gene. What is the most likely result?
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B: No protein is made from the gene
No mRNA is made, so the protein is not made, though the gene itself is normal.
Asexual and Sexual Reproduction
Just this lesson-
1 State [1 mark]
State what is meant by asexual reproduction.
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Model answer
Reproduction by one parent, with no gametes, which produces genetically identical offspring.
Mark scheme
- One parent, identical offspring — 1 mark
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2 Give [2 marks]
Give two advantages of asexual reproduction.
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Model answer
Any two from: only one parent is needed; many offspring are made quickly; the offspring are well adapted to the same environment; it uses less energy.
Mark scheme
- First — 1 mark
- Second — 1 mark
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3 Explain [3 marks]
Explain why a population of genetically identical organisms is at risk from a new disease.
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Model answer
All of the individuals have the same genes, so they all have the same susceptibility to the pathogen. If none of them is resistant, the disease can affect all of them, and the whole population could die.
Mark scheme
- All have the same genes — 1 mark
- None may be resistant — 1 mark
- The whole population could be killed — 1 mark
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4 Calculate [3 marks]
A bacterium divides every 30 minutes. Calculate the number of bacteria that would be produced from one cell after 4 hours, if none die.
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Model answer
4 hours is 240 minutes, so there are 240 ÷ 30 = 8 divisions. 2 to the power 8 is 256 bacteria.
Mark scheme
- 8 divisions — 1 mark
- 2 to the power 8 — 1 mark
- 256 — 1 mark
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5 Explain [3 marks]
Explain why an organism that reproduces sexually is more likely to survive a change in the environment than one that only reproduces asexually.
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Model answer
Sexual reproduction produces offspring that are genetically different. If there is an environmental pressure, some of them are likely to have variants that help them to survive. The survivors reproduce, so the species survives. Clones would all have the same genes, and the whole population could die.
Mark scheme
- Offspring are genetically different — 1 mark
- Some are likely to survive — 1 mark
- Clones would all be affected in the same way — 1 mark
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6 Evaluate [6 marks]
Aphids reproduce asexually in the summer and sexually in the autumn. Evaluate the advantages to the aphids of using both methods of reproduction.
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Model answer
In the summer the conditions are good, with plenty of food. Asexual reproduction is fast, and makes many offspring without the need for a mate, so the population grows very quickly and takes advantage of the food. The offspring are clones of well-adapted parents, which will do well in the same conditions. In the autumn the conditions are going to change, as the winter approaches. Sexual reproduction produces variation, so some of the eggs are likely to survive the cold or other pressures, such as a different host plant or predators. It is slower and uses more energy, but this is not a problem when the population is not growing. By using both methods, the aphids get the benefit of speed in the summer, and variation for the winter.
Mark scheme
- Summer: good conditions, so fast growth — 1 mark
- Asexual: many offspring, no mate needed — 1 mark
- Clones are well adapted — 1 mark
- Autumn: conditions change — 1 mark
- Sexual: variation, so some survive — 1 mark
- A reasoned conclusion on using both methods — 1 mark
Quick check
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1
Which type of cell division is used in asexual reproduction?
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B: Mitosis
Mitosis.
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2
How are the offspring of asexual reproduction related to the parent?
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C: They are genetically identical
They are genetically identical clones.
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3
Which is an advantage of sexual reproduction?
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A: It produces variation
It produces variation.
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4
Which is an advantage of asexual reproduction?
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D: It makes many offspring fast, without a mate
It can make many offspring quickly, with no mate.
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5
Why were the potatoes in Ireland all affected by blight?
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B: They were genetically identical
They were genetically identical, so none was resistant.
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6
A bacterium divides every 20 minutes. How many bacteria are there after 1 hour, from one cell?
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C: 8
3 divisions gives \(2^3 = 8\).
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7
Which organism uses both asexual and sexual reproduction?
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A: A strawberry plant
A strawberry plant makes runners and seeds.
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8
How many babies are usually born at one time to a human?
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D: One
Usually one.
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9
A population of clones is in an environment that suddenly gets hotter. What is most likely to happen?
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B: They could all be affected in the same way
All the clones have the same tolerance, so if it is not high enough, they will all die.
Meiosis, Haploid and Diploid
Just this lesson-
1 State [2 marks]
State the number of chromosomes in a human body cell and in a human egg cell.
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Model answer
46 in a body cell, and 23 in an egg cell.
Mark scheme
- 46 — 1 mark
- 23 — 1 mark
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2 Give [2 marks]
Give the meaning of the terms haploid and diploid.
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Model answer
Haploid means having one set of chromosomes. Diploid means having two sets of chromosomes.
Mark scheme
- Haploid: one set — 1 mark
- Diploid: two sets — 1 mark
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3 Calculate [2 marks]
A potato plant has 48 chromosomes in each body cell. Calculate the number of chromosomes in a pollen grain.
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Model answer
48 ÷ 2 = 24 chromosomes.
Mark scheme
- Halves the number — 1 mark
- 24 — 1 mark
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4 Explain [3 marks]
Explain why gametes are made by meiosis, and not by mitosis.
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Model answer
Meiosis halves the chromosome number, so that the gametes are haploid. When they fuse, the zygote is diploid. If mitosis were used, the gametes would be diploid, and the number of chromosomes would double with each generation. Meiosis also makes the gametes genetically different.
Mark scheme
- Meiosis gives haploid gametes — 1 mark
- Fertilisation maintains the diploid number — 1 mark
- Mitosis would double the number each generation — 1 mark
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5 Describe [4 marks]
Describe the differences between mitosis and meiosis.
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Model answer
Mitosis makes two cells, but meiosis makes four. The cells made by mitosis are diploid, but those made by meiosis are haploid. The cells from mitosis are genetically identical, but those from meiosis are genetically different. Mitosis is used for growth and repair, but meiosis is used to make gametes.
Mark scheme
- Two cells and four cells — 1 mark
- Diploid and haploid — 1 mark
- Identical and different — 1 mark
- Growth and repair, and gametes — 1 mark
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6 Explain [6 marks]
Explain how meiosis and fertilisation maintain the diploid number, and how they are sources of genetic variation.
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Model answer
Meiosis halves the number of chromosomes, so each gamete is haploid, with one set of chromosomes. At fertilisation a male and a female gamete fuse to make a zygote, which is diploid, with two sets of chromosomes. This maintains the diploid number in every generation. In meiosis the pairs of chromosomes are shared out at random, so each gamete has a different mixture of the chromosomes that came from the parents' own mother and father. A person can make millions of different gametes. Any one of the gametes can fertilise any egg, so the offspring have a different combination of genes from their parents and from each other.
Mark scheme
- Meiosis halves the number, so gametes are haploid — 1 mark
- Fertilisation makes a diploid zygote — 1 mark
- So the diploid number is maintained — 1 mark
- Chromosomes shared out at random — 1 mark
- Millions of different gametes — 1 mark
- Fertilisation is random, so the offspring are different — 1 mark
Quick check
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1
Which cells are made by meiosis?
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C: Gametes
Meiosis makes gametes.
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2
How many chromosomes does a human sperm cell have?
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B: 23
Gametes are haploid, so they have 23 chromosomes.
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3
How many cells are made when one cell divides by meiosis?
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D: 4
Four.
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4
What is a zygote?
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A: The cell formed at fertilisation
The diploid cell formed when two gametes fuse.
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5
What does haploid mean?
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B: Having one set of chromosomes
Having one set of chromosomes.
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6
Which type of cell division is used for growth and repair?
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C: Mitosis
Mitosis.
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7
A body cell has 16 chromosomes. How many are in a gamete?
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A: 8
\(16 \div 2 = 8\).
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8
Why do brothers and sisters look different?
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D: Each gamete has a different mixture of genes
Each gamete has a different mixture of chromosomes, and fertilisation is random.
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9
Identical twins have the same genome. Which statement explains this?
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B: They come from one fertilised egg that splits
They come from one fertilised egg that splits in two.
Single Gene Inheritance and Punnett Squares
Just this lesson-
1 State [1 mark]
State the genotype of a female human, in terms of sex chromosomes.
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Model answer
XX.
Mark scheme
- XX — 1 mark
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2 Give [2 marks]
A plant that is heterozygous for height is crossed with a plant of the same genotype. Give the ratio of phenotypes expected in the offspring if tall (T) is dominant to dwarf (t).
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Model answer
3 tall : 1 dwarf.
Mark scheme
- Tt x Tt gives TT, Tt, Tt, tt — 1 mark
- 3 tall : 1 dwarf — 1 mark
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3 Explain [3 marks]
Explain why a woman who is a carrier of cystic fibrosis (Ff) and a man who is a carrier (Ff) can have a child who does not carry the allele.
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Model answer
Each parent makes F and f gametes. If both of them pass on the F allele, the child is FF, which does not carry the f allele. There is a 1 in 4 chance of this.
Mark scheme
- Each makes F and f gametes — 1 mark
- Both pass on F — 1 mark
- Child is FF, with a 1 in 4 chance — 1 mark
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4 Calculate [3 marks]
Two heterozygous black rabbits (Bb) have 80 offspring. Black is dominant to white. Calculate the number of white rabbits that you would expect.
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Model answer
Bb x Bb gives 1 in 4 bb, so \(\frac{1}{4} \times 80 = 20\) white rabbits.
Mark scheme
- 1 in 4 are white — 1 mark
- 80 ÷ 4 — 1 mark
- 20 — 1 mark
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5 Describe [4 marks]
Use a genetic cross to show that the probability that a baby is a girl is 50%.
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Model answer
The mother is XX, so every egg has an X. The father is XY, so half of the sperm carry X and half carry Y. A Punnett square gives XX, XY, XX and XY. Two of the four are XX, which is a girl, so the probability is 2 in 4, which is 50%.
Mark scheme
- Mother XX, eggs all X — 1 mark
- Father XY, sperm X or Y — 1 mark
- Punnett square XX, XY, XX, XY — 1 mark
- 2 in 4, which is 50% girls — 1 mark
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6 Evaluate [6 marks]
A couple already have three daughters. They think that their fourth child is much more likely to be a boy. Evaluate this idea, using a genetic cross.
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Model answer
The mother is XX, so all of her eggs carry an X. The father is XY, so half of his sperm carry an X and half carry a Y. The Punnett square gives two XX and two XY, so the chance of a boy is 1 in 2, or 50%. Each fertilisation is a separate event, and the sperm that fertilises the egg is random. The earlier babies do not change the chance, because the gametes have no memory. So the idea is wrong, and the chance of a fourth child being a boy is still 50%, the same as for each of the earlier children. The chance of four girls in a row is 1 in 16, but this was only a matter of chance.
Mark scheme
- Mother XX, all eggs carry X — 1 mark
- Father XY, half of the sperm carry X and half Y — 1 mark
- Chance of a boy is 50% — 1 mark
- Fertilisation is random — 1 mark
- Earlier children do not change the probability — 1 mark
- A reasoned conclusion — 1 mark
Quick check
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1
What are the gametes of an individual with the genotype Tt?
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C: T and t
T and t.
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2
What is the genotype ratio of Tt x Tt?
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A: 1 TT : 2 Tt : 1 tt
1 TT : 2 Tt : 1 tt.
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3
What is the probability of a recessive phenotype in Tt x Tt?
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D: 25%
1 in 4, which is 25%.
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4
What are all of the offspring of TT x tt?
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B: Tt
All are Tt.
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5
What are the sex chromosomes of a male?
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C: XY
XY.
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6
What proportion of sperm carry a Y chromosome?
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A: Half
Half.
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7
Two carriers (Ff) have 40 children. How many would you expect to have the disorder (ff)?
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D: 10
1 in 4 of 40 is 10.
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8
What is a Punnett square used for?
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B: To predict the results of a cross
To predict the results of a genetic cross.
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9
A tall pea plant is crossed with a dwarf plant, and about half of the offspring are dwarf. What is the genotype of the tall plant?
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C: Tt
Half dwarf means the tall plant must be heterozygous, Tt.
Mendel and the Development of Genetics
Just this lesson-
1 State [1 mark]
State the organism that Mendel used in his experiments on inheritance.
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Model answer
The pea plant.
Mark scheme
- Pea plant — 1 mark
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2 Give [2 marks]
Give two reasons why pea plants were a good choice for Mendel's experiments.
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Model answer
Any two from: they grow quickly; they have clear differences, such as tall or dwarf; they can be cross-pollinated by hand; they are cheap and easy to grow in large numbers.
Mark scheme
- First — 1 mark
- Second — 1 mark
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3 Describe [3 marks]
Describe what Mendel found when he crossed pure-breeding tall pea plants with pure-breeding dwarf pea plants, and then allowed the offspring to self-pollinate.
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Model answer
All of the offspring in the first generation were tall. When these were self-pollinated, the second generation had about three tall plants to every one dwarf plant. The dwarf feature came back.
Mark scheme
- F1 all tall — 1 mark
- F2 about 3 tall to 1 dwarf — 1 mark
- Dwarf feature reappeared — 1 mark
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4 Calculate [3 marks]
In another experiment, Mendel obtained 705 plants with purple flowers and 224 plants with white flowers. Calculate the ratio of purple to white, in the form x : 1, to 2 decimal places.
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Model answer
705 ÷ 224 = 3.15, so the ratio is 3.15 : 1.
Mark scheme
- Division of 705 by 224 — 1 mark
- 3.15 — 1 mark
- In the form 3.15 : 1 — 1 mark
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5 Explain [3 marks]
Explain why Mendel's work was not accepted by scientists when it was published.
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Model answer
Most scientists believed in blending inheritance, which did not fit his results. Nobody knew about chromosomes or how they were passed on, so there was no mechanism to explain his factors. His use of mathematics in biology was unusual, and his paper was not widely read.
Mark scheme
- Blending inheritance was believed — 1 mark
- No mechanism known — 1 mark
- Unusual mathematics, or few readers — 1 mark
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6 Evaluate [6 marks]
Evaluate the importance of Mendel's work, and of the later discoveries of chromosomes and DNA, to our understanding of inheritance.
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Model answer
Mendel showed that features are controlled by separate factors that do not blend, and that one factor can be dominant over another, with a ratio of about 3 to 1 in the second generation. This was a major advance, because it replaced blending inheritance and used a careful, mathematical method. But he could not say what the factors were, or where they were, so his work was ignored. Later, scientists saw chromosomes, and Sutton and Boveri suggested that the factors were carried on them, which explained how they could be separated into gametes. The structure of DNA in 1953 showed what genes are made of and how information is copied, and the sequencing of the genome showed all of the genes. Each stage depended on the one before, and together they explain inheritance. Mendel's work was the foundation, but the later discoveries were needed to explain it.
Mark scheme
- Mendel showed separate, non-blending factors — 1 mark
- Ratio and dominance — 1 mark
- He could not explain what the factors were — 1 mark
- Chromosomes carry the factors — 1 mark
- DNA shows what genes are made of — 1 mark
- A reasoned judgement of the importance — 1 mark
Quick check
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1
Which organism did Mendel use?
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B: The pea plant
The pea plant.
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2
What did all of the F1 plants look like when tall and dwarf pure-breeding plants were crossed?
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A: Tall
All were tall.
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3
What was the approximate ratio of tall to dwarf plants in the F2 generation?
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C: 3 : 1
About 3 : 1.
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4
What is blending inheritance?
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D: The idea that the features of the parents mix
The idea that the features of the parents mix in the offspring.
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5
What did Mendel call what we now call genes?
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B: Inherited factors
Inherited factors.
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6
Why was Mendel's work not accepted at first?
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C: It did not fit the ideas of the time
It did not fit blending inheritance, and no mechanism was known.
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7
Who suggested that Mendel's factors were carried on chromosomes?
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A: Sutton and Boveri
Sutton and Boveri, in 1902.
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8
Mendel obtained 750 tall and 250 dwarf plants. What percentage were tall?
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D: 75%
\(750 \div 1000 \times 100 = 75\%\).
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9
What did the reappearance of dwarf plants in the F2 generation show?
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B: The dwarf factor had been hidden, not lost
The recessive factor was hidden, but not lost, in the F1 plants.