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Exam questions · Biology · Inheritance

Genes, the Genome and Phenotype

  • 6 exam questions
  • 16 marks
  • 9 quick checks
  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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

  1. 1

    What is a gene?

    1. AA type of cell
    2. BA section of DNA that codes for a protein
    3. CA protein
    4. DA type of chromosome
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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.

  2. 2

    What is an allele?

    1. AA kind of chromosome
    2. BA type of protein
    3. CA different version of the same gene
    4. DA gamete
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    C: A different version of the same gene

    An allele, or variant, is a different version of the same gene.

  3. 3

    A plant has the genotype Tt. Which word describes it?

    1. AHeterozygous
    2. BHomozygous
    3. CHaploid
    4. DRecessive
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    A: Heterozygous

    It has two different alleles, so it is heterozygous.

  4. 4

    Which genotype shows the recessive phenotype?

    1. ATT
    2. BTt
    3. CtT
    4. Dtt
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    D: tt

    The recessive phenotype only shows when there are two recessive alleles.

  5. 5

    What is the genome?

    1. AA single gene
    2. BAll of the genetic material of an organism
    3. CA chromosome
    4. DA protein
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    B: All of the genetic material of an organism

    The entire genetic material of an organism.

  6. 6

    What decides the phenotype of an organism?

    1. AThe genome only
    2. BThe environment only
    3. CThe genome and the environment
    4. DThe mutations only
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    C: The genome and the environment

    The genome and its interaction with the environment.

  7. 7

    Which feature is controlled by a single gene?

    1. ACystic fibrosis
    2. BHeight
    3. CSkin colour
    4. DBody mass
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    A: Cystic fibrosis

    Cystic fibrosis is controlled by one gene. Height and skin colour involve many genes.

  8. 8

    About how many genes are in the human genome?

    1. A20
    2. B200
    3. C2000
    4. D20 000
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    D: 20 000

    About 20 000.

  9. 9

    Two plants of the genotype Tt have different heights. What is the most likely explanation?

    1. AThey have different genotypes
    2. BThe environment has affected their growth
    3. COne has a recessive allele
    4. DThey are haploid
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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.