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Exam questions · Biology · Inheritance

Alleles, Genotype and Phenotype

  • 6 exam questions
  • 20 marks
  • 9 quick checks
  1. 1 State [2 marks]

    State what is meant by (a) an allele and (b) a dominant allele.

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    Model answer

    (a) A different version of the same gene. (b) An allele that shows its effect even when only one copy is present.

    Mark scheme

    • Different version of a gene — 1 mark
    • Shows its effect with one copy — 1 mark
  2. 2 Name [2 marks]

    In pea plants, tall (T) is dominant to dwarf (t). Name the genotypes that give a tall plant.

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    Model answer

    TT and Tt.

    Mark scheme

    • TT — 1 mark
    • Tt — 1 mark
  3. 3 Explain [3 marks]

    Explain what is meant by the terms homozygous and heterozygous, using the genotypes TT and Tt as examples.

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    Model answer

    Homozygous means that the two alleles of the gene are the same, such as TT. Heterozygous means that the two alleles are different, such as Tt.

    Mark scheme

    • Homozygous: two identical alleles — 1 mark
    • Heterozygous: two different alleles — 1 mark
    • Correct example of each — 1 mark
  4. 4 Explain [3 marks]

    A heterozygous tall plant has the genotype Tt. Explain why it is tall and not dwarf.

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    Model answer

    The allele for tall (T) is dominant, so it shows its effect even though only one copy is present. The allele for dwarf (t) is recessive, so it only has an effect if there are two copies. The plant therefore has the phenotype tall.

    Mark scheme

    • T is dominant — 1 mark
    • Shows with one copy — 1 mark
    • t is recessive and only shows with two copies — 1 mark
  5. 5 Describe [4 marks]

    Cystic fibrosis is caused by a recessive allele (f). Describe the genotypes of a person who has cystic fibrosis, a carrier and a person who does not have the allele.

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    Model answer

    A person with cystic fibrosis has the genotype ff, with two recessive alleles. A carrier has the genotype Ff, with one dominant and one recessive allele, and does not have the disorder. A person who does not have the allele has the genotype FF.

    Mark scheme

    • ff has cystic fibrosis — 1 mark
    • Ff is a carrier — 1 mark
    • Carrier does not have the disorder — 1 mark
    • FF has no recessive allele — 1 mark
  6. 6 Explain [6 marks]

    Explain how the alleles of a gene are passed from parents to their children.

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    Model answer

    Each parent has two alleles of a gene. In meiosis the pair of chromosomes is separated, so each gamete carries only one allele of the gene. A heterozygous parent produces two types of gamete. At fertilisation, one gamete from the mother fuses with one gamete from the father to form a zygote. The zygote therefore has two alleles again, one from each parent. Which gametes fuse is a matter of chance, so the combination of alleles in the child is random.

    Mark scheme

    • Each parent has two alleles — 1 mark
    • Meiosis separates the alleles, one per gamete — 1 mark
    • Heterozygous parent makes two types of gamete — 1 mark
    • Fertilisation fuses one gamete from each parent — 1 mark
    • The zygote has two alleles again — 1 mark
    • Random / chance — 1 mark

Quick check

  1. 1

    What is an allele?

    1. AA type of chromosome
    2. BA cell
    3. CA different version of a gene
    4. DA protein
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    C: A different version of a gene

    An allele is a different version of the same gene.

  2. 2

    Which genotype is homozygous recessive?

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

    Two lower-case letters, tt, show two copies of the recessive allele.

  3. 3

    What is the phenotype of a plant with the genotype Tt, if T (tall) is dominant?

    1. ATall
    2. BDwarf
    3. CMedium
    4. DIt cannot be told
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    A: Tall

    The dominant allele shows, so the plant is tall.

  4. 4

    What does genotype mean?

    1. AThe characteristic it shows
    2. BThe alleles it has
    3. CThe number of chromosomes
    4. DIts gametes
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    B: The alleles it has

    The genotype is the combination of alleles an organism has.

  5. 5

    How many alleles of a gene are in a gamete?

    1. AOne
    2. BTwo
    3. CThree
    4. DFour
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    A: One

    A gamete contains only one allele of each gene.

  6. 6

    Which describes a heterozygous genotype?

    1. ABB
    2. Bbb
    3. CBb
    4. DB
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    C: Bb

    Heterozygous means that the two alleles are different, such as Bb.

  7. 7

    A person with genotype Ff does not have cystic fibrosis, but has a recessive allele. What is the person called?

    1. AA mutant
    2. BA zygote
    3. CA clone
    4. DA carrier
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    D: A carrier

    A person with one recessive allele who does not show the disorder is a carrier.

  8. 8

    A plant is tall. Which statement is correct?

    1. AIts genotype must be TT
    2. BIts genotype could be TT or Tt
    3. CIts genotype must be Tt
    4. DIts genotype must be tt
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    B: Its genotype could be TT or Tt

    The plant could be TT or Tt, because both have the dominant phenotype.

  9. 9

    Why does the recessive phenotype only appear in the homozygous recessive genotype?

    1. AA dominant allele would hide the recessive allele
    2. BRecessive alleles are rare
    3. CRecessive alleles are in the gametes
    4. DRecessive alleles are on a different chromosome
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    A: A dominant allele would hide the recessive allele

    If there is a dominant allele, it shows its effect and hides the recessive allele, so the recessive phenotype needs two recessive alleles.