Genes, Alleles and Genetic Terms

1. 1 State [2 marks]

   State what is meant by the terms allele and genotype.

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   Model answer

   An allele is a different form of the same gene. The genotype is the combination of alleles that an organism has.

   Mark scheme

   • Allele: a different form of a gene — 1 mark
   • Genotype: the alleles present — 1 mark
2. 2 Describe [2 marks]

   Describe the difference between a dominant allele and a recessive allele.

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   Model answer

   A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present.

   Mark scheme

   • Dominant is expressed with one copy — 1 mark
   • Recessive needs two copies — 1 mark
3. 3 Give [3 marks]

   In sweet peas, the allele for purple flowers (P) is dominant to the allele for white flowers (p). Give the phenotype of a plant with each of the genotypes PP, Pp and pp.

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   Model answer

   PP is purple. Pp is purple. pp is white.

   Mark scheme

   • PP: purple — 1 mark
   • Pp: purple — 1 mark
   • pp: white — 1 mark
4. 4 Explain [3 marks]

   The diagram shows the alleles for fur colour in mice. Explain why the mouse with the genotype Bb has black fur, but the mouse with the genotype bb has brown fur.

   

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   Model answer

   The allele for black fur (B) is dominant, so it is expressed even when only one copy is present. In Bb there is one B, so the fur is black. The allele for brown fur (b) is recessive, so it is only expressed when two copies are present, as in bb.

   Mark scheme

   • B is dominant and expressed with one copy — 1 mark
   • Bb has one B allele, so black fur — 1 mark
   • b is recessive, so only expressed when two copies are present (bb) — 1 mark
5. 5 Explain [2 marks]

   A student says that a dominant allele is always the most common allele in a population. Explain why the student is wrong.

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   Model answer

   Dominant only means that the allele is expressed when only one copy is present. It says nothing about how common the allele is. A dominant allele can be rare, and a recessive allele can be very common.

   Mark scheme

   • Dominant refers to expression, not how common — 1 mark
   • A dominant allele can be rare (or recessive common) — 1 mark
6. 6 Explain [3 marks]

   Most human characteristics, such as height, are not controlled by a single gene. Explain why a cross cannot predict a person's height as simply as it can predict fur colour in mice.

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   Model answer

   Height is controlled by many genes interacting, not by one gene with two alleles. The environment, such as diet, also affects height. So the offspring do not fall into simple ratios such as three to one.

   Mark scheme

   • Height is controlled by many genes — 1 mark
   • The environment also affects it — 1 mark
   • So no simple ratio can be predicted — 1 mark
7. 7 Explain [4 marks]

   Two black mice are both heterozygous for fur colour. Explain how they can have a brown offspring. Use the letters B and b in your answer.

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   Model answer

   Each parent has the genotype Bb. Each parent can make gametes carrying B or b. If an egg with b is fertilised by a sperm with b, the offspring has the genotype bb. With no dominant allele present, the recessive allele is expressed, so the offspring has brown fur.

   Mark scheme

   • Both parents are Bb — 1 mark
   • Gametes carry B or b — 1 mark
   • A b gamete from each parent gives bb — 1 mark
   • No dominant allele, so brown fur is expressed — 1 mark

Quick check

1. 1

   What is an allele?

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

   Alleles are the different forms 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

   Homozygous recessive means two copies of the recessive allele, written as two lower case letters.

3. 3

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

   1. AHomozygous
   2. BHeterozygous
   3. CRecessive only
   4. DHaploid
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   B: Heterozygous

   The two alleles are different, so it is heterozygous.

4. 4

   When is a recessive allele expressed?

   1. AWhen two copies are present
   2. BWhenever it is present
   3. COnly when one copy is present
   4. DOnly in gametes
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   A: When two copies are present

   A recessive allele is only expressed when two copies are present, so no dominant allele is there to mask it.

5. 5

   In mice B is dominant to b. Which genotype gives brown fur?

   1. ABB
   2. BBb
   3. Cbb
   4. DB
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   C: bb

   Brown fur is the recessive phenotype, so it needs the genotype bb.

6. 6

   What does phenotype mean?

   1. AThe alleles in a cell
   2. BThe number of chromosomes
   3. CThe type of gamete made
   4. DThe characteristic that is expressed
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   D: The characteristic that is expressed

   The phenotype is the characteristic that is expressed, such as the colour of fur.

7. 7

   Which of these is controlled by a single gene?

   1. AHeight
   2. BRed-green colour blindness
   3. CBody mass
   4. DIntelligence
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   B: Red-green colour blindness

   Red-green colour blindness is an example of a single gene characteristic, whereas height and skin colour involve many genes.

8. 8

   How many alleles of one gene are in each gamete?

   1. AOne
   2. BTwo
   3. CFour
   4. DTwenty-three
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   A: One

   Gametes have one copy of each chromosome, so one allele of each gene.