Family Pedigrees

1. 1 State [2 marks]

   In a pedigree chart, state what is shown by (a) a circle, (b) a shaded symbol.

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   Model answer

   (a) A female. (b) A person who has the characteristic or disorder.

   Mark scheme

   • Female — 1 mark
   • Has the disorder — 1 mark
2. 2 Name [2 marks]

   A genetic disorder is caused by a recessive allele (n). Name the genotype of a carrier, and the genotype of a person with the disorder.

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   Model answer

   A carrier is Nn. A person with the disorder is nn.

   Mark scheme

   • Nn — 1 mark
   • nn — 1 mark
3. 3 Explain [3 marks]

   Two parents without cystic fibrosis have a daughter who has the disorder. Explain how this is possible.

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   Model answer

   Cystic fibrosis is caused by a recessive allele. Both parents are carriers, Ff, so each has one dominant allele that hides the effect of the recessive one. The daughter inherited a recessive allele from each parent, so she is ff and has the disorder.

   Mark scheme

   • The allele is recessive — 1 mark
   • Both parents are carriers (Ff) — 1 mark
   • She inherited f from each parent — 1 mark
4. 4 Calculate [3 marks]

   A man has Huntington's disease (Hh) and his partner does not have it (hh). Draw a Punnett square, and state the percentage chance that their child has Huntington's disease.

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   Model answer

   Gametes from the man are H and h, and from the woman h and h. The offspring are Hh, Hh, hh, hh. Half have Hh, so the chance is 50%.

   Mark scheme

   • Correct gametes — 1 mark
   • Correct genotypes Hh, Hh, hh, hh — 1 mark
   • 50% — 1 mark
5. 5 Explain [4 marks]

   In a pedigree for a disorder, two parents who are both affected have a child who is unaffected. Explain whether the disorder is caused by a dominant or a recessive allele.

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   Model answer

   The disorder is caused by a dominant allele. If it were recessive, both affected parents would be homozygous recessive, and could only have affected children. Since they have an unaffected child, the parents must have a dominant allele that is hidden in the child. The parents are heterozygous, and the child has two recessive alleles.

   Mark scheme

   • Dominant allele — 1 mark
   • If recessive, all children would be affected — 1 mark
   • Parents are heterozygous — 1 mark
   • Child received the recessive allele from each — 1 mark
6. 6 Evaluate [6 marks]

   People with a family history of Huntington's disease can have a genetic test to find out whether they carry the allele. Evaluate the advantages and disadvantages of taking the test.

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   Model answer

   An advantage is that people who test positive can make decisions about their future, such as whether to have children, and they can plan for their care. They can also avoid the uncertainty of not knowing. People who test negative are relieved of worry. A disadvantage is that there is currently no cure, so a positive result could cause great distress and depression. The result might affect insurance or employment, and it also has implications for relatives, who might not want to know. Overall the decision is personal, and counselling should be available before and after the test.

   Mark scheme

   • Can plan or make decisions about children — 1 mark
   • Ends uncertainty — 1 mark
   • No cure, so distress — 1 mark
   • Insurance or employment — 1 mark
   • Implications for relatives — 1 mark
   • A reasoned conclusion — 1 mark

Quick check

1. 1

   In a pedigree, what does a filled square show?

   1. AAn unaffected male
   2. BAn affected male
   3. CAn affected female
   4. DA carrier
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   B: An affected male

   A square is a male and a filled symbol means the person has the disorder.

2. 2

   Two unaffected parents have an affected child. What type of allele causes the condition?

   1. ADominant
   2. BMutated
   3. CRecessive
   4. DSex-linked
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   C: Recessive

   The allele is hidden in the parents, so it is recessive.

3. 3

   What is the genotype of a person with cystic fibrosis?

   1. AFF
   2. BFf
   3. CfF only
   4. Dff
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   D: ff

   Cystic fibrosis is recessive, so a person with it has two recessive alleles, ff.

4. 4

   Which genotype is a carrier of cystic fibrosis?

   1. AFf
   2. BFF
   3. Cff
   4. DF
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   A: Ff

   A carrier has one dominant and one recessive allele, Ff.

5. 5

   Huntington's disease is caused by a dominant allele. What is the chance that a child of one affected parent (Hh) and one unaffected parent (hh) is affected?

   1. A25%
   2. B50%
   3. C75%
   4. D100%
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   B: 50%

   Half of the children inherit H from the affected parent, so the chance is 50%.

6. 6

   In a pedigree, what does a horizontal line between a square and a circle show?

   1. ABrother and sister
   2. BParent and child
   3. CTwo partners
   4. DTwins
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   C: Two partners

   A horizontal line joins two partners.

7. 7

   Two carriers (Ff) have a child. What is the probability that the child is a carrier?

   1. A50%
   2. B25%
   3. C75%
   4. D100%
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   A: 50%

   The Punnett square gives FF, Ff, Ff and ff, so 2 out of 4 are carriers: 50%.

8. 8

   In a family, every affected person has an affected parent, and the condition appears in every generation. What type of allele is most likely?

   1. ARecessive
   2. BSex-linked recessive
   3. CIt cannot be inherited
   4. DDominant
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   D: Dominant

   This pattern points to a dominant allele.

9. 9

   Why can the genotype of an unaffected child of two carriers not be known for certain?

   1. AThe child is a carrier
   2. BThe child could be FF or Ff
   3. CThe child must be ff
   4. DThe genotype changes with age
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   B: The child could be FF or Ff

   The child could be FF or Ff, because both look the same, so the pedigree cannot tell which one it is.