Cystic fibrosis is an autosomal recessive trait that affects many parts of the body, particularly the lungs and other organs. Parents who show none of the characteristics of cystic fibrosis have an affected child. The chance that their next child will be phenotypically normal is: A. three in four. B. one in four. C. one in two. D. zero.

SSCE VCE Biology Mechanisms of Inheritance: Cystic fibrosis is an autosomal

Cystic fibrosis is an autosomal recessive trait that affects many parts of the body, particularly the lungs and other organs - VCE - SSCE Biology - Question 23 - 2013 - Paper 1

Question 23

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Cystic fibrosis is an autosomal recessive trait that affects many parts of the body, particularly the lungs and other organs. Parents who show none of the character... show full transcript

Worked Solution & Example Answer:Cystic fibrosis is an autosomal recessive trait that affects many parts of the body, particularly the lungs and other organs - VCE - SSCE Biology - Question 23 - 2013 - Paper 1

Step 1

Determine the genotype of the parents.

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Answer

Since the parents show no characteristics of cystic fibrosis but have one affected child, we can conclude that both parents are carriers of the cystic fibrosis gene. This means their genotypes are both 'Cc' (C = normal allele, c = cystic fibrosis allele).

Step 2

Use a Punnett square to predict the probabilities.

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By setting up a Punnett square with the alleles from both parents:

        C        c
      ___________
    C |   CC   |  Cc  |
      |_________|______|
    c |   Cc   |  cc  |
      |_________|______|

The genotypes of the offspring could be:

CC (normal): 1
Cc (carrier, normal): 2
cc (affected): 1

This results in 3 out of 4 outcomes being phenotypically normal (CC or Cc).

Step 3

Calculate the probability.

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The chance that their next child will be phenotypically normal is therefore:

3 normal (CC or Cc)
1 affected (cc)

Thus, the probability is:

$P(normal) = rac{3}{4}$ .

Step 4

Select the correct answer.

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Answer

The correct answer is A. three in four.

Cystic fibrosis is an autosomal recessive trait that affects many parts of the body, particularly the lungs and other organs - VCE - SSCE Biology - Question 23 - 2013 - Paper 1

Worked Solution & Example Answer:Cystic fibrosis is an autosomal recessive trait that affects many parts of the body, particularly the lungs and other organs - VCE - SSCE Biology - Question 23 - 2013 - Paper 1

Determine the genotype of the parents.

Use a Punnett square to predict the probabilities.

Calculate the probability.

Select the correct answer.

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