Cystic Fibrosis

• What inherited disorder is discussed in this video?

  Cystic fibrosis
• What should you be able to do by the end of this video?

  Describe cystic fibrosis and complete Punnett squares
• What are alleles?

  Different versions of a gene
• What is the difference between dominant and recessive alleles?

  Dominant alleles are expressed with one copy
• What is the symbol for the dominant allele for normal cell membrane function?

  Capital C
• What is the symbol for the recessive allele for defective cell membranes?

  Lowercase c
• How many copies of the defective allele must a person inherit to have cystic fibrosis?

  Two copies
• What do we call a person with one defective allele and one normal allele?

  A carrier of the cystic fibrosis allele
• What is the genotype of a man who is a carrier of the cystic fibrosis allele?

  Heterozygous (one normal, one defective)
• What is the genotype of a woman who does not have cystic fibrosis and is not a carrier?

  Homozygous for the normal allele
• What is the first step in constructing a Punnett square for cystic fibrosis?

  Write down the genotypes of the gametes
• What percentage of offspring will be carriers when a heterozygous man and a homozygous normal woman have children?

  50%
• What is the genotype of the offspring that are carriers of cystic fibrosis?

  Capital C lowercase c
• What is the ratio of carriers to non-carriers from the cross of a heterozygous man and a homozygous normal woman?

  1:1
• What is the probability that all offspring could be carriers or all could be unaffected?

  These are just probabilities
• What happens when two carriers of the cystic fibrosis allele have children?

  25% will have cystic fibrosis
• What is the ratio of offspring from two carriers of cystic fibrosis?

  1:2:1 ratio
• What is the percentage of offspring that are not affected at all from two carriers?

  25%
• What is the percentage of offspring that are carriers from two carriers?

  50%
• What is the percentage of offspring that have cystic fibrosis from two carriers?

  25%
• What are the key points about cystic fibrosis inheritance?

  • Cystic fibrosis is a disorder of cell membranes.
  • Controlled by a single gene with two alleles.
  • Dominant allele (normal function): Capital C.
  • Recessive allele (defective function): Lowercase c.
  • Two copies of the defective allele are needed for cystic fibrosis.
  • Carriers have one normal and one defective allele.
• What are the outcomes of genetic crosses involving cystic fibrosis?

  • Heterozygous man and homozygous normal woman:
  • 50% carriers, 50% not affected (1:1 ratio).
  • Two carriers:
  • 25% not affected, 50% carriers, 25% with cystic fibrosis (1:2:1 ratio).
• What should students practice regarding genetic crosses?

  • Complete Punnett square diagrams.
  • Express outcomes as percentages and ratios.
  • Understand probabilities in genetic outcomes.