Cystic fibrosis

• 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?

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

  Heterozygous (C, c)
• What is the genotype of a woman who is homozygous for the normal allele?

  Homozygous (C, C)
• What is the first step in constructing a Punnett square?

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

  50%
• What is the ratio of carriers to non-carriers from the cross of a heterozygous man and a homozygous woman?

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

  These are just probabilities
• What is the expected outcome 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
• What percentage of offspring are expected to be homozygous for the normal allele from two carriers?

  25%
• What is the expected percentage of offspring who are carriers from two carriers of cystic fibrosis?

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

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

  • Cystic fibrosis is caused by a single gene.
  • The normal allele (C) is dominant.
  • The defective allele (c) is recessive.
  • 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 x Homozygous normal:
  • 50% carriers, 50% non-carriers
  • Carrier x Carrier:
  • 25% homozygous normal
  • 50% carriers
  • 25% homozygous for cystic fibrosis
• What are the probabilities in genetic crosses?

  • Probabilities indicate expected outcomes.
  • Actual results may vary from probabilities.
  • All offspring could be carriers or unaffected.