Cystic Fibrosis

• What inherited disorder is discussed in this video?

  Cystic fibrosis
• What are the key characteristics of cystic fibrosis?

  • Disorder of cell membranes
  • Controlled by a single gene
  • Two alleles: dominant (normal function) and recessive (defective function)
• What is the difference between dominant and recessive alleles?

  A dominant allele is expressed with one copy; a recessive allele needs two copies.
• 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 of the defective allele
• What does it mean to be a carrier of the cystic fibrosis allele?

  A carrier has one defective allele and one normal allele.
• How do you construct a Punnett square for cystic fibrosis inheritance?

  • Write down the genotypes of the gametes
  • Combine male and female gametes
  • Determine offspring genotypes and phenotypes
• What is the expected genotype ratio of offspring from a cross between a heterozygous male and a homozygous normal female?

  1:1 ratio of carriers to non-carriers
• What percentage of offspring from a cross between a heterozygous male and a homozygous normal female will be carriers?

  50%
• What are the expected outcomes of a cross between two carriers of the cystic fibrosis allele?

  • 25% homozygous normal (not affected)
  • 50% heterozygous carriers
  • 25% homozygous for defective allele (have cystic fibrosis)
• What is the ratio of offspring from a cross between two carriers of cystic fibrosis?

  1:2:1 ratio
• Why are the numbers in genetic crosses considered probabilities?

  They are expected outcomes, not certainties.