Cystic Fibrosis

• What inherited disorder is described in this video?

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

  Describe cystic fibrosis, complete Punnett square diagrams, and express outcomes as percentages or ratios
• What are different versions of genes called?

  Alleles
• How can alleles be classified?

  As dominant or recessive
• What is the expression of a dominant allele in the phenotype?

  A dominant allele is always expressed even if only one copy is present
• When is a recessive allele expressed?

  A recessive allele is expressed only if two copies are present
• What type of disorder is cystic fibrosis?

  A disorder of cell membranes
• How is cystic fibrosis controlled genetically?

  By a single gene with two alleles
• What is the symbol for the allele for normal cell membrane function?

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

  Lowercase c
• What must a person inherit to have cystic fibrosis?

  Two copies of the defective allele
• 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 copy of each allele)
• What is the genotype of a woman who is homozygous for the normal allele?

  She has two copies of the normal allele
• What is the first step in constructing a Punnett square for cystic fibrosis?

  Write down the genotypes of the gametes
• What is the outcome when the male gametes combine with the female gametes in the example given?

  50% of the offspring will be carriers and 50% will not be affected
• What is the ratio of carriers to non-carriers in the offspring from the first cross?

  1. 1
• What do the numbers in the Punnett square represent?

  Probabilities of the offspring's genotypes
• What happens when two carriers of the cystic fibrosis allele have children?

  On average, 25% of the offspring will have cystic fibrosis
• What is the expected percentage of offspring that are heterozygous carriers from two carrier parents?

  50%
• What is the expected percentage of offspring that are homozygous for the defective allele?

  25%
• What are the genotypes and phenotypes of the offspring from two carriers of the cystic fibrosis allele?

  • 25% homozygous for normal allele (not affected)
  • 50% heterozygous (carriers)
  • 25% homozygous for defective allele (have cystic fibrosis)
• What should you remember about the probabilities in genetic crosses?

  They are averages and actual outcomes can vary
• What will be covered in the next video?

  Inherited disorder polyductyly