Family Trees

• Recessive allele


  A person must inherit the faulty allele from both parents to have the disorder
• Dominant allele


  A person only needs to inherit the faulty allele from one parent to have the disorder
• A family tree only shows phenotypes, not genotypes
• Cystic fibrosis


  • Caused by a recessive allele
  • A person with cystic fibrosis must inherit the faulty allele from both parents
• Polydactyly


  • Caused by a dominant allele
  • A person only needs to inherit the faulty allele from one parent to have polydactyly
• Determining genotype of person 2 with cystic fibrosis

  Person 2 must have the genotype lowercase e lowercase e
• Evidence that cystic fibrosis is caused by a recessive allele
• Probability of child of carriers 7 and 8 having cystic fibrosis

  25% or 1 in 4
• Determining genotype of person 3 with polydactyly

  Person 3 must have the genotype capital P lowercase P
• Probability of child of person 6 (unaffected) and person 7 (with polydactyly) having polydactyly

  50% or 1 in 2