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
• Genotype of person 2


  Lowercase e lowercase e (cystic fibrosis)
• If cystic fibrosis was dominant, at least one parent of person 11 would have to have cystic fibrosis</b>
• Probability of child having cystic fibrosis if parents are carriers

  25% (1 in 4)
• Genotype of person 3

  Capital P lowercase P (polydactyly)
• Probability of child having polydactyly if one parent is affected and one is unaffected

  50% (1 in 2)