Pedigree

• Pedigree


  Chart that shows the presence or absence of a trait within a family across generations
• Genotype


  The genetic makeup of an organism (e.g.: TT)
• Phenotype


  The physical characteristics of an organism (e.g.: tall)
• Dominant allele


  Allele that is phenotypically expressed over another allele
• Recessive allele


  Allele that is only expressed in absence of a dominant allele
• Autosomal trait


  Trait that is located on an autosome (non-sex chromosome)
• Sex-linked trait


  Trait that is located on one of the two sex chromosomes
• Homozygous


  Having two identical alleles for a particular gene
• Heterozygous


  Having two different alleles for a particular gene
• Pedigree Charts


  Pedigrees are family trees that show the occurrence of a particular characteristic over several generations.
• Pedigree Symbols


  • II 1 and III 2
  • II 2
  • II 2 and II 3
  • I 2 and III 5
  • III 1 and III 6
• By analysing a pedigree, we can determine genotypes and identify phenotypes, and predict how a trait will be passed on in the future.
• The information from a pedigree makes it possible to determine how certain alleles are inherited: whether they are dominant, recessive, autosomal, or sex-linked (inherited from a sex chromosome X or Y).
• To start reading a pedigree:

  1. Determine whether the trait is dominant or recessive.

  2. Determine if the chart shows an autosomal or sex-linked (usually X-linked) trait.
• The BEST QUESTIONS to ask of a pedigree that shows autosomal inheritance

  • Q1. Does the trait skip a generation?
  • Q2. Do any affected children have unaffected parents?
  • Q3. Is there an affected individual in every generation?
  • Q4. Do 2 affected parents have any unaffected children?
  • Q5. If the trait disappears from a branch, does it reappear?
• Pedigrees of Autosomal Dominant traits/diseases


  • Both males and females can be affected (autosomal)
  • All affected individuals have at least one affected parent (dominant)
  • Transmission can be father to daughter/son, or mother to daughter/son (autosomal)
  • If a trait disappears from a branch it does not reappear (dominant; no carriers)
  • Approximately equal males : females affected in a large sample (autosomal)
• Pedigrees of Autosomal Recessive traits/diseases


  • Both males and females can be affected (autosomal)
  • Two unaffected parents can have an affected child (recessive)
  • Two affected parents will always have affected children (recessive)
  • If a trait disappears from a branch it may reappear later (recessive: skip generations)
  • Approximately equal males : females affected in a large sample (autosomal)
• Pedigree for three generations of a family

  • Person 8 is the aunt of person 13
  • Person 5 is related to person 9 as brother in-law
• The trait in the pedigree is recessive. Individuals 8 and 9 do not have the characteristic yet one of their children does, so they must each have one copy of the allele for the condition, but they are not displaying symptoms as the condition is recessive.
• The gene for cystic fibrosis is likely to be recessive based on the pedigree.
• If individuals 12 and 13 have more children, the chance that any future child will have cystic fibrosis is 1/4 or 25%.


  1. P Bb x Bb

  2. B b

  3. B BB Bb

  4. b Bb bb
• Huntington's disease


  • Approx equal distribution males and females
  • Every affected person has at least one affected parent
  • Affected males can have affected sons or daughters
  • Autosomal Dominant
• The BEST QUESTIONS to ask of a pedigree that might show X linked inheritance

  • Q1. Does an affected male (with an unaffected partner) have all affected daughters and no affected sons?
  • Q2. Does an affected female (with an unaffected partner) have all affected sons and no affected daughters?
• Pedigrees of X-Linked Dominant traits/diseases

  • An affected male passes the trait to all his daughters and none of his sons (X-linked)
  • A female with the trait may pass it on to both her daughters or sons (X-linked, dom)
  • Every affected person has at least one parent with the trait (dominant)
  • The trait cannot skip generations (dominant: no carriers)
  • More affected females than males over a large sample (X-linked, dominant)
• Pedigrees of X-Linked Recessive traits/diseases

  • An affected female passes the trait to all her sons (X-linked, recessive)
  • A male with the trait will have daughters who are carriers (X-linked, recessive)
  • A male with the trait will have unaffected sons unless the mother is a carrier (X-linked)
  • Two affected parents will always have affected children (recessive)
  • There will be more affected males than females in a large sample (X-linked, recessive)
• The inheritance pattern for a trait with more affected females than males, where an affected male has all affected daughters and no affected sons, and every affected person has at least one affected parent, is X-Linked Dominant.
• The inheritance pattern for colour-blindness, which affects more males than females and skips a generation (carrier females, not indicated), is X-Linked Recessive.