Determining Mode of Inheritance Using Pedigrees

Understanding Pedigrees

• A pedigree is a chart similar to a family tree.
• It shows relationships between family members and traces specific traits through generations.
• Traits traced can be genetic disorders like hemophilia or sickle cell anemia.
• Symbols:
  • Circles = Females
  • Squares = Males
  • Diamonds = Unknown sex
  • Shaded = Affected by the trait
  • Unshaded = Unaffected by the trait

Relationships in Pedigrees

• Biological parents connected by a marriage line.
• Offspring connected by a horizontal sib-ship line below parents.
• Offspring listed in order of birth from left to right.
• Generations denoted by Roman numerals.

Modes of Inheritance

1. Autosomal Dominant

   • Carried on a dominant allele of an autosomal chromosome.
   • Affected individuals: Homozygous dominant or heterozygous.
   • Unaffected parents cannot have affected children.
   • Affected parents can have unaffected children if heterozygous.

2. Autosomal Recessive

   • Carried on a recessive allele of an autosomal chromosome.
   • Affected individuals: Homozygous recessive.
   • Affected parents have all affected children.
   • Unaffected parents can have affected children if heterozygous.

3. X-linked Dominant

   • Carried on the X chromosome, behaves like a dominant allele.
   • Affected females: One dominant X allele.
   • Affected males: One X allele.
   • Affected father has all affected daughters.

4. X-linked Recessive

   • Carried on a recessive allele on the X chromosome.
   • Affected females: Both recessive alleles.
   • Affected males: One affected X allele.
   • Affected mothers have all affected sons.
   • Affected parents have all affected children.

5. Y-linked

   • Carried on the Y chromosome.
   • Only males affected.
   • Affected father has all affected sons.

Analyzing Pedigrees

• List all possible modes of inheritance, rule out based on pedigree patterns.
• Start by ruling out Y-linked based on affected individuals.
• Use unaffected parents having affected children to rule out autosomal dominant and X-linked dominant.
• Check for recessive inheritance by confirming if parents can be heterozygous.
• Rule out X-linked recessive if not all sons are affected.

Practice Examples

• Use specific families to rule out modes of inheritance.
• Observations like skipping generations help in ruling out dominantly inherited disorders.
• Common Strategy: First rule out impossible inheritance modes, then verify possible ones with pedigree data.

Conclusion

• Practice helps in accurately identifying the mode of inheritance in pedigrees.
• Additional information may be needed to confirm the inheritance mode in complex pedigrees.
• Online resources can provide blank pedigrees for practice.