Pedigree PPT

Pedigrees

12-2: A Genetic Family Tree

  • Pedigrees are genetic family trees used to study inheritance patterns.

Review Questions

  • Autosomes: Non-sex chromosomes (humans have 22 pairs).
  • Sex-chromosomes: Chromosomes determining sex (X and Y in humans).
  • X & Y Chromosomes: X chromosomes carry many genes, while Y chromosomes carry fewer.
  • Carrier: An individual heterozygous for a recessive allele, not expressing the trait but capable of passing it to offspring.

Pedigree Symbols

  • Male: Represented by a square.
  • Female: Represented by a circle.
  • Mating: Represented by a horizontal line connecting a male and female.
  • Parents and Children: Vertical lines connect parents to their children.
    • Children are typically represented in birth order from left to right.
  • Dizygotic (Non-identical) Twins: Represented by two lines stemming from the same point connected to the parents and not connected to each other with a horizontal line.
  • Monozygotic (Identical) Twins: Represented by two lines stemming from the same point connected to the parents and connected to each other with a horizontal line.
  • Number of Children of Sex Indicated: A number next to a symbol representing the sex of the children.
  • Affected Individuals: Represented by shaded symbols.
  • Heterozygotes for Autosomal Recessive: Represented by half-shaded symbols.
  • Carrier of Sex-linked Recessive: Represented by a circle with a dot in the center.
  • Death: Represented by a diagonal line through the symbol.
  • Abortion or Stillbirth (Sex Unspecified): Represented by a small diamond.
  • Propositus: The individual who first brings the pedigree to the attention of a geneticist, indicated by an arrow.
  • Sex Unspecified: Represented by a diamond shape.
  • Method of Identifying Persons: Individuals are identified by generation and individual number (e.g., II.2).
  • Consanguineous Marriage: Represented by a double line connecting mating individuals, indicating a marriage between close relatives.

Patterns of Inheritance: Autosomal Dominant

  • Trait appears in every generation.
  • When one parent is affected (heterozygous), approximately 50% of children will be affected.
  • Affected individuals are usually heterozygous.
  • Unaffected parents do not produce affected children.
  • Two affected parents can have unaffected children (if both are heterozygous).
  • Transmitted from fathers to daughters and mothers to daughters.

Patterns of Inheritance: Autosomal Recessive

  • Parents are generally unaffected (carriers).
  • Approximately 25% of children are affected if both parents are carriers.
  • Two affected parents will always have an affected child.
  • Recessive traits often result from incestuous mating.

Patterns of Inheritance: Sex-linked Recessive (X-linked)

  • Children of an affected male will not be affected.
  • 100% of female offspring will be carriers of the disease.
  • An unaffected carrier female will have approximately 50% affected male offspring and approximately 50% carrier female offspring.

Patterns of Inheritance: Sex-linked Dominant (X-linked)

  • Affected male will always produce affected female offspring and unaffected male offspring.
  • Affected female (heterozygous) will produce approximately 50% affected male and 50% affected female offspring.
  • If the affected female is homozygous, 100% of offspring will be affected.

Pedigree Examples

  • Pedigree 1: Neurofibromatosis: Autosomal Dominant Inheritance
    • Includes symbols for elective termination of pregnancy, miscarriage
  • Pedigree 2: Cystic Fibrosis: Autosomal Recessive Inheritance
    • Includes symbols for affected male, carrier, and deceased.
  • Pedigree 3: Hemophilia: X-Linked Recessive Inheritance

Rules for Pedigree Problems

  • Rule #1: Every gamete carries exactly one allele for every gene. Each parent passes on 1 allele.
    • Exception: Sex-linked genes
      • If X-linked, the Y chromosome will not pass on an allele for that gene.
      • If Y-linked, the X chromosome will not pass on an allele for that gene.
  • Rule #2: Any individual with the recessive phenotype must be homozygous recessive. Assign two recessive alleles to each shaded symbol.
  • Rule #3: Assign one dominant allele to any unshaded individual. Determine if this person is homozygous dominant or heterozygous.
  • Rule #4: Any individual with a homozygous recessive offspring must have at least one recessive allele.
  • Rule #5: Any individual with a homozygous recessive parent must have at least one recessive allele.

Practice Problem

  • Determine the genotypes of individuals labeled a, b, c, and d:
    • Lung Disease (shaded symbol)
    • Normal (unshaded symbol)
    • a, b, c, d are individuals in the pedigree

Solution

  • The trait must be recessive because all of individual a’s children are normal.
    • a) aa
    • b) Aa (Some of the children are affected, so b must be a carrier)
    • c) Aa (Her mom is aa, so she has at least one a, but she is unaffected, so she is a carrier)
    • d) AA or Aa (Unaffected so at least one A, but we can’t determine the other allele)