Pedigree ICA T1

Bismillah

Ultimate Pedigree Cheat Sheet: Year 10 Biology (AIC)

This sheet incorporates information from your specific task description and example Punnett squares.

I. The Basics: Reading the Chart

• Symbols:
  • □ = Male
  • ○ = Female
  • ◇ = Sex unspecified
  • Shaded symbol (■ or ●) = Affected individual (Expresses the trait/disease)
  • Unshaded symbol (□ or ○) = Unaffected individual
  • Half-shaded or Dot symbol (◐/◑ or ⊡/⊙) = Carrier (Usually heterozygous for a recessive trait; doesn’t show trait but can pass it on)
• Lines:
  • Horizontal line (—) = Mating/Marriage
  • Double horizontal line (═) = Consanguineous mating (mating between relatives)
  • Vertical line (|) = Offspring line
  • Siblings connected by horizontal sibship line.
• Generations: Roman numerals (I, II, III…).
• Individuals: Arabic numerals (1, 2, 3…). (e.g., II-3).
• Proband: Arrow (→) points to the first affected individual studied.

II. Key Concepts: Alleles & Genotypes

• Alleles: Different versions of a gene (e.g., A for dominant, a for recessive).
• Genotype: The allele combination (e.g., AA, Aa, aa).
• Phenotype: The observable trait (e.g., affected/unaffected).
• Homozygous: Two identical alleles (AA or aa).
• Heterozygous: Two different alleles (Aa) - often called a “carrier” for recessive traits.

III. The Four Main Inheritance Patterns & How to Spot Them

A. Autosomal Recessive (AR)

• Genotype: Affected = aa; Unaffected = AA or Aa (carrier).
• Key Pedigree Clues:
  • ✅ Can skip generations.
  • ✅ Unaffected parents CAN have affected offspring (Classic sign: Aa x Aa → aa). This is a very strong indicator.
  • ✅ Affects males and females roughly equally.
  • ✅ If both parents are affected (aa x aa), ALL offspring MUST be affected (aa).
  • ✅ More likely with consanguineous mating.
• Example Punnett Square (Carrier Parents - Aa x Aa):

      A   a
    +---+---+
  A | AA| Aa|  -> 25% AA (Unaffected, non-carrier)
    +---+---+
  a | Aa| aa|  -> 50% Aa (Unaffected, carrier)
    +---+---+      -> 25% aa (Affected)

• Examples: Cystic Fibrosis, Sickle Cell Anemia, PKU.

B. Autosomal Dominant (AD)

• Genotype: Affected = AA or Aa; Unaffected = aa.
• Key Pedigree Clues:
  • ❌ Usually does NOT skip generations. (Trait appears in every generation).
  • ✅ Affected individuals MUST have at least one affected parent (unless a new mutation).
  • ✅ Affects males and females roughly equally.
  • ✅ Male-to-male transmission IS possible.
  • ✅ Unaffected individuals (aa) cannot transmit the trait.
• Example Punnett Square (Heterozygous Affected x Unaffected - Aa x aa):

      A   a
    +---+---+
  a | Aa| aa|  -> 50% Aa (Affected)
    +---+---+
  a | Aa| aa|  -> 50% aa (Unaffected)
    +---+---+

• Examples: Huntington’s Disease, Achondroplasia.
• Why Cystic Fibrosis isn’t Dominant: CF requires two recessive alleles (aa). People with Aa are healthy carriers. Dominant traits show with only one dominant allele (Aa).

C. X-linked Recessive (XR)

• Genotype Notation: Xᴬ = dominant, Xª = recessive allele on X.
  • Affected Male = XªY
  • Affected Female = XªXª (rare)
  • Carrier Female = XᴬXª
  • Unaffected Male = XᴬY
  • Unaffected Female = XᴬXᴬ
• Key Pedigree Clues:
  • ✅ Affects males MUCH more often than females.
  • ✅ Can skip generations (passed via carrier females).
  • ❌ NO male-to-male transmission (Fathers pass Y to sons).
  • ✅ An affected father (XªY) passes the allele to ALL daughters (making them at least carriers) and NO sons.
  • ✅ Affected mothers (XªXª) have ALL affected sons.
• Example Punnett Square (Carrier Mother x Unaffected Father - XᴬXª x XᴬY):

       Xᴬ    Xª
    +-----+-----+
  Xᴬ| XᴬXᴬ| XᴬXª| -> Daughters: 50% Unaffected (XᴬXᴬ), 50% Carrier (XᴬXª)
    +-----+-----+
  Y | XᴬY | XªY | -> Sons: 50% Unaffected (XᴬY), 50% Affected (XªY)
    +-----+-----+

• Examples: Hemophilia, Red-Green Color Blindness.

D. X-linked Dominant (XD) Less common

• Genotype Notation: Xᴬ= dominant/affected allele on X.
  • Affected Male = XᴬY
  • Affected Female = XᴬXᴬ or XᴬXª
  • Unaffected Male = XªY
  • Unaffected Female = XªXª
• Key Pedigree Clues:
  • ❌ Usually does NOT skip generations.
  • ✅ Affects both sexes, often females more frequently.
  • ❌ NO male-to-male transmission.
  • ✅ Affected father (XᴬY) transmits the trait to ALL daughters and NO sons. (THE MOST IMPORTANT CLUE FOR XD).
  • ✅ Affected heterozygous mother (XᴬXª) transmits to 50% of daughters AND 50% of sons.
• Example Punnett Square (Affected Father x Unaffected Mother - XᴬY x XªXª):

       Xᴬ    Y
    +-----+-----+
  Xª| XᴬXª| XªY | -> Daughters: 100% Affected (Carrier XᴬXª)
    +-----+-----+
  Xª| XᴬXª| XªY | -> Sons: 100% Unaffected (XªY)
    +-----+-----+

• Examples: Hypophosphatemic Rickets.

IV. Strategy for Analyzing a Pedigree:

1. Dominant or Recessive?
   • Look for affected individuals having at least one affected parent (suggests DOMINANT).
   • Look for unaffected parents having affected offspring (confirms RECESSIVE).
   • Does it skip generations? (Suggests RECESSIVE).
2. Autosomal or X-linked?
   • Are males and females affected about equally? (Suggests AUTOSOMAL).
   • Are significantly more males affected? (Suggests X-LINKED RECESSIVE).
   • Is there NO male-to-male transmission? (Suggests X-LINKED).
   • Crucial Check: If you suspect XD, check if ALL daughters of affected fathers are affected. If yes, strong evidence for XD. If NO, rule out XD.
3. Test Your Hypothesis: Assign genotypes based on your suspected pattern. Start with individuals whose genotypes you know (e.g., aa if recessive; aa if dominant & unaffected; XªY males if XR). Check for consistency.

V. Quick Summary Table (Integrates Punnett Logic)

Feature	Autosomal Recessive (AR)	Autosomal Dominant (AD)	X-linked Recessive (XR)	X-linked Dominant (XD)
Skips Generations?	Often	Rarely	Often	Rarely
Unaffected Parents -> Affected Child?	YES	No	Yes (if Mother carrier)	No
Sex Bias?	No	No	Males >> Females	Females ≥ Males
Male-to-Male Transmission?	Yes	Yes	NO	NO
Affected Father has:	Depends on Mother	50% affected kids (if Aa)	All daughters carriers, NO sons affected	ALL daughters affected, NO sons affected
Carrier Mother (het) has:	25% affected kids (w/ Aa dad)	N/A	50% affected sons	50% affected sons & daughters

VI. Final Tips for the Validation Test

• Practice: Work through example pedigrees.
• Process of Elimination: Rule out impossible patterns.
• Focus on the Definites: Unaffected parents having affected kids = RECESSIVE. Affected father -> ALL daughters affected = X-LINKED DOMINANT. No male-to-male transmission = X-LINKED.
• Read Carefully: Note shaded symbols, generations, relationships.
• Trust the Patterns: Stick with clear evidence unless contradicted.