GCSE Biology - How to Solve: Pedigree Analysis (Family Trees, Sex-Linked Inheritance) – Complete Guide

How to Solve: Pedigree Analysis (Family Trees, Sex-Linked Inheritance) – Complete Guide

Introduction "Mastering pedigree analysis unlocks 6–8 marks in your GCSE/A-Level Biology exam—enough to boost your grade by a full level. It’s how doctors predict genetic disorders like haemophilia or colour blindness before a baby is born."

WHAT YOU NEED TO KNOW FIRST

1. Basic inheritance patterns – Dominant vs. recessive alleles, genotypes vs. phenotypes.
2. Sex determination – XX (female) vs. XY (male) chromosomes.
3. Punnett squares – How to predict offspring genotypes from parents.

KEY TERMS & FORMULAS

Key Terms

Key Rules (MEMORISE THIS)

1. Autosomal dominant – Affected individuals appear in every generation. Unaffected parents cannot have affected children.
2. Autosomal recessive – Can skip generations. Two unaffected parents can have affected children (if both are carriers).
3. X-linked recessive – More males affected than females. Affected fathers cannot pass it to sons (they give Y chromosome).
4. X-linked dominant – Affected fathers pass it to all daughters (but no sons). Affected mothers pass it to 50% of children.

STEP-BY-STEP METHOD

Step 1: Label the Generations & Individuals

• Generations = Roman numerals (I, II, III…).
• Individuals = Arabic numbers (1, 2, 3…) left to right.

Step 2: Identify the Proband (Arrow)

• The first affected person in the pedigree.

Step 3: Check for Sex Bias

• More males affected? → Likely X-linked recessive.
• Equal males/females? → Likely autosomal.

Step 4: Look for Skipped Generations

• Trait skips a generation? → Likely recessive.
• Trait in every generation? → Likely dominant.

Step 5: Check Parent-Offspring Patterns

• Unaffected parents → affected child? → Recessive (both parents must be carriers).
• Affected parent → unaffected child? → Dominant (parent must be heterozygous).

Step 6: Assign Genotypes

• Autosomal recessive (aa) – Affected = aa, carriers = Aa, unaffected = AA.
• Autosomal dominant (A-) – Affected = Aa or AA, unaffected = aa.
• X-linked recessive (XᵃY or XᵃXᵃ) – Affected males = XᵃY, carrier females = XᴬXᵃ.

Step 7: Verify with Punnett Squares (if needed)

• Cross parents to check if offspring genotypes match the pedigree.

WORKED EXAMPLES

Example 1 – Basic (Autosomal Recessive)

Pedigree: - Generation I: Unaffected parents (I-1, I-2) → 2 affected children (II-1, II-2). - Generation II: II-1 (affected) marries unaffected (II-3) → 1 affected child (III-1).

Step-by-Step:
1. Label generations & individuals (done above).
2. Proband = II-1 (arrow).
3. Equal males/females affected → Autosomal.
4. Trait skips generation I → Recessive.
5. Unaffected parents (I-1, I-2) → affected children (II-1, II-2) → Both parents must be carriers (Aa).
6. Genotypes: - I-1 = Aa, I-2 = Aa - II-1 = aa (affected), II-3 = AA (unaffected) - III-1 = aa (affected)
7. Verify: - I-1 (Aa) × I-2 (Aa) → 25% aa (matches II-1, II-2). - II-1 (aa) × II-3 (AA) → 100% Aa (no affected children, but III-1 is aa → contradiction!). - Correction: II-3 must be Aa (carrier) for III-1 to be aa.

What we did and why: - Used skipped generations and unaffected parents → affected children to confirm autosomal recessive. - Fixed a mistake by realising II-3 must be a carrier (Aa).

Example 2 – Medium (X-Linked Recessive)

Pedigree: - Generation I: Unaffected father (I-1), carrier mother (I-2) → 1 affected son (II-1), 1 carrier daughter (II-2). - Generation II: II-1 (affected) marries unaffected (II-3) → 2 unaffected sons (III-1, III-2).

Step-by-Step:
1. Label generations & individuals (done above).
2. Proband = II-1 (arrow).
3. More males affected → X-linked recessive.
4. Trait not in every generation → Recessive.
5. Genotypes: - I-1 = XᴬY (unaffected), I-2 = XᴬXᵃ (carrier). - II-1 = XᵃY (affected), II-2 = XᴬXᵃ (carrier), II-3 = XᴬY (unaffected). - III-1, III-2 = XᴬY (unaffected).
6. Verify: - I-1 (XᴬY) × I-2 (XᴬXᵃ) → 25% XᵃY (matches II-1). - II-1 (XᵃY) × II-3 (XᴬY) → No affected sons (matches III-1, III-2).

What we did and why: - Used male bias and father → son transmission to confirm X-linked recessive. - Confirmed affected fathers cannot pass X-linked traits to sons (they give Y chromosome).

Example 3 – Exam-Style (Disguised Autosomal Dominant)

Question: "A pedigree shows a trait appearing in every generation. Two affected parents (II-3 and II-4) have an unaffected child (III-2). What is the inheritance pattern?"

Step-by-Step:
1. Trait in every generation → Dominant.
2. Affected parents → unaffected child → Dominant (heterozygous parents).
3. Equal males/females affected → Autosomal dominant.
4. Genotypes: - II-3 = Aa, II-4 = Aa (both affected). - III-2 = aa (unaffected).
5. Verify: - Aa × Aa → 25% aa (matches III-2).

What we did and why: - Used dominant pattern clues (every generation, affected parents → unaffected child). - Confirmed autosomal (no sex bias).

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is how you ace pedigree analysis in under 60 seconds:
1. Label generations (I, II, III…) and individuals (1, 2, 3…).
2. Find the proband (arrow)—this is your starting point.
3. Check for sex bias: - More males? → X-linked recessive. - Equal males/females? → Autosomal.
4. Look for skipped generations: - Skips? → Recessive. - Every generation? → Dominant.
5. Check parent-offspring patterns: - Unaffected parents → affected child? → Recessive (both parents carriers). - Affected parent → unaffected child? → Dominant (parent is Aa).
6. Assign genotypes: - Autosomal recessive: aa = affected, Aa = carrier. - X-linked recessive: XᵃY = affected male, XᴬXᵃ = carrier female.
7. Verify with Punnett squares—if it doesn’t match, you made a mistake!

Remember: - X-linked recessive: Affected fathers never pass to sons. - Autosomal dominant: Affected parents can have unaffected children (if heterozygous). - Carriers are key—always check if parents could be Aa!

Now go crush that exam!"