Preparation of Pedigree Chart in a Case of Hereditary Disorder (10 marks) Scenario A 6-year-old boy is diagnosed with β-thalassemia major. His parents are healthy. He has one healthy younger sister. The father's brother also has a child with thalassemia. Candidate Task Draw a three-generation pedigree. (3 marks) Use standard pedigree symbols. (2 marks) Identify the inheritance pattern. (3 marks) Mention recurrence risk for the next pregnancy. (2 marks)
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Pedigree Chart - β-Thalassemia Major (10-Mark Answer)
1. Three-Generation Pedigree Chart (3 marks)
The pedigree spans three generations (Generation I = grandparents, Generation II = parents + paternal uncle, Generation III = children).
STANDARD PEDIGREE SYMBOLS KEY
■ = Affected male (filled square)
□ = Unaffected male (open square)
○ = Unaffected female (open circle)
◑ = Carrier female (half-filled circle)
◐ = Carrier male (half-filled square, dot inside)
▲ = Proband arrow (↓)
══════════════════════════════════════════════════════════
GENERATION I (Paternal grandparents presumed carriers)
□(Gg) ── ○(Gg) □(Gg) ── ○(Gg)
[Pat. GF] [Pat. GM] [Mat. GF] [Mat. GM]
══════════════════════════════════════════════════════════
GENERATION II
□(Gg) ──────────────── ○(Gg) □(Gg) ── ○(?)
[Father, carrier] [Mother, carrier] [Pat. Uncle]
══════════════════════════════════════════════════════════
GENERATION III
■(gg) ○ ■(gg)
[Proband [Healthy [Uncle's affected
6-yr-old boy younger child]
↑ PROBAND] sister]
Properly formatted ASCII pedigree:
Generation I:
[Gg]□───○[Gg] [Gg]□───○[Gg]
| |
__________|___________________ |
| | |
Generation II:
[Gg]□ ──────────── [Gg]○ [Gg]□───○[?]
Father (carrier) Mother(carrier) Pat. Uncle
| |
_______|_______ ____|____
| | |
Generation III:
■ [gg] ○ [?] ■ [gg]
Proband(6yr) Healthy sister Uncle's child
↑
(Arrow = Proband)
2. Standard Pedigree Symbols (2 marks)
| Symbol | Meaning |
|---|---|
| □ (open square) | Unaffected male |
| ○ (open circle) | Unaffected female |
| ■ (filled square) | Affected male |
| ● (filled circle) | Affected female |
| ◑ / ◐ (half-filled) | Obligate carrier (heterozygous) |
| ─── (horizontal line) | Mating/union |
| ─── (vertical line) | Line of descent |
| ─── (horizontal sibship line) | Siblings |
| ↑ or arrow | Proband (index case) |
| Gg written below symbol | Genotype notation: G = normal allele, g = mutant allele |
Carriers (father and mother, Generation II) should be shown with half-filled symbols or a dot inside, as they are obligate carriers deduced from having an affected child.
3. Identification of Inheritance Pattern (3 marks)
Pattern: Autosomal Recessive
Evidence supporting this pattern:
| Feature | Observation in this family |
|---|---|
| Both parents unaffected | Father and mother are phenotypically healthy yet have an affected child - classic carrier x carrier mating |
| Autosomal (not X-linked) | Males and females can be affected; the affected child is a male but the pattern is not sex-limited |
| Consanguinity not required | Carriers are distributed in the general population at high frequency for thalassemia |
| Paternal uncle also has an affected child | Father and his brother are both carriers (both received the mutant allele from Generation I grandparents), and the uncle's partner is also a carrier - consistent with autosomal recessive transmission down the paternal line |
| No generation skipping in pattern | Skip-generation appearance (healthy Gen I → affected Gen III) is typical of recessive disorders |
Genotypes:
- Both parents (Gen II): Aa (heterozygous carriers; A = normal β-globin allele, a = mutant allele)
- Proband (Gen III): aa (homozygous affected)
- Healthy sister: AA or Aa (unaffected; probability 1/3 AA, 2/3 Aa)
- Paternal uncle: Aa (obligate carrier)
- Uncle's partner: Aa (obligate carrier, deduced from their affected child)
This is consistent with a mutation in the HBB gene (chromosome 11p15.4), the gene encoding the β-globin chain.
4. Recurrence Risk for the Next Pregnancy (2 marks)
Since both parents are confirmed carriers (Aa × Aa), the Punnett square gives:
A (normal) a (mutant)
A(normal) | AA (normal) | Aa (carrier) |
a(mutant) | Aa (carrier) | aa (affected)|
| Outcome | Probability |
|---|---|
| Normal (AA) | 25% |
| Carrier (Aa) - phenotypically healthy | 50% |
| Affected (aa) - β-thalassemia major | 25% |
For each pregnancy:
- 1 in 4 (25%) risk of β-thalassemia major (affected child)
- 2 in 4 (50%) chance of a carrier child (clinically normal but can pass the gene)
- 1 in 4 (25%) chance of a completely normal child
- 2 in 3 (67%) chance that any unaffected child is a carrier
This 25% risk applies independently to EVERY pregnancy - previous births do not influence the next.
Prenatal Diagnosis Options to Offer:
- Chorionic villus sampling (CVS) at 10-12 weeks
- Amniocentesis at 15-18 weeks
- Pre-implantation genetic testing (PGT) with IVF
Summary Table
| Item | Answer |
|---|---|
| Disorder | β-Thalassemia major |
| Gene | HBB (β-globin), chromosome 11 |
| Inheritance | Autosomal recessive |
| Parents' genotype | Both carriers (Aa × Aa) |
| Proband's genotype | Homozygous affected (aa) |
| Recurrence risk (next pregnancy) | 25% per pregnancy |
| Risk of next child being carrier | 50% |
Sources: Harrison's Principles of Internal Medicine 22E, p. 3834 (autosomal recessive inheritance, Punnett square, recurrence risks); Emery's Elements of Medical Genetics and Genomics (pedigree drawing, carrier notation); Pedigree inheritance diagram from Harrison's Fig. 479-13B above.
A couple planning marriage asks about non-consanguinity (Atulya Gotra) and its importance in Ayurveda. Candidate Task Explain the following: Meaning of Atulya Gotra (2 marks) Importance in preventing hereditary disorders (3 marks) Relevance in modern genetics (3 marks) Practical advice before marriage (2 marks)
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