Pedigree Analysis, Mendelian Disorders & Chromosomal Disorders – NEET Quick Guide

🔍 1. Pedigree Analysis: Reading Family Trees

Symbol Meaning Tip

⬜ / ⬛ Unaffected / Affected male Squares = **“M”**en

⚪ / ⚫ Unaffected / Affected female Circles = **“F”**emales

◇ Unknown sex 

⃝–⃝ Consanguineous marriage (double line) Look for recessive traits

Arrow (→) Proband / index case First diagnosed individual

How to identify inheritance pattern:

1. Autosomal Dominant

• Appears in every generation

• Both sexes equally affected

• Affected × unaffected → ≈ 50 % affected offspring

Mnemonic: “AD = Always Displayed”

2. Autosomal Recessive

• Skips generations, often after consanguinity

• Both sexes equally affected

• Unaffected parents can have affected child

Mnemonic: “AR = Accidentally Reveals”

3. X-linked Recessive

• Mostly males; no male-to-male transmission

• Affected mother → all sons affected

Mnemonic: “XR = Xtra Risk for guys”

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🧬 2. Common Mendelian (Single-Gene) Disorders

Disorder Inheritance Defect / Gene Key Symptoms / NEET Facts

Hemophilia A X-linked recessive Factor VIII Prolonged bleeding; “Royal Disease”

Color Blindness X-linked recessive Opsin gene Red-green discrimination loss

Sickle-Cell Anemia Autosomal recessive β-globin (Glu→Val) HbS, sickled RBCs; heterozygote ↑ malaria resistance

Phenylketonuria (PKU) Autosomal recessive PAH enzyme Musty odor, mental retardation; avoid phenylalanine

Thalassemia β Autosomal recessive β-globin deletions Microcytic anemia, target cells

🧠 Memory Hook: “Help Colour Sick Patients Thrive”

H = Hemophilia, C = Color blindness, S = Sickle cell, P = PKU, T = Thalassemia.

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🧬 3. Major Chromosomal Disorders (Numerical Anomalies)

Syndrome Karyotype Cause Hallmarks / Board Highlights

Down’s (Trisomy 21) 47, +21 Non-disjunction Flat face, simian crease, mental retardation; ↑ risk with maternal age

Klinefelter’s 47, XXY Extra X in male Tall, sterile male, gynecomastia, Barr body present

Turner’s 45, X0 Missing X in female Short, webbed neck, sterile female, no Barr body

Edward’s (Trisomy 18) 47, +18 Non-disjunction Clenched fists, rocker-bottom feet

Patau’s (Trisomy 13) 47, +13 Non-disjunction Cleft lip/palate, polydactyly

🧠 Mnemonic (Trisomies): “Drinking Age = 21, Election = 18, Puberty = 13.”

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✅ Quick Decision Tree for NEET MCQs

1. Only males affected, skips generations → X-linked recessive

2. Affected child from unaffected parents → Autosomal recessive

3. Every generation, both sexes → Autosomal dominant

4. Multiple traits travel together → Consider linkage or chromosomal disorder.