High-Frequency NEET Topics in GEN-02
1. X-linked recessive cross calculations (tested almost every year)
- Most common format: carrier female × normal male → find P(affected son), P(carrier daughter), P(haemophilic first child)
- Key trap: P(haemophilic child) = 1/4, NOT 1/2. The 1/2 is only true if asking "among sons."
- Secondary trap: affected father cannot pass X-linked trait to sons.
2. Chromosomal disorder identification (yearly)
- Present karyotype (47, XXY / 45, XO / 47, +21) → identify syndrome
- OR present clinical features → identify karyotype
- Most tested confusion: XXY = male (not female); XO = female (not male)
3. Sex determination systems (high frequency)
- "In which organism is the female heterogametic?" → Birds/butterflies (ZW-ZZ)
- "Which sex determines offspring sex in birds?" → Female (mother, ZW)
4. Sickle cell molecular details (medium-high frequency)
- Which codon change? GAG→GUG
- Which amino acids? Glutamic acid → Valine
- Which position? 6th position of beta-globin
- Which gene/chromosome? HBB, chromosome 11
5. Pedigree inheritance pattern identification (yearly)
- Provide a pedigree → identify autosomal dominant / recessive / X-linked recessive / X-linked dominant
- Key clue: father-to-son transmission rules out X-linked; every-generation appearance rules out recessive
Marking Scheme Insights
- Each GEN-02 question is 4 marks (NEET 2024 format)
- Typically 3–4 questions per year from this chapter
- Assertion-reason format increasingly common for chromosomal disorders and X-linked inheritance
- Multi-step probability (cross + sex ratio + conditional probability) questions are high scorers but also most error-prone