Chromosome Numbers
| Condition | Total Chromosomes |
|---|---|
| Normal human (diploid) | 46 (44 autosomes + 2 sex chromosomes) |
| Down syndrome | 47 (+21 = extra autosome) |
| Turner syndrome | 45 (XO = missing sex chromosome) |
| Klinefelter syndrome | 47 (XXY = extra sex chromosome) |
| Super Female | 47 (XXX = extra sex chromosome) |
| Normal egg/sperm (haploid) | 23 |
Gene Locations (Must Memorise for NEET)
| Disorder | Gene | Chromosome |
|---|---|---|
| Sickle cell anaemia | HBB (beta-globin) | 11 |
| Beta-thalassemia | HBB (beta-globin) | 11 |
| Alpha-thalassemia | HBA1/HBA2 (alpha-globin) | 16 |
| Phenylketonuria (PKU) | PAH (phenylalanine hydroxylase) | 12 |
| Haemophilia A | F8 (clotting factor VIII) | X chromosome |
| Colour blindness | OPN1LW / OPN1MW (photopigments) | X chromosome |
Key Genetic Ratios (Standard Crosses)
| Cross | Ratio | Application |
|---|---|---|
| Aa × Aa | 3:1 (affected:unaffected) | Autosomal recessive parents |
| Aa × Aa | 1:2:1 (AA:Aa:aa) | Autosomal recessive genotype ratio |
| Aa × aa | 1:1 (carrier:affected) | Test cross / carrier × affected |
| × Y | 1:1 (carrier ♀ or normal ♂ : affected ♂) | X-linked carrier cross |
| 2/3 probability | Unaffected sibling is carrier in autosomal recessive | Conditional probability |
Mnemonic: Barr Body Rule
Number of Barr bodies = Total X chromosomes − 1
- Normal female (XX) → 1 Barr body
- Turner (XO) → 0 Barr bodies
- Klinefelter (XXY) → 1 Barr body
- Super Female (XXX) → 2 Barr bodies
- Normal male (XY) → 0 Barr bodies