Genetics & Inheritance
DNA, genes, alleles, Mendelian inheritance and genetic crosses
DNA (deoxyribonucleic acid) is the molecule of inheritance. It is a double helix made of nucleotides (base + sugar + phosphate). The four bases are: A (adenine), T (thymine), G (guanine), C (cytosine). A pairs with T; G pairs with C.
A gene is a section of DNA that codes for a protein. Humans have approximately 20,000 genes on 23 pairs of chromosomes (46 total). Alleles are different versions of a gene.
Dominant allele (capital letter) is expressed when present. Recessive allele (lowercase) is only expressed when two copies are present (homozygous recessive).
Genetic crosses using Punnett squares:
- Homozygous dominant (BB) × Homozygous recessive (bb) → all Bb (carriers, all show dominant trait)
- Heterozygous (Bb) × Heterozygous (Bb) → BB:Bb:Bb:bb = 3 dominant : 1 recessive
Sex determination: Females = XX, Males = XY. Father's sperm (X or Y) determines sex.
Genetic disorders:
- Sickle cell anaemia: recessive (ss). Common in populations from malaria-endemic areas including Pakistan.
- Down's syndrome: extra chromosome 21 (trisomy 21) — caused by non-disjunction during meiosis.
Key Points to Remember
- 1DNA: double helix, A-T and G-C base pairs
- 2Dominant expressed over recessive; recessive needs two copies
- 3Punnett square: Bb × Bb gives 3:1 ratio
- 4Father determines sex (X or Y sperm)
Pakistan Example
Thalassaemia — Pakistan's Genetic Crisis
Pakistan has one of the highest rates of thalassaemia (a blood disorder) in the world — approximately 9 million carriers. Thalassaemia major occurs when a child inherits the recessive allele (tt) from both carrier parents (Tt × Tt). This gives a 25% chance for each pregnancy. AKU-EB Biology Punnett square questions regularly use thalassaemia as the Pakistani context.
Quick Revision Infographic
Biology — Quick Revision
Genetics & Inheritance
Key Concepts
Formulas to Know
Punnett square: Bb × Bb gives 3:1 ratioThalassaemia — Pakistan's Genetic Crisis
Pakistan has one of the highest rates of thalassaemia (a blood disorder) in the world — approximately 9 million carriers. Thalassaemia major occurs when a child inherits the recessive allele (tt) from both carrier parents (Tt × Tt). This gives a 25% chance for each pregnancy. AKU-EB Biology Punnett square questions regularly use thalassaemia as the Pakistani context.