Reproduction & Genetics
Sexual/asexual reproduction, DNA, inheritance and genetic crosses
Asexual reproduction: One parent, no gametes, offspring are clones (genetically identical). Examples: binary fission (bacteria), budding (yeast), runners (strawberry).
Sexual reproduction: Two parents produce **gametes** (sex cells) that fuse at **fertilisation**. Offspring are genetically varied.
DNA (deoxyribonucleic acid) is a double helix carrying genetic information. A gene = section of DNA coding for a protein. Chromosomes = coiled DNA + protein. Humans have 46 chromosomes (23 pairs).
Mitosis: One cell → 2 identical diploid cells (growth/repair).
Meiosis: One cell → 4 different haploid gametes (produces sex cells).
Inheritance: **Alleles** are different versions of a gene. **Dominant** alleles (capital letter) are expressed over **recessive** (lowercase). **Homozygous** = same alleles (BB or bb). **Heterozygous** = different (Bb).
Genetic crosses: Use Punnett squares. Bb × Bb → BB:Bb:Bb:bb = 3 dominant : 1 recessive.
Sex determination: XX = female, XY = male. Father determines sex (X or Y sperm).
Key Points to Remember
- 1Asexual = clones, sexual = genetic variation
- 2Mitosis: 2 identical cells; Meiosis: 4 different gametes
- 3Dominant alleles expressed over recessive
- 4Punnett square for genetic crosses
Pakistan Example
Eye Colour in Pakistani Families — Mendelian Genetics
Brown eyes (B) are dominant over blue/green (b) in Pakistani families. Two brown-eyed parents who are both Bb can have a green-eyed child (bb) — 25% chance. This explains why some children look different from both parents. The father's sperm determines the baby's sex (X or Y chromosome).