Reproduction (B6.1)
Sexual vs asexual reproduction
| Feature | Sexual | Asexual |
|---|---|---|
| Parents | Two | One |
| Genetic variation | Yes — gametes fuse; meiosis creates variety | No — offspring are clones (mitosis) |
| Speed | Slower | Faster |
| Examples | Humans, most plants | Bacteria, some plants, fungi (by spores) |
Meiosis
Meiosis produces gametes (sex cells — sperm and eggs). Unlike mitosis, meiosis involves two divisions and produces four haploid cells that are genetically different.
Why is variation produced?
- Independent assortment of homologous chromosomes (which of each pair goes to which cell).
- Crossing over — exchange of DNA segments between homologous chromosomes.
- Random fertilisation.
Result: 4 genetically different haploid cells (23 chromosomes each in humans).
DNA and the genome
Gene: a section of DNA coding for a specific protein (or sequence of amino acids).
Allele: alternative versions of a gene (same locus, different sequence).
Genome: the entire DNA of an organism.
Advantages of knowing the human genome: identify genes causing inherited diseases, develop personalised medicines, understand evolutionary relationships.
Genetic terminology
- Homozygous: two identical alleles (e.g. BB or bb)
- Heterozygous: two different alleles (e.g. Bb)
- Dominant: allele always expressed (capital letter, e.g. B)
- Recessive: only expressed when homozygous (lower-case, e.g. b)
- Genotype: the alleles an organism carries (e.g. Bb)
- Phenotype: the observable characteristics (e.g. brown eyes)
Genetic crosses — Punnett squares
Example: cystic fibrosis (autosomal recessive, F = normal, f = carrier)
Cross: Ff × Ff:
F f
F | FF | Ff |
f | Ff | ff |
Ratios: 1 FF : 2 Ff : 1 ff → 25% chance of being affected (ff), 50% carrier, 25% unaffected non-carrier.
Inherited disorders
- Cystic fibrosis: recessive; thick sticky mucus (lungs, gut, pancreas); CFTR gene mutation.
- Polydactyly: dominant; extra fingers/toes; only ONE parent needs to carry allele.
- Sickle cell disease: recessive; abnormal haemoglobin; red blood cells sickle-shaped; protective against malaria.
Sex determination
Humans have 23 pairs of chromosomes. The 23rd pair determines sex:
- Female: XX
- Male: XY (Y chromosome carries SRY gene → testes development)
All eggs carry X; sperm carry either X or Y → 50:50 sex ratio at fertilisation.
Common exam errors
- Confusing meiosis (4 haploid, different) with mitosis (2 diploid, identical).
- Forgetting that a dominant disorder (polydactyly) only needs ONE copy of the allele.
- Missing the 50% carrier proportion in recessive disorder crosses.
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