Cell division and inheritance
Mitosis
Mitosis is cell division for growth, repair and asexual reproduction. It produces two genetically identical daughter cells, each with the same chromosome number as the parent cell (diploid → diploid; 2n → 2n).
Stages of mitosis (CCEA requires awareness of four stages)
- Prophase: chromosomes condense and become visible; nuclear membrane breaks down.
- Metaphase: chromosomes line up at the cell equator; spindle fibres attach to centromeres.
- Anaphase: spindle fibres contract; chromatids pulled to opposite poles.
- Telophase: nuclear membranes reform; cytoplasm divides (cytokinesis) → two identical cells.
Before mitosis: DNA replication occurs (each chromosome is copied → two identical chromatids joined at the centromere).
Meiosis
Meiosis is cell division for sexual reproduction. It occurs in the gonads (testes/ovaries in animals). It produces four genetically different daughter cells (gametes), each with half the chromosome number (diploid → haploid; 2n → n).
In humans: diploid = 46 chromosomes; haploid gametes = 23 chromosomes.
Why meiosis produces genetic variation:
- Crossing over (chiasma formation): homologous chromosomes exchange segments during prophase I → new combinations of alleles.
- Independent assortment: homologous pairs line up randomly at metaphase I → random combination of chromosomes in gametes.
- Random fertilisation: any gamete can fertilise any other gamete.
Mitosis vs Meiosis — key differences
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Genetic content | Identical to parent | Different from parent |
| Chromosome number | Same (2n → 2n) | Halved (2n → n) |
| Purpose | Growth, repair, asexual reproduction | Gamete production (sexual reproduction) |
Mendelian genetics — dihybrid crosses (extension)
A dihybrid cross involves two genes on different chromosomes. Because the genes assort independently (Mendel's Second Law), the expected ratio in an F2 cross from two heterozygous parents is 9:3:3:1.
Example: seed colour (Y=yellow, y=green) and seed shape (R=round, r=wrinkled). Cross YyRr × YyRr → 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled.
Codominance: both alleles are expressed in the heterozygote. Example: ABO blood groups — I^A and I^B are codominant → blood group AB has both A and B antigens.
Sex-linked inheritance: genes on the X chromosome. Males (XY) have only one X, so a single recessive allele on X is expressed (no second allele to mask it). Example: red-green colour blindness. Alleles: X^B (normal), X^b (colour blind). Carrier female: X^B X^b (unaffected but carries allele). Affected male: X^b Y.
⚠Common mistakes
- Confusing mitosis and meiosis in terms of output — mitosis = 2 identical cells; meiosis = 4 different haploid cells.
- Forgetting that DNA replication must occur before either mitosis or meiosis.
- In sex-linked crosses, only males are hemizygous — the Y carries no allele for most X-linked genes.
- Codominance ≠ incomplete dominance — in codominance, BOTH phenotypes are shown; in incomplete dominance, a blend occurs.
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