Cambridge A-Level Biology – Passage of Information from Parents to Offspring
Passage of Information from Parents to Offspring
Objective
Explain why a reduction division (meiosis) is required in the production of gametes.
Key Concepts
Organisms are either diploid (\$2n\$) or haploid (\$n\$).
Gametes must be haploid so that fertilisation restores the species‑specific chromosome number.
Meiosis reduces the chromosome number by half while maintaining genetic diversity.
Why a Reduction Division is Necessary
Restoration of Chromosome Number at Fertilisation
When a sperm (\$n\$) fuses with an egg (\$n\$), the resulting zygote has \$2n\$ chromosomes, the normal complement for that species.
Avoidance of Chromosome Doubling in Successive Generations
If gametes were produced by mitosis (retaining \$2n\$), each generation would double the chromosome number: \$2n \rightarrow 4n \rightarrow 8n\$, leading to inviability.
Generation of Genetic \cdot ariation
Meiosis introduces variation through:
Independent assortment of homologous chromosomes.
Cross‑over (recombination) between non‑sister chromatids.
Random fertilisation of gametes.
Chromosome Numbers in Humans (Example)
The human diploid number is \$2n = 46\$ (23 pairs). Gametes contain \$n = 23\$ chromosomes.
Comparison of Mitosis and Meiosis
Feature
Mitosis
Meiosis
Purpose
Growth, repair, asexual reproduction
Production of gametes (sexual reproduction)
Number of divisions
One
Two (Meiosis I and Meiosis II)
Resulting cells
Two genetically identical diploid cells
Four genetically distinct haploid cells
Chromosome number in daughter cells
\$2n\$ (unchanged)
\$n\$ (reduced)
Cross‑over
Absent
Occurs in Prophase I (recombination)
Independent assortment
None (sister chromatids separate)
Homologous chromosomes separate in Meiosis I
Stages of Meiosis – Where Reduction Occurs
Meiosis I – Reductional Division
Prophase I – Synapsis and crossing‑over.
Metaphase I – Homologous pairs line up at the equatorial plate.
Anaphase I – Homologous chromosomes (each still consisting of two sister chromatids) are pulled to opposite poles, halving the chromosome number.
Telophase I – Two haploid cells are formed.
Meiosis II – Equational Division
Similar to mitosis; sister chromatids separate.
Result: Four haploid gametes, each with a unique set of chromosomes.
Consequences of Skipping Reduction Division
If gametes were produced without reduction:
Fertilisation would produce a zygote with \$4n\$ chromosomes.
Successive generations would experience exponential chromosome number increase, leading to genetic imbalance and developmental failure.
Loss of the mechanisms that generate genetic diversity, reducing adaptability.
Mathematical Representation of Chromosome Reduction
\$\text{After Meiosis I: } \frac{2n}{2} = n\$
Thus each gamete receives one set of chromosomes, ensuring the diploid number is restored at fertilisation.
Suggested diagram: Schematic of Meiosis I showing homologous chromosome pairing, crossing‑over, and segregation to produce haploid cells.
Summary
Reduction division during meiosis is essential to:
Maintain a constant species‑specific chromosome number across generations.
Prevent the detrimental accumulation of chromosomes.
Introduce genetic variation that underpins evolution and adaptation.