D3.1 Reproduction

Meiosis has two essential roles in sexual reproduction: it must reduce the chromosome number from diploid (2n) to haploid (n) so that, upon fertilisation, the diploid number is restored; and it must generate genetic variation so offspring are not identical.

Crossing over during prophase I exchanges segments between homologous chromosomes, creating new allele combinations. Independent assortment during metaphase I randomly distributes maternal and paternal chromosomes into gametes, giving $2^{23}$ combinations in humans from one parent alone.

The option stating that meiosis reduces the chromosome number by half and generates genetic diversity through recombination and independent assortment correctly captures both essential roles.

The question asks for the correct description of meiosis's role in sexual reproduction — both the outcome of the division and any contribution to genetic variation.

Meiosis produces four haploid cells, not two diploid ones, and these cells are genetically distinct, not identical. This describes mitosis, not meiosis.

Meiosis does not double the chromosome number in the zygote — that would be lethal. Meiosis halves the chromosome number precisely so fertilisation restores the diploid number.

Meiosis occurs before fertilisation to produce gametes. Mitosis in the zygote is initiated after fertilisation and is a separate process driven by different signals.

The remaining option — that meiosis reduces the chromosome number by half and generates genetic diversity — is correct and accounts for both functions.

During the follicular phase, oestradiol rises as the dominant follicle matures. At low to moderate levels, oestradiol exerts negative feedback on LH. However, when oestradiol reaches a threshold concentration, the feedback switches.

At high concentrations, oestradiol acts on the anterior pituitary (and hypothalamus) to amplify LH release rather than suppress it — this is positive feedback. The result is a sharp LH surge around Day 14.

The LH surge causes the dominant follicle to rupture, releasing the mature egg into the fallopian tube — this event is ovulation.

The correct answer is that high oestradiol triggers a positive feedback surge in LH, leading to ovulation.

The question asks what happens as a consequence of the sharp oestradiol rise just before Day 14 — specifically what hormonal event follows and what type of feedback is involved.

The corpus luteum forms only after ovulation from the ruptured follicle. Oestradiol does not directly cause its formation or progesterone release at this stage.

While oestradiol does suppress LH at low concentrations, the sharp rise before Day 14 causes the opposite effect — positive feedback. This option incorrectly applies negative feedback to the mid-cycle oestradiol surge.

Rising oestradiol actually suppresses FSH, which causes the less dominant follicles to regress. It does not stimulate FSH to sustain multiple follicles beyond Day 14.

The correct answer is that the high oestradiol surge triggers positive feedback, causing an LH surge that results in ovulation around Day 14.