Reproductive System & Development

Question 1 (12)

(a) Lineages (6) — 3 each

Spermatogenesis: Spermatogonium ($2n$, diploid) → primary spermatocyte ($2n$, after DNA replication = sister chromatids) → [Meiosis I] → two secondary spermatocytes ($n$, but still 2 chromatids each) → [Meiosis II] → four spermatids ($n$, single chromatid) → spermiogenesis → four spermatozoa ($n$). Marks: correct sequence 1; correct ploidy labels 1; spermatid→sperm maturation noted 1.

Oogenesis: Oogonium ($2n$) → primary oocyte ($2n$, replicated, arrested prophase I) → [Meiosis I, unequal] → secondary oocyte ($n$, 2 chromatids) + first polar body → [Meiosis II, completes only if fertilised] → ovum ($n$) + second polar body. Marks: sequence 1; ploidy 1; unequal division / polar bodies 1.

(b) Numbers & mechanism (3)

• Spermatogenesis: 4 functional gametes per primary spermatocyte (1).
• Oogenesis: 1 functional gamete per primary oocyte (1).
• Why: meiotic divisions in oogenesis are cytoplasmically unequal — the egg retains almost all cytoplasm/nutrients while the discarded polar bodies get minimal cytoplasm, conserving reserves for the embryo; spermatogenesis divides cytoplasm equally (1).

(c) Timing / arrest (3)

• Spermatogenesis begins at puberty and continues continuously throughout adult life (1).
• Oogenesis begins in the fetus; all primary oocytes are arrested in prophase I before birth (1st arrest) (1).
• At each cycle one resumes to become a secondary oocyte arrested in metaphase II (2nd arrest); MII completes only on fertilisation (1).

Question 2 (12)

(a) Male HPG axis (6) Hypothalamus → GnRH → anterior pituitary → FSH and LH.

• LH → Leydig (interstitial) cells → testosterone (1+1).
• FSH → Sertoli cells → support spermatogenesis; Sertoli secrete inhibin (1+1).
• Feedback: testosterone inhibits hypothalamus/pituitary (−); inhibin selectively inhibits FSH (−) (1 for arrows, 1 for correct diagram layout).

(b) 28-day timeline (4) — 1 mark per correct phase

• Days 1–5 Menstruation (follicular start): FSH rising; low oestrogen/progesterone; endometrium shed.
• Days 6–13 Follicular: FSH then rising oestrogen; follicle matures; endometrium rebuilds.
• Day ~14 Ovulation: LH surge; oestrogen peak triggers it; oocyte released.
• Days 15–28 Luteal: LH→corpus luteum→progesterone (+oestrogen); endometrium maintained/secretory. If no implantation, corpus luteum degenerates → hormones fall → menstruation.

(c) Positive feedback (2)

• Around ovulation, high sustained oestrogen switches from negative to positive feedback, driving the LH surge that triggers ovulation (1).
• If the switch fails: no LH surge → no ovulation (anovulation) → infertility; cycle may become irregular (1).

Question 3 (12)

(a) Fertilisation (6)

1. Sperm binds zona pellucida; acrosome reaction — acrosomal enzymes (hyaluronidase, acrosin) digest a path through zona/corona (2).
2. Sperm & egg membranes fuse; sperm nucleus enters (1).
3. Cortical reaction — cortical granules release contents into perivitelline space, hardening the zona (zona reaction) (1).
4. This is the slow block to polyspermy; a fast membrane depolarisation gives the fast block (1).
5. Egg completes meiosis II; male & female pronuclei fuse → diploid zygote (1).

(b) Stages (4) — 1 each + 1 for correct contrast

• Cleavage: rapid mitotic divisions with no growth — cell number ↑, cell size ↓, total mass ~constant.
• Blastula/blastocyst: hollow ball with fluid cavity (blastocoel); mammal = inner cell mass + trophoblast.
• Gastrula: cell movement/invagination reorganises into layered embryo — establishes germ layers.
• Contrast mark: cleavage = number↑/size↓; gastrulation = movement/differentiation not just division.

(c) Germ layers (2) — ½ each

• Ectoderm → skin epidermis / nervous system.
• Mesoderm → muscle / bone / blood.
• Endoderm → gut lining / lungs / liver.

Question 4 (10)

(a) Functions (4) — 1 each

• Gas exchange — O₂ to fetus, CO₂ away.
• Nutrient supply — glucose, amino acids.
• Waste removal — urea.
• Endocrine — secretes hCG (maintains corpus luteum), progesterone, oestrogen. (Also accept passive immunity via IgG antibodies.)

(b) Structure (3)

• Barrier = placental membrane separating maternal blood in intervillous space from fetal capillaries in chorionic villi (1).
• Adaptation 1: large surface area (branching villi) (1).
• Adaptation 2: thin barrier / short diffusion distance + counter/close blood flow maintaining concentration gradient (1).

(c) Compensations (3) — 1 each, max 2 named + 1 explanation

• Fetal haemoglobin (HbF) has higher O₂ affinity → loads O₂ at lower pO₂ (1+1 explain).
• Higher fetal haematocrit / [Hb] raises O₂ carrying capacity (1).
• (Accept: Bohr effect — fetal blood picks up O₂ as CO�2 leaves.)

Question 5 (14)

(a) Labour (5)

• Stage 1 – dilation: cervix dilates (~10 cm); contractions begin (1).
• Stage 2 – expulsion: delivery of baby (1).
• Stage 3 – placental: delivery of placenta/afterbirth (1).
• Hormone = oxytocin drives uterine contractions (1).
• Ferguson positive-feedback reflex: fetal head stretches cervix → stretch receptors signal hypothalamus/posterior pituitary → more oxytocin → stronger contractions → more stretch → escalating cycle until birth (1).

(b) Combined pill (3)

• Contains oestrogen + progesterone which exert negative feedback on hypothalamus/pituitary (1).
• Suppresses FSH (no follicle maturation) and blocks the LH surge → no ovulation (1).
• Also thickens cervical mucus / thins endometrium as backup (1).

(c) Classification (4) — 1 each

(d) Combined use (2)

• Failure occurs only if both independently fail; probabilities multiply (1).
• e.g. if each fails 5% of the time, combined ≈ 0.05×0.05 = 0.25% — far lower; also cover different failure modes (mistimed pill vs. barrier tear) (1).

[
  {"claim":"Spermatogenesis yields 4 gametes per primary cell vs 1 for oogenesis",
   "code":"sperm=4; oo=1; result = (sperm==4 and oo==1)"},
  {"claim":"One primary spermatocyte gives 4 spermatids via 2 meiotic divisions (2->4)",
   "code":"after_MI=2; after_MII=after_MI*2; result = (after_MII==4)"},
  {"claim":"Combined method multiplicative failure: 5% and 5% give 0.25%",
   "code":"f = 0.05*0.05; result = abs(f-0.0025) < 1e-9"},
  {"claim":"Placental O2 shortfall: 30 needed minus 20 delivered = 10 mL/min deficit",
   "code":"deficit = 30-20; result = (deficit==10)"}
]