Reproduction
What Reproduction covers
This domain tests how organisms make gametes and offspring: flowering-plant structures, pollination, double fertilisation, seed formation, plus human gametogenesis, the menstrual cycle, fertilisation, implantation and contraception. Expect diagram-free questions on ploidy, hormone timing and events in sequence.
Core rules
- In angiosperms the embryo sac is usually 8-nucleate and 7-celled: 3 antipodals, 2 synergids, 1 egg and a central cell with 2 polar nuclei.
- Double fertilisation: one male gamete fuses with the egg (2n zygote); the other fuses with the two polar nuclei (3n primary endosperm nucleus).
- Spermatogenesis yields four haploid sperm from one primary spermatocyte; oogenesis yields one ovum and three polar bodies.
- The menstrual cycle: menstrual phase, follicular/proliferative phase, LH surge around day 14, ovulation, luteal/secretory phase; corpus luteum secretes progesterone.
- Contraceptive methods act at different points: barrier, hormonal (ovulation suppression), IUD (sperm/fertilisation), surgical (gamete transport).
Worked example
A plant has 2n = 24. State the chromosome number in the endosperm, embryo and an antipodal cell. Reasoning: endosperm forms after triple fusion, so it is triploid: 3 × 12 = 36. The embryo is diploid: 24. The antipodal cell is part of the haploid female gametophyte: 12. Answer: 36, 24, 12.
Traps
- Endosperm is not always triploid: it is triploid in typical angiosperms but can be diploid or polyploid in some taxa.
- Pollen grains are released 2-celled in about 60% of angiosperms and 3-celled in others; do not assume one pattern.
- Progesterone dominates the luteal phase, not oestrogen.
- The zona pellucida and corona radiata surround the ovum at fertilisation; the acrosome is on the sperm.