D3.1 Reproduction. Practice questions with markscheme.

• B: superovulation (FSH), harvesting, mixing with sperm (or ICSI) in vitro, then embryo transfer;

• D: progesterone (with oestradiol) sustains the endometrium and suppresses further follicle development; its fall triggers menstruation if no embryo implants;

• C: human fertilization is internal, normally in the oviduct, beginning with fusion of the sperm's cell membrane with the egg cell membrane; implantation happens days later, and only a single sperm participates: its nucleus enters the egg but the nuclei do not fuse (their membranes dissolve and the chromosomes join in a joint mitosis producing two diploid nuclei);

• A: wind flowers invest in pollen quantity and exposure; showy, rewarding, scented flowers advertise to animal pollinators;

• (a) [1]
  • 50 %;
• (b) [3]
  • self-pollination means both gametes come from the same plant / is inbreeding;
  • the self-pollinated line gets shorter and produces fewer seeds each generation (44 to 31 cm, 220 to 110 seeds), whereas the cross-pollinated line stays about the same (44 to 45 cm, about 220 seeds);
  • inbreeding reduces genetic diversity (within the line) each generation / the offspring become more genetically uniform (homozygous);
  • this loss of diversity reduces vigour, shown by shorter plants producing fewer seeds;
  • cross-pollination combines alleles from two different plants, so genetic diversity and vigour are maintained, OWTTE;

  Beyond this subtopic. Award if given; the marks above can be earned without it.

  • D3.2 harmful recessive alleles are more often homozygous in inbred plants, so they are expressed in the phenotype;
• (c) [2]
  • a lone self-incompatible plant cannot fertilize itself (its own pollen is rejected);
  • it needs pollen from another, genetically different plant, which is absent, so it produces no seeds;
  • so the species cannot establish a population in the new habitat (by sexual reproduction), whereas a self-compatible plant could produce seeds on its own, OWTTE;

  Accept "unless it can reproduce asexually" as part of a correct answer.

• sexual: generates genetic variation (meiosis + fertilization);
• (variation) allows adaptation to changing conditions / disease resistance in populations;
• asexual: rapid reproduction / no mate needed / all individuals can reproduce;
• asexual preserves successful (well-adapted) genotypes in stable environments, OWTTE;

• I, testa / seed coat;
• II, cotyledon;
• III, plumule / embryo shoot;
• IV, radicle / embryo root;

• ovary, annotated as producing eggs / secondary oocytes and secreting oestradiol and progesterone;
• oviduct (fallopian tube), annotated as the site of fertilization, carrying the egg / embryo towards the uterus;
• uterus (with muscular wall), annotated as the site of implantation and development of the embryo / fetus;
• endometrium, annotated as the lining that thickens each cycle and supports the embryo / is shed at menstruation;
• cervix, annotated as the ring of muscle / opening between the uterus and vagina that widens during childbirth;
• vagina, annotated as receiving the penis and sperm during intercourse and forming the birth canal;

Each mark needs a correctly placed label and a correct function. Ovaries must be shown to the sides of the uterus with the oviducts leading from near them to the top of the uterus.

• A: fission and vegetative propagation clone the parent; fertilization defines sexual reproduction;

• B: each gamete receives one chromosome of each pair, and the two pairs separate independently of each other, so meiosis breaks up the parental combinations AB and ab and also gives Ab and aB, which are new combinations;

• D: superovulation (FSH), harvesting, mixing with sperm (or ICSI) in vitro, then embryo transfer;

• C: genetic self-incompatibility: a shared S-allele halts the pollen tube, so a plant (and its close relatives) cannot fertilize it, enforced outbreeding;

• A: pollen transfer (wind/animal) precedes pollen-tube growth and gamete fusion; the fertilized ovule becomes the seed;

• both start from diploid germ cells that multiply by mitosis, include a growth phase, and use meiosis to produce haploid gametes;
• spermatogenesis produces four functional sperm per meiosis, whereas oogenesis produces one functional egg (plus polar bodies that degenerate);
• cytoplasmic division is equal in spermatogenesis but unequal in oogenesis (concentrating reserves in the single egg);
• spermatogenesis occurs continuously from puberty onwards, whereas oogenesis begins before birth, arrests, and releases usually one egg per cycle;
• sperm are small and motile, whereas the egg is large and non-motile, OWTTE;

Award converse comparative points.

• progesterone maintains pregnancy by inhibiting contraction of the uterine muscle / myometrium (and inhibiting oxytocin secretion);
• blocking progesterone receptors mimics the natural fall in progesterone (level / effect) at the end of pregnancy, OWTTE;
• the removal of progesterone's inhibition allows oxytocin secretion / oxytocin receptors in the uterus to increase;
• oxytocin stimulates contraction of the uterine muscle / myometrium;
• contractions stimulate further oxytocin secretion (positive feedback), so an infusion starts a cycle of increasingly strong contractions that continues until birth, OWTTE;

Accept 'progesterone secretion is switched off' as equivalent to a fall in progesterone. Award the final point only if positive feedback (or the idea of contractions increasing oxytocin) is stated.

• (early cycle) oestradiol at moderate levels inhibits FSH/LH secretion / negative feedback;
• (late follicular phase) high oestradiol stimulates LH (and FSH) secretion / positive feedback, causing the LH surge and ovulation;
• (luteal phase) progesterone (and oestradiol) inhibit FSH and LH, negative feedback preventing further follicles;
• falling progesterone (if no pregnancy) releases the inhibition, starting a new cycle, OWTTE;

• ovary, annotated as producing eggs / secondary oocytes and secreting oestradiol and progesterone;
• oviduct (fallopian tube), annotated as the site of fertilization, carrying the egg / embryo towards the uterus;
• uterus (with muscular wall), annotated as the site of implantation and development of the embryo / fetus;
• endometrium, annotated as the lining that thickens each cycle and supports the embryo / is shed at menstruation;
• cervix, annotated as the ring of muscle / opening between the uterus and vagina that widens during childbirth;
• vagina, annotated as receiving the penis and sperm during intercourse and forming the birth canal;

Each mark needs a correctly placed label and a correct function. Ovaries must be shown to the sides of the uterus with the oviducts leading from near them to the top of the uterus.