Reproductive biology

In short

Reproductive biology is the study of how organisms produce gametes, fertilize, develop embryos and regulate all of it hormonally — in zoology it spans invertebrate strategies, vertebrate cycles and detailed human reproductive physiology. It matters because reproduction links physiology, endocrinology, genetics and development, and because it underpins fertility medicine, animal breeding, wildlife conservation and population policy. It is examined in UGC-CBCS B.Sc Zoology core papers (Animal Physiology, Developmental Biology, Reproductive Biology electives), CSIR-UGC NET Life Sciences Unit VI/VII, GATE Biotechnology/XL (Zoology section), ICAR, and entrance tests for M.Sc and embryology diplomas. Employers who test it include IVF and ART clinics (embryologist and andrology-lab roles), animal-husbandry and dairy breeding organisations (NDDB, BAIF, state semen stations), pharma reproductive-toxicology labs, CROs, and conservation bodies such as CCMB-LaCONES and zoo breeding programmes.

Explained

Reproductive strategies and the gonads

• Asexual modes (invertebrates): binary fission (*Amoeba*, *Paramecium*), budding (*Hydra*, sponges), fragmentation and regeneration (*Planaria*, starfish), gemmule formation (sponges), parthenogenesis — development of an unfertilized egg.
• Parthenogenesis types: arrhenotoky (haploid males from unfertilized eggs — honeybee drones), thelytoky (females only — some rotifers, *Rhabditis*), paedogenesis (larva reproduces — *Miastor*); artificial parthenogenesis induced by pricking or ionic shock (Loeb's experiments on sea-urchin eggs).
• Sexual reproduction terms: gonochorism (separate sexes), hermaphroditism — simultaneous (earthworm, *Taenia*) or sequential (protandry in *Crepidula*, protogyny in some fish); cross-fertilization is the rule even in hermaphrodites to avoid inbreeding.
• Oviparity (egg-laying — most fish, birds, reptiles), viviparity (placental nourishment — eutherian mammals, some sharks), ovoviviparity (eggs hatch inside, yolk-nourished — *Squalus* (dogfish), many snakes; note *Scoliodon* is truly viviparous, with a yolk-sac placenta).
• Male system (human): paired testes in the scrotum (~2 °C below core temperature, essential for spermatogenesis), seminiferous tubules, rete testis, epididymis (sperm maturation and storage), vas deferens, seminal vesicles (~60% of semen, fructose), prostate (~25–30%, citrate, acid phosphatase, PSA), bulbourethral glands.
• Testis cells: Sertoli cells — nurse cells inside tubules, form the blood–testis barrier, secrete androgen-binding protein and inhibin, FSH-responsive; Leydig (interstitial) cells — outside tubules, secrete testosterone under LH.
• Female system: ovaries (cortex with follicles), fallopian tubes (infundibulum–ampulla–isthmus; fertilization occurs in the ampulla), uterus (perimetrium, myometrium, endometrium), cervix, vagina; accessory Bartholin's glands.
• Ovarian follicle stages: primordial → primary → secondary → tertiary/antral → Graafian (mature) follicle; after ovulation the remnant becomes the corpus luteum, degenerating to corpus albicans if no pregnancy.

Gametogenesis

• Spermatogenesis (in seminiferous tubules, ~64–74 days in humans, continuous from puberty):
  1. Multiplication phase: spermatogonia (type A self-renew, type B commit) divide mitotically — diploid, 2n = 46.
  2. Growth phase: type B becomes primary spermatocyte (2n, 46).
  3. Maturation phase: meiosis I → two secondary spermatocytes (n, 23); meiosis II → four spermatids (n, 23).
  4. Spermiogenesis: spermatid → spermatozoon — nucleus condenses, Golgi forms the acrosome, centriole forms the flagellum, mitochondria form the middle-piece spiral, cytoplasm shed as the residual body.
  5. Spermiation: release into the lumen; sperm are immotile until epididymal maturation, and gain fertilizing ability only after capacitation in the female tract.
• Sperm structure: head (nucleus + acrosome with hyaluronidase and acrosin), neck, middle piece (mitochondria — ATP for motility), principal and end pieces of tail (9+2 axoneme).
• Oogenesis (begins in fetal life, discontinuous):
  1. Oogonia multiply in the fetal ovary; no oogonia are formed after birth (classical view).
  2. Primary oocytes (2n) enter meiosis I and arrest at diplotene of prophase I — arrest lasts years to decades (dictyate stage).
  3. At each cycle, one dominant follicle completes meiosis I just before ovulation → a large secondary oocyte (n) + a tiny first polar body (unequal cytokinesis conserves cytoplasm).
  4. Meiosis II arrests at metaphase II; it completes only on sperm entry, giving the ovum and the second polar body. So one oogonium → one ovum + 3 polar bodies (vs 4 sperm per spermatogonium).
• Germ-cell counts (approximate, textbook figures): ~7 million oogonia at 5 months of gestation, ~1–2 million at birth, ~400,000 at puberty, ~400–500 ovulated in a lifetime; the rest undergo atresia.
• Egg types by yolk: alecithal (eutherian mammals), microlecithal, mesolecithal (frog), macrolecithal/polylecithal (bird, reptile); by yolk distribution — isolecithal, telolecithal, centrolecithal (insects), discoidal.
• Egg membranes: primary — vitelline membrane/zona pellucida (secreted by the oocyte); secondary — chorion, follicle cells, corona radiata (by follicle cells); tertiary — albumen, shell membranes, shell (by the oviduct in birds).
• Vitellogenesis: yolk precursor vitellogenin is made in the liver (vertebrates) or fat body (insects) under oestrogen/juvenile hormone control and taken up by the oocyte by receptor-mediated endocytosis.

Hormonal control — the hypothalamo–pituitary–gonadal (HPG) axis

• GnRH from the hypothalamus is released in pulses into the hypophyseal portal system → anterior pituitary gonadotropes secrete FSH and LH (glycoprotein hormones); pulse frequency decides the FSH:LH ratio (slow pulses favour FSH).
• In the male: LH → Leydig cells → testosterone (which also feeds back negatively on GnRH/LH); FSH + testosterone → Sertoli cells → support spermatogenesis; inhibin from Sertoli cells selectively suppresses FSH. Testosterone drives secondary sexual characters, libido, muscle and bone mass, and spermatogenesis.
• In the female: FSH → granulosa cells → aromatase converts androgens to oestradiol; LH → theca interna cells → androgen precursors. This is the two-cell, two-gonadotropin model.
• Feedback switch: low–moderate oestradiol inhibits LH; sustained high oestradiol from the dominant follicle flips to positive feedback, producing the LH surge that triggers ovulation ~36 h after the onset of the LH surge and ~10–12 h after the LH peak.
• Menstrual cycle (~28 days, day 1 = first day of bleeding):
  • *Menstrual phase* (days 1–5): corpus luteum regression → progesterone fall → endometrial shedding.
  • *Follicular/proliferative phase* (≈days 1–13): FSH recruits follicles, one becomes dominant, oestrogen rebuilds the endometrium.
  • *Ovulation* (≈day 14): LH surge ruptures the Graafian follicle; the secondary oocyte is swept into the tube.
  • *Luteal/secretory phase* (days 15–28): LH converts granulosa and theca cells into the corpus luteum; progesterone makes the endometrium secretory, thickens cervical mucus, raises basal body temperature by ~0.3–0.5 °C, and suppresses GnRH.
• Oestrous versus menstrual cycle: in oestrous mammals (rat, cow, dog) there is no menstrual bleeding — the endometrium is resorbed; phases are proestrus, oestrus (heat, receptivity), metoestrus, dioestrus (and anoestrus). Rats are polyoestrous (4–5 day cycle), dogs monoestrous/dioestrous, cattle polyoestrous (~21 days).
• Seasonal breeders use photoperiod via the pineal gland and melatonin; long-day breeders (horse, hamster) versus short-day breeders (sheep, goat, deer).
• Puberty: reactivation of GnRH pulsatility, gated by kisspeptin (KISS1/KISS1R) neurons and permissive leptin levels; adrenarche precedes gonadarche.
• Menopause: follicle exhaustion → oestradiol and inhibin fall → FSH and LH rise markedly (high FSH is the diagnostic marker); typical age ~45–52 years in Indian women, somewhat earlier than Western averages in several Indian cohort studies.

Fertilization, implantation and pregnancy

• Capacitation: removal of cholesterol and glycoproteins from the sperm membrane in the female tract (hours) → hyperactivated motility and competence to undergo the acrosome reaction.
• Steps of fertilization: sperm–zona binding (ZP3 and ZP2 glycoproteins; IZUMO1 on sperm binds JUNO on the egg for fusion) → acrosome reaction releasing acrosin/hyaluronidase → penetration of the zona → membrane fusion → cortical reaction (Ca²⁺ wave releases cortical granules, zona hardening = slow block to polyspermy; membrane depolarization = fast block) → completion of meiosis II → amphimixis (fusion of pronuclei) → zygote (2n, 46).
• Cleavage and early development: holoblastic (equal in mammals, unequal in frog) or meroblastic (discoidal in birds, superficial in insects) → morula (~day 4) → blastocyst with trophoblast and inner cell mass (~day 5) → hatching from the zona.
• Implantation (~day 6–7 after ovulation, in the upper posterior uterine wall): apposition → adhesion → invasion; trophoblast differentiates into cytotrophoblast and syncytiotrophoblast; interstitial implantation in humans. Endometrium undergoes decidualization.
• Placenta: human placenta is chorioallantoic, discoidal, deciduate and haemochorial (chorion bathed directly in maternal blood); functions — exchange of O₂/CO₂, nutrients and wastes; barrier; endocrine organ secreting hCG, hPL (human placental lactogen), progesterone, oestrogens and relaxin.
• Placenta types by contact: epitheliochorial (pig, horse), synepitheliochorial/syndesmochorial (ruminants), endotheliochorial (dog, cat), haemochorial (human, rodents); by shape — diffuse, cotyledonary, zonary, discoidal.
• hCG: secreted by the syncytiotrophoblast from about implantation, rescues the corpus luteum so progesterone continues; basis of urine and serum pregnancy tests (detects the β-subunit); peaks around 8–10 weeks then falls as the placenta takes over progesterone output (luteo-placental shift, ~8–10 weeks).
• Gestation and extra-embryonic membranes: human gestation ~280 days (40 weeks) from LMP, ~266 days from conception; amnion (fluid cushion), chorion, allantois (vessels), yolk sac (early haematopoiesis and germ cells).
• Parturition: fetal hypothalamo–pituitary–adrenal activation and placental CRH, rising oestrogen:progesterone ratio, more oxytocin receptors in the myometrium; oxytocin and prostaglandins (PGF2α, PGE2) drive contractions in a positive-feedback neuroendocrine reflex (Ferguson reflex); relaxin softens the cervix and pubic symphysis. Stages: dilatation, expulsion, placental delivery.
• Lactation: oestrogen and progesterone build the duct–alveolar system in pregnancy while blocking secretion; after placental delivery progesterone falls and prolactin initiates milk secretion (lactogenesis); oxytocin causes milk ejection (let-down) via myoepithelial contraction. Colostrum is rich in IgA and antibodies. Suckling-induced prolactin suppresses GnRH → lactational amenorrhoea.

Sex determination, differentiation and reproductive health

• Mechanisms: XX–XY (mammals, *Drosophila* — though *Drosophila* uses the X:autosome ratio, not Y), XX–XO (grasshopper), ZZ–ZW (birds), haplodiploidy (bees), environmental/temperature-dependent (TSD in turtles and crocodiles), genic balance; Barr body — the inactivated X (Lyon hypothesis), one less than the X number.
• Gonadal differentiation: SRY on Yp → SOX9 → testis determination; Sertoli cells secrete anti-Müllerian hormone (AMH) causing Müllerian duct regression; Leydig testosterone retains the Wolffian ducts; dihydrotestosterone (DHT) via 5α-reductase masculinizes the external genitalia. In the absence of SRY, the WNT4/RSPO1/FOXL2 pathway gives an ovary and Müllerian derivatives (tubes, uterus, upper vagina).
• Disorders (commonly asked): Klinefelter (47,XXY), Turner (45,X), androgen insensitivity (AR mutation), 5α-reductase deficiency, congenital adrenal hyperplasia, freemartin in cattle.
• Infertility: male factors — oligozoospermia, azoospermia, varicocele, Y-microdeletions, heat and toxin exposure; female factors — tubal block (pelvic inflammatory disease, genital tuberculosis — significant in India), PCOS (hyperandrogenism, oligo-anovulation, polycystic ovaries, insulin resistance; Rotterdam criteria — any two of three), endometriosis, hyperprolactinaemia, premature ovarian insufficiency. WHO 2021 (6th edition) semen reference lower limits: volume ≥1.4 mL, concentration ≥16 million/mL, total count ≥39 million, total motility ≥42%, progressive motility ≥30%, normal forms ≥4%.
• Contraception: natural (rhythm, withdrawal, lactational amenorrhoea); barrier (condom, diaphragm, cervical cap); IUDs — copper (Cu-T 380A, spermicidal copper ions) and hormonal (LNG-IUS); hormonal — combined oral pills (Mala-N, Mala-D in the national programme), progestin-only pills, injectable DMPA (Antara programme), implants; emergency contraception (levonorgestrel, Ezy-Pill/i-Pill type products, within 72 h); sterilization — vasectomy and tubectomy/laparoscopic ligation. Centchroman (ormeloxifene, Chhaya) is a non-steroidal weekly pill developed at CDRI Lucknow and used in India's programme.
• ART: IUI, IVF, ICSI (single sperm injected — for severe male factor), GIFT and ZIFT (largely historical), embryo and oocyte cryopreservation (vitrification), blastocyst culture, PGT, surrogacy. India's first scientifically documented IVF baby, Harsha Chawda (Kanupriya), was born in Mumbai in 1986 under Dr Indira Hinduja; Dr Subhas Mukhopadhyay's 1978 Kolkata case (Durga) received recognition later.
• Indian law and practice (as applicable; details are amended periodically — verify current rules):
  • ART (Regulation) Act, 2021 — registration of ART clinics and banks in a National Registry, donor and commissioning-couple eligibility, consent and record rules.
  • Surrogacy (Regulation) Act, 2021 — only altruistic surrogacy; commercial surrogacy prohibited; eligibility certificates required (several provisions have been subject to Supreme Court challenge and amendment of rules).
  • PCPNDT Act, 1994 (Pre-Conception and Pre-Natal Diagnostic Techniques, amended 2003) — bans sex selection and disclosure of fetal sex; mandatory Form F records and clinic registration.
  • MTP Act, 1971 as amended 2021 — abortion up to 20 weeks on one RMP's opinion, 20–24 weeks for specified categories on two RMPs' opinion, beyond 24 weeks only for substantial fetal abnormality via a Medical Board.
  • Transplantation/tissue and drug rules, CPCSEA norms for animal experiments, and ICMR National Ethical Guidelines (2017) govern reproductive research; National Family Health Survey (NFHS-5, 2019–21) reported India's total fertility rate at 2.0, below the replacement level of 2.1.

Key points

• Spermatogenesis - one spermatogonium → 4 sperm; continuous, ~64–74 days, needs a temperature below core body temperature.
• Oogenesis - one oogonium → 1 ovum + 3 polar bodies; starts in fetal life, arrested at diplotene, completed only after fertilization.
• Sertoli cells - FSH target; blood–testis barrier, nutrition, inhibin (FSH-specific negative feedback).
• Leydig cells - LH target; testosterone synthesis and secondary sexual characters.
• Two-cell two-gonadotropin model - theca makes androgens under LH, granulosa aromatizes them to oestradiol under FSH.
• LH surge - caused by high sustained oestradiol switching feedback from negative to positive; ovulation follows ~36 h later.
• Corpus luteum - progesterone source of the luteal phase; rescued by hCG if implantation occurs, else becomes corpus albicans in ~12–14 days.
• Progesterone - secretory endometrium, thick cervical mucus, myometrial quiescence, slight basal temperature rise.
• Capacitation and acrosome reaction - capacitation happens in the female tract; the acrosome reaction occurs at the zona and releases lytic enzymes.
• Block to polyspermy - fast electrical depolarization plus slow cortical-granule-mediated zona hardening.
• Implantation - blastocyst attaches ~6–7 days after ovulation; syncytiotrophoblast invades and secretes hCG.
• Human placenta - discoidal, deciduate, haemochorial, chorioallantoic; endocrine as well as nutritive.
• Luteo-placental shift - progesterone source moves from corpus luteum to placenta at about 8–10 weeks.
• Parturition - rising oestrogen:progesterone ratio plus oxytocin and prostaglandins; Ferguson positive-feedback reflex.
• Lactation - prolactin for secretion, oxytocin for ejection; colostrum carries IgA.
• SRY → AMH + testosterone + DHT - the male pathway; its absence gives the female pathway.
• Oestrous versus menstrual - oestrous mammals resorb the endometrium and show behavioural heat; menstruating primates shed it.
• Indian statutes to name - ART Act 2021, Surrogacy Act 2021, PCPNDT Act 1994, MTP Act 1971 (amended 2021).

Formulas and examples

• Example 1 — EDD and ovulation date. LMP = 12 March 2026, regular 30-day cycles.
  • Naegele's rule: 12 March + 9 months = 12 December 2026; + 7 days = 19 December 2026.
  • Cross-check by 280 days: March remainder 19 + April 30 + May 31 + June 30 + July 31 + Aug 31 + Sep 30 + Oct 31 + Nov 30 = 263 days to 30 November; 280 − 263 = 17 → 17 December 2026. The two differ by a couple of days because calendar months are unequal — clinics use the 280-day/ultrasound dating.
  • Ovulation ≈ day 30 − 14 = day 16, i.e. 27 March 2026 (12 March is day 1, so day 16 = 12 + 15 = 27 March).

• Example 2 — semen analysis against WHO 2021 limits. Volume 2.0 mL; haemocytometer count 60 sperm in 2 large WBC squares at 1:20 dilution; motility: 30 progressive, 12 non-progressive, 58 immotile out of 100.
  • Concentration = N × dilution × 10⁴ / squares = 60 × 20 × 10⁴ / 2 = 6.0 × 10⁶ per mL = 6 million/mL.
  • Total count = 6 × 2.0 = 12 million per ejaculate.
  • Total motility = 30 + 12 = 42%; progressive = 30%.
  • Verdict: motility is just at the reference limits, but concentration (6 vs ≥16 million/mL) and total count (12 vs ≥39 million) are below them → oligozoospermia; ICSI is typically indicated over conventional IVF.

• Example 3 — Pearl Index of a contraceptive. 1,200 women are followed for 18 months each on a copper IUD; 9 unintended pregnancies occur.
  • Woman-months = 1,200 × 18 = 21,600.
  • Pearl Index = (9 × 1200)/21,600 = 10,800/21,600 = 0.50 pregnancies per 100 woman-years — i.e. about 0.5% failure per year, consistent with published Cu-T 380A performance.

• Example 4 — TFR from age-specific fertility rates. ASFRs per 1,000 women: 15–19: 40; 20–24: 160; 25–29: 120; 30–34: 50; 35–39: 20; 40–44: 6; 45–49: 2.
  • Sum = 40 + 160 + 120 + 50 + 20 + 6 + 2 = 398 per 1,000 = 0.398 per woman per year in each group.
  • TFR = 0.398 × 5 = 1.99 children per woman — below the replacement level of 2.1, matching the NFHS-5 figure of 2.0 for India.

• Example 5 — hCG doubling in early pregnancy. Serum β-hCG is 120 mIU/mL on day 0 and 460 mIU/mL 96 hours later.
  • Fold rise = 460/120 = 3.83 over 4 days; doublings = log₂(3.83) ≈ 1.94 → doubling time = 96/1.94 ≈ 49.5 hours.
  • A normal early intrauterine pregnancy doubles roughly every 48–72 h, so this rise is reassuring; a plateau or a slow rise raises suspicion of ectopic pregnancy or non-viability.

Where it is used

• IVF and ART clinics (Nova IVF, Indira IVF, Milann, Cloudnine, government ART units): embryologist, andrologist and ART-lab technician roles — oocyte retrieval handling, ICSI, vitrification, semen analysis per WHO 2021, record-keeping under the ART Act 2021.
• Dairy and animal breeding: NDDB, BAIF, state semen stations and Sabarmati/Gujarat cooperatives — oestrus synchronization and timed AI, frozen-semen quality control (motility, CASA), embryo transfer and IVF in buffalo and cattle, sex-sorted semen.
• Poultry, fisheries and aquaculture: induced breeding of carps with Ovaprim/GnRH analogues and pituitary extract at ICAR-CIFA and CIFE; hatchery management; photoperiod control in layer birds.
• Pharma and CRO reproductive toxicology: OECD 414/416/443 segment I–III studies on fertility, teratogenicity and perinatal effects in rats and rabbits, under CPCSEA-registered animal facilities (Syngene, Sai Life, Jai Research Foundation).
• Wildlife conservation and zoos: CCMB-LaCONES, Central Zoo Authority programmes — non-invasive faecal hormone monitoring, gamete and genome resource banking, assisted reproduction in deer and wild cats.
• Public health and policy: National Family Planning Programme and Mission Parivar Vikas (Antara injectable, Chhaya/ormeloxifene pill), RMNCH+A, NFHS survey analysis, ASHA and counsellor training; diagnostics companies making hCG, LH-surge and AMH test kits.

Interview questions

• Q: Why are the testes located outside the abdominal cavity? A: Spermatogenesis needs a temperature about 2–3 °C below core body temperature; the scrotum, pampiniform plexus and dartos muscle maintain it, which is why cryptorchidism causes infertility.
• Q: What triggers ovulation? A: Sustained high oestradiol from the dominant follicle switches hypothalamo-pituitary feedback from negative to positive, producing an LH surge that ruptures the Graafian follicle about 36 hours later.
• Q: Why does one oogonium yield only one ovum while a spermatogonium yields four sperm? A: Oogenesis has unequal cytokinesis so nearly all cytoplasm, yolk and maternal mRNA stay in one cell; the other three products become polar bodies and degenerate.
• Q: Where exactly does fertilization occur, and where implantation? A: Fertilization in the ampulla of the fallopian tube; implantation in the endometrium of the upper posterior uterine wall about 6–7 days later.
• Q: What is capacitation and why does it matter clinically? A: It is the destabilizing of the sperm membrane in the female tract that enables hyperactivated motility and the acrosome reaction; in vitro it must be mimicked with capacitation media before IVF.
• Q: How does the body prevent polyspermy? A: A fast electrical block by egg-membrane depolarization, then a slow block in which the Ca²⁺-triggered cortical reaction hardens the zona pellucida and removes sperm receptors.
• Q: Why does a pregnancy test detect hCG and not progesterone? A: hCG is secreted only by trophoblast, appearing within days of implantation and rising steeply, so it is specific; progesterone is high in every luteal phase and cannot distinguish pregnancy.
• Q: What keeps the corpus luteum alive in pregnancy? A: hCG from the syncytiotrophoblast acts on LH receptors to maintain progesterone output until the placenta takes over at around 8–10 weeks.
• Q: Differentiate the oestrous and menstrual cycles. A: Both are ovarian–uterine cycles, but oestrous mammals resorb the endometrium and show behavioural heat, while menstruating primates shed it as visible bleeding and are receptive throughout.
• Q: What is the role of SRY, and what happens if it is absent or non-functional? A: SRY drives SOX9 and testis formation with AMH and testosterone; without functional SRY the gonad follows the ovarian pathway, so a 46,XY individual with SRY deletion develops as phenotypically female (Swyer syndrome).
• Q: What are the WHO 2021 lower reference limits you would use to report a semen sample? A: Volume ≥1.4 mL, concentration ≥16 million/mL, total count ≥39 million, total motility ≥42%, progressive ≥30%, normal morphology ≥4% — they are population reference limits, not a pass/fail fertility test.
• Q: Which Indian laws govern an ART laboratory, and what does each control? A: The ART (Regulation) Act 2021 (clinic and bank registration, donor and consent rules), the Surrogacy (Regulation) Act 2021 (altruistic surrogacy only), the PCPNDT Act 1994 (no sex selection or sex disclosure) and the MTP Act 1971 as amended in 2021 (gestational limits for termination).

Common mistakes

• Saying the human oocyte completes meiosis before ovulation — meiosis I finishes just before ovulation, but meiosis II only on sperm entry.
• Confusing the cell types: writing that FSH acts on Leydig cells or that Sertoli cells make testosterone.
• Claiming the LH surge is caused by progesterone or by low oestrogen; it is sustained *high* oestradiol switching to positive feedback.
• Stating ovulation is always on day 14 — only the luteal phase is near-fixed at ~14 days; in a 35-day cycle ovulation is around day 21.
• Calling the human placenta epitheliochorial or non-deciduate; it is discoidal, deciduate and haemochorial.
• Mixing up prolactin and oxytocin in lactation, or forgetting that high progesterone blocks milk secretion until the placenta is delivered.
• Unit and dilution errors in semen counts (forgetting the dilution factor or the 10⁴ chamber factor), and reporting concentration as if it were total count.
• Treating parthenogenesis as cloning, or calling all hermaphrodites self-fertilizing.

Related topics

Nervous and endocrine systems · Developmental biology: cleavage, gastrulation and organogenesis · Genetics: sex-linked inheritance and chromosomal disorders · Cell biology: mitosis and meiosis · Animal behaviour: mating systems and parental care · Comparative anatomy of vertebrate urinogenital systems